From 43a97878ce14b72f0981164f87f2e35e14151312 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Sun, 7 Apr 2024 11:22:09 +0200
Subject: Adding upstream version 110.0.1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 .../web-platform/tests/scroll-animations/META.yml  |   4 +
 .../css/animation-duration-auto.tentative.html     |  56 ++
 .../scroll-animations/css/animation-shorthand.html |  43 ++
 .../css/animation-timeline-computed.html           |  76 ++
 .../css/animation-timeline-ignored.tentative.html  | 145 ++++
 .../css/animation-timeline-in-keyframe.html        |  27 +
 .../css/animation-timeline-multiple.html           |  90 +++
 ...e-named-scroll-progress-timeline.tentative.html | 662 ++++++++++++++++++
 .../css/animation-timeline-none.html               |  41 ++
 .../css/animation-timeline-parsing.html            |  88 +++
 ...eline-scroll-functional-notation.tentative.html | 148 ++++
 ...imeline-view-functional-notation.tentative.html | 263 +++++++
 .../css/get-animations-inactive-timeline.html      |  87 +++
 ...keyframes-with-document-timeline.tentative.html |  54 ++
 ...imeline-none-with-progress-print.tentative.html |  56 ++
 .../animation-timeline-none-with-progress-ref.html |  38 +
 .../scroll-timeline-default-iframe-print.html      |  65 ++
 .../scroll-timeline-default-print.tentative.html   |  62 ++
 .../scroll-timeline-specified-scroller-print.html  |  58 ++
 .../scroll-timeline-specified-scroller-ref.html    |  38 +
 ...on-animation-longhand-properties.tentative.html | 255 +++++++
 .../css/progress-based-animation-timeline.html     |  52 ++
 .../scroll-timeline-axis-computed.tentative.html   |  35 +
 .../scroll-timeline-axis-parsing.tentative.html    |  27 +
 .../css/scroll-timeline-axis-writing-mode.html     | 126 ++++
 .../css/scroll-timeline-default-iframe-ref.html    |  33 +
 .../css/scroll-timeline-default-iframe.html        |  73 ++
 .../css/scroll-timeline-default-quirks-mode.html   |  63 ++
 .../css/scroll-timeline-default-ref.html           |  31 +
 ...croll-timeline-default-writing-mode-rl-ref.html |  32 +
 .../scroll-timeline-default-writing-mode-rl.html   |  65 ++
 .../css/scroll-timeline-default.html               |  63 ++
 .../scroll-timeline-document-scroller-quirks.html  |  36 +
 .../css/scroll-timeline-dynamic.tentative.html     | 267 +++++++
 .../scroll-timeline-frame-size-changed-ref.html    |  31 +
 .../css/scroll-timeline-frame-size-changed.html    |  67 ++
 .../css/scroll-timeline-in-container-query.html    |  74 ++
 .../css/scroll-timeline-inactive.html              |  51 ++
 .../scroll-timeline-inline-orientation-ref.html    |  32 +
 .../css/scroll-timeline-inline-orientation.html    |  68 ++
 .../css/scroll-timeline-multi-pass.tentative.html  | 109 +++
 .../scroll-timeline-name-computed.tentative.html   |  34 +
 .../scroll-timeline-name-parsing.tentative.html    |  30 +
 .../css/scroll-timeline-name-tree-scoped.html      | 144 ++++
 .../css/scroll-timeline-nearest-dirty.html         |  42 ++
 .../css/scroll-timeline-paused-animations.html     |  95 +++
 ...roll-timeline-responsiveness-from-endpoint.html |  62 ++
 .../css/scroll-timeline-root-dirty.html            |  35 +
 .../css/scroll-timeline-sampling.html              |  46 ++
 .../css/scroll-timeline-shorthand.tentative.html   |  41 ++
 .../css/scroll-timeline-sibling-gcs.html           |  52 ++
 .../scroll-animations/css/support/testcommon.js    |  19 +
 ...e-range-name-offset-in-keyframes.tentative.html |  94 +++
 .../css/view-timeline-animation.html               | 219 ++++++
 .../css/view-timeline-axis-computed.html           |  37 +
 .../css/view-timeline-axis-parsing.html            |  29 +
 .../css/view-timeline-delay-animation.html         | 144 ++++
 .../css/view-timeline-dynamic.html                 | 183 +++++
 .../css/view-timeline-inset-animation.html         | 768 +++++++++++++++++++++
 .../css/view-timeline-inset-computed.html          |  41 ++
 .../css/view-timeline-inset-parsing.html           |  34 +
 .../css/view-timeline-lookup.html                  | 301 ++++++++
 .../css/view-timeline-name-computed.html           |  36 +
 .../css/view-timeline-name-parsing.html            |  30 +
 .../css/view-timeline-name-tree-scoped.html        | 145 ++++
 .../css/view-timeline-shorthand.tentative.html     | 104 +++
 .../css/view-timeline-used-values.html             | 104 +++
 .../scroll-timelines/animation-ref.html            |  45 ++
 .../animation-with-animatable-interface.html       |  66 ++
 .../animation-with-delay-crash.html                |  31 +
 .../animation-with-display-none.html               |  75 ++
 .../animation-with-offsets-crash.html              |  32 +
 .../animation-with-overflow-hidden-ref.html        |  45 ++
 .../animation-with-overflow-hidden.html            |  64 ++
 .../animation-with-root-scroller-ref.html          |  37 +
 .../animation-with-root-scroller.html              |  60 ++
 .../scroll-timelines/animation-with-transform.html |  68 ++
 .../scroll-timelines/cancel-animation.html         | 214 ++++++
 .../scroll-timelines/constructor-no-document.html  |  19 +
 .../scroll-timelines/constructor.html              |  95 +++
 .../scroll-timelines/current-time-nan.html         |  80 +++
 .../current-time-root-scroller.html                |  49 ++
 .../current-time-writing-modes.html                | 148 ++++
 .../scroll-timelines/effect-updateTiming.html      | 602 ++++++++++++++++
 .../scroll-timelines/finish-animation.html         | 449 ++++++++++++
 .../scroll-timelines/idlharness.window.js          |  16 +
 ...ayout-changes-on-percentage-based-timeline.html |  84 +++
 .../scroll-timelines/null-scroll-source-crash.html |  24 +
 .../scroll-timelines/pause-animation.html          | 210 ++++++
 .../scroll-timelines/play-animation.html           | 233 +++++++
 .../progress-based-effect-delay-ref.html           |  45 ++
 .../progress-based-effect-delay.tentative.html     |  69 ++
 .../scroll-timelines/reverse-animation.html        | 240 +++++++
 ...roll-animation-effect-fill-modes.tentative.html | 137 ++++
 .../scroll-animation-effect-phases.tentative.html  | 480 +++++++++++++
 .../scroll-animation-inactive-timeline.html        | 170 +++++
 .../scroll-timelines/scroll-animation.html         | 216 ++++++
 .../scroll-timeline-in-removed-iframe-crash.html   |  20 +
 .../scroll-timeline-invalidation.html              | 128 ++++
 .../scroll-timeline-snapshotting.html              |  44 ++
 .../set-current-time-before-play.html              |  69 ++
 .../scroll-timelines/setting-current-time.html     | 286 ++++++++
 .../scroll-timelines/setting-playback-rate.html    | 323 +++++++++
 .../scroll-timelines/setting-start-time.html       | 439 ++++++++++++
 .../setting-timeline.tentative.html                | 392 +++++++++++
 .../scroll-timelines/source-quirks-mode.html       |  36 +
 .../scroll-timelines/testcommon.js                 | 124 ++++
 ...-attach-to-same-scroll-timeline-cancel-one.html |  84 +++
 ...-animations-attach-to-same-scroll-timeline.html |  79 +++
 .../scroll-timelines/update-playback-rate.html     | 181 +++++
 .../updating-the-finished-state.html               | 565 +++++++++++++++
 ...imeline-current-time-vertical-rl.tentative.html |  97 +++
 ...block-view-timeline-current-time.tentative.html | 205 ++++++
 ...ock-view-timeline-nested-subject.tentative.html | 113 +++
 ...nline-view-timeline-current-time.tentative.html | 289 ++++++++
 .../scroll-animations/view-timelines/testcommon.js | 137 ++++
 .../view-timeline-delay-large-subject.html         |  93 +++
 .../view-timelines/view-timeline-delay.html        | 103 +++
 .../view-timeline-get-current-time-range-name.html | 145 ++++
 .../view-timelines/view-timeline-inset.html        | 226 ++++++
 .../view-timelines/view-timeline-snapport.html     |  58 ++
 .../view-timeline-source.tentative.html            |  94 +++
 .../view-timeline-subject-size-changes.html        |  79 +++
 123 files changed, 15072 insertions(+)
 create mode 100644 testing/web-platform/tests/scroll-animations/META.yml
 create mode 100644 testing/web-platform/tests/scroll-animations/css/animation-duration-auto.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/animation-shorthand.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/animation-timeline-computed.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/animation-timeline-ignored.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/animation-timeline-in-keyframe.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/animation-timeline-multiple.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/animation-timeline-named-scroll-progress-timeline.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/animation-timeline-none.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/animation-timeline-parsing.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/animation-timeline-scroll-functional-notation.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/animation-timeline-view-functional-notation.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/get-animations-inactive-timeline.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/named-range-keyframes-with-document-timeline.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/printing/animation-timeline-none-with-progress-print.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/printing/animation-timeline-none-with-progress-ref.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-default-iframe-print.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-default-print.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-specified-scroller-print.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-specified-scroller-ref.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/progress-based-animation-animation-longhand-properties.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/progress-based-animation-timeline.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-axis-computed.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-axis-parsing.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-axis-writing-mode.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-iframe-ref.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-iframe.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-quirks-mode.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-ref.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-writing-mode-rl-ref.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-writing-mode-rl.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-default.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-document-scroller-quirks.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-dynamic.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-frame-size-changed-ref.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-frame-size-changed.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-in-container-query.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-inactive.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-inline-orientation-ref.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-inline-orientation.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-multi-pass.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-name-computed.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-name-parsing.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-name-tree-scoped.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-nearest-dirty.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-paused-animations.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-responsiveness-from-endpoint.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-root-dirty.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-sampling.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-shorthand.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/scroll-timeline-sibling-gcs.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/support/testcommon.js
 create mode 100644 testing/web-platform/tests/scroll-animations/css/timeline-range-name-offset-in-keyframes.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/view-timeline-animation.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/view-timeline-axis-computed.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/view-timeline-axis-parsing.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/view-timeline-delay-animation.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/view-timeline-dynamic.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/view-timeline-inset-animation.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/view-timeline-inset-computed.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/view-timeline-inset-parsing.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/view-timeline-lookup.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/view-timeline-name-computed.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/view-timeline-name-parsing.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/view-timeline-name-tree-scoped.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/view-timeline-shorthand.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/css/view-timeline-used-values.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/animation-ref.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-animatable-interface.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-delay-crash.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-display-none.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-offsets-crash.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-overflow-hidden-ref.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-overflow-hidden.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-root-scroller-ref.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-root-scroller.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-transform.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/cancel-animation.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/constructor-no-document.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/constructor.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/current-time-nan.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/current-time-root-scroller.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/current-time-writing-modes.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/effect-updateTiming.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/finish-animation.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/idlharness.window.js
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/layout-changes-on-percentage-based-timeline.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/null-scroll-source-crash.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/pause-animation.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/play-animation.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/progress-based-effect-delay-ref.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/progress-based-effect-delay.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/reverse-animation.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation-effect-fill-modes.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation-effect-phases.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation-inactive-timeline.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-timeline-in-removed-iframe-crash.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-timeline-invalidation.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-timeline-snapshotting.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/set-current-time-before-play.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/setting-current-time.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/setting-playback-rate.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/setting-start-time.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/setting-timeline.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/source-quirks-mode.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/testcommon.js
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/two-animations-attach-to-same-scroll-timeline-cancel-one.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/two-animations-attach-to-same-scroll-timeline.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/update-playback-rate.html
 create mode 100644 testing/web-platform/tests/scroll-animations/scroll-timelines/updating-the-finished-state.html
 create mode 100644 testing/web-platform/tests/scroll-animations/view-timelines/block-view-timeline-current-time-vertical-rl.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/view-timelines/block-view-timeline-current-time.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/view-timelines/block-view-timeline-nested-subject.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/view-timelines/inline-view-timeline-current-time.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/view-timelines/testcommon.js
 create mode 100644 testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-delay-large-subject.html
 create mode 100644 testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-delay.html
 create mode 100644 testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-get-current-time-range-name.html
 create mode 100644 testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-inset.html
 create mode 100644 testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-snapport.html
 create mode 100644 testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-source.tentative.html
 create mode 100644 testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-subject-size-changes.html

(limited to 'testing/web-platform/tests/scroll-animations')

diff --git a/testing/web-platform/tests/scroll-animations/META.yml b/testing/web-platform/tests/scroll-animations/META.yml
new file mode 100644
index 0000000000..c7f0e4903b
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/META.yml
@@ -0,0 +1,4 @@
+spec: https://drafts.csswg.org/scroll-animations/
+suggested_reviewers:
+  - birtles
+  - graouts
diff --git a/testing/web-platform/tests/scroll-animations/css/animation-duration-auto.tentative.html b/testing/web-platform/tests/scroll-animations/css/animation-duration-auto.tentative.html
new file mode 100644
index 0000000000..f44a682890
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/animation-duration-auto.tentative.html
@@ -0,0 +1,56 @@
+<!DOCTYPE html>
+<title>animation-duration: auto</title>
+<link rel="help" src="https://github.com/w3c/csswg-drafts/issues/6530">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="./support/testcommon.js"></script>
+<script src="/css/support/parsing-testcommon.js"></script>
+<script src="/css/support/computed-testcommon.js"></script>
+<style>
+  #scroller {
+    overflow: hidden;
+    width: 100px;
+    height: 100px;
+  }
+  #scroller > div {
+    height: 200px;
+    width: 200px;
+  }
+
+  @keyframes anim {
+    from { z-index: 0; }
+    to { z-index: 100; }
+  }
+
+  #scroller {
+    scroll-timeline: timeline;
+  }
+
+  #element {
+    z-index: -1;
+    animation-name: anim;
+    animation-duration: auto;
+    animation-timeline: timeline;
+  }
+</style>
+<main>
+  <div id=scroller>
+    <div></div>
+  </div>
+  <div id=element></div>
+</main>
+<script>
+  promise_test(async (t) => {
+    await waitForCSSScrollTimelineStyle();
+    assert_equals(getComputedStyle(element).zIndex, '0');
+  }, 'A value of auto can be specified for animation-duration');
+</script>
+
+<div id="target"></div>
+<script>
+  test_valid_value('animation-duration', 'auto');
+  test_computed_value('animation-duration', 'auto');
+  test_valid_value('animation', 'auto cubic-bezier(0, -2, 1, 3) -3s 4 reverse both paused anim');
+  test_computed_value('animation', 'auto cubic-bezier(0, -2, 1, 3) -3s 4 reverse both paused anim');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/animation-shorthand.html b/testing/web-platform/tests/scroll-animations/css/animation-shorthand.html
new file mode 100644
index 0000000000..60d6c76646
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/animation-shorthand.html
@@ -0,0 +1,43 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#animation-shorthand">
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#animation-timeline">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/computed-testcommon.js"></script>
+<script src="/css/support/parsing-testcommon.js"></script>
+<script src="/css/support/shorthand-testcommon.js"></script>
+<div id="target"></div>
+<script>
+test_valid_value('animation',
+  '1s linear 1s 2 reverse forwards paused anim');
+
+test_invalid_value('animation',
+  '1s linear 1s 2 reverse forwards paused anim initial');
+test_invalid_value('animation',
+  '1s linear 1s 2 reverse forwards paused anim 2000');
+test_invalid_value('animation',
+  '1s linear 1s 2 reverse forwards paused anim scroll()');
+test_invalid_value('animation',
+  '1s linear 1s 2 reverse forwards paused anim view()');
+test_invalid_value('animation',
+  '1s linear 1s 2 reverse forwards paused anim timeline');
+
+test_computed_value('animation',
+  '1s linear 1s 2 reverse forwards paused anim');
+
+test_shorthand_value('animation',
+  `1s linear 1s 2 reverse forwards paused anim1,
+   1s linear 1s 2 reverse forwards paused anim2,
+   1s linear 1s 2 reverse forwards paused anim3`,
+{
+  'animation-duration': '1s, 1s, 1s',
+  'animation-timing-function': 'linear, linear, linear',
+  'animation-delay': '1s, 1s, 1s',
+  'animation-iteration-count': '2, 2, 2',
+  'animation-direction': 'reverse, reverse, reverse',
+  'animation-fill-mode': 'forwards, forwards, forwards',
+  'animation-play-state': 'paused, paused, paused',
+  'animation-name': 'anim1, anim2, anim3',
+  'animation-timeline': 'auto, auto, auto'
+});
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/animation-timeline-computed.html b/testing/web-platform/tests/scroll-animations/css/animation-timeline-computed.html
new file mode 100644
index 0000000000..9cabaab1fe
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/animation-timeline-computed.html
@@ -0,0 +1,76 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#animation-timeline">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/computed-testcommon.js"></script>
+</head>
+<style>
+  #outer { animation-timeline: foo; }
+  #target { animation-timeline: bar; }
+</style>
+<div id="outer">
+  <div id="target"></div>
+</div>
+<script>
+test_computed_value('animation-timeline', 'initial', 'auto');
+test_computed_value('animation-timeline', 'inherit', 'foo');
+test_computed_value('animation-timeline', 'unset', 'auto');
+test_computed_value('animation-timeline', 'revert', 'auto');
+test_computed_value('animation-timeline', 'auto');
+test_computed_value('animation-timeline', 'none');
+test_computed_value('animation-timeline', 'auto, auto');
+test_computed_value('animation-timeline', 'none, none');
+test_computed_value('animation-timeline', 'auto, none');
+test_computed_value('animation-timeline', 'none, auto');
+test_computed_value('animation-timeline', '"test"', 'test');
+test_computed_value('animation-timeline', '"none"');
+test_computed_value('animation-timeline', '"auto"');
+test_computed_value('animation-timeline', '"initial"');
+test_computed_value('animation-timeline', '"inherit"');
+test_computed_value('animation-timeline', '"unset"');
+test_computed_value('animation-timeline', '"revert"');
+test_computed_value('animation-timeline', '"revert-layer"');
+test_computed_value('animation-timeline', 'test');
+test_computed_value('animation-timeline', 'test1, test2');
+test_computed_value('animation-timeline', 'test1, "test2", none, test3, auto', 'test1, test2, none, test3, auto');
+
+test(() => {
+  let style = getComputedStyle(document.getElementById('target'));
+  assert_not_equals(Array.from(style).indexOf('animation-timeline'), -1);
+}, 'The animation-timeline property shows up in CSSStyleDeclaration enumeration');
+
+test(() => {
+  let style = document.getElementById('target').style;
+  assert_not_equals(style.cssText.indexOf('animation-timeline'), -1);
+}, 'The animation-timeline property shows up in CSSStyleDeclaration.cssText');
+
+// https://drafts.csswg.org/scroll-animations-1/#scroll-notation
+//
+// animation-timeline: scroll(<axis>? <scroller>?);
+// <axis> = block | inline | vertical | horizontal
+// <scroller> = root | nearest
+test_computed_value('animation-timeline', 'scroll()');
+test_computed_value('animation-timeline', 'scroll(block)', 'scroll()');
+test_computed_value('animation-timeline', 'scroll(inline)');
+test_computed_value('animation-timeline', 'scroll(horizontal)');
+test_computed_value('animation-timeline', 'scroll(vertical)');
+test_computed_value('animation-timeline', 'scroll(root)');
+test_computed_value('animation-timeline', 'scroll(nearest)', 'scroll()');
+test_computed_value('animation-timeline', 'scroll(inline nearest)', 'scroll(inline)');
+test_computed_value('animation-timeline', 'scroll(vertical root)');
+
+// https://drafts.csswg.org/scroll-animations-1/#view-notation
+test_computed_value('animation-timeline', 'view()');
+test_computed_value('animation-timeline', 'view(block)', 'view()');
+test_computed_value('animation-timeline', 'view(inline)', 'view(inline)');
+test_computed_value('animation-timeline', 'view(horizontal)', 'view(horizontal)');
+test_computed_value('animation-timeline', 'view(vertical)', 'view(vertical)');
+test_computed_value('animation-timeline', 'view(vertical 1px)');
+test_computed_value('animation-timeline', 'view(1px auto)');
+test_computed_value('animation-timeline', 'view(auto 1px)');
+test_computed_value('animation-timeline', 'view(vertical 1px auto)');
+test_computed_value('animation-timeline', 'view(1px vertical)', 'view(vertical 1px)');
+test_computed_value('animation-timeline', 'view(vertical auto)', 'view(vertical)');
+test_computed_value('animation-timeline', 'view(vertical auto auto)', 'view(vertical)');
+
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/animation-timeline-ignored.tentative.html b/testing/web-platform/tests/scroll-animations/css/animation-timeline-ignored.tentative.html
new file mode 100644
index 0000000000..32cb89c4ef
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/animation-timeline-ignored.tentative.html
@@ -0,0 +1,145 @@
+<!DOCTYPE html>
+<link rel="help" src="https://github.com/w3c/csswg-drafts/pull/5666">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<style>
+  main {
+    overflow: hidden;
+    height: 0px;
+  }
+  main > div {
+    overflow: hidden;
+    width: 100px;
+    height: 100px;
+  }
+  main > div > div {
+    height: 200px;
+  }
+  @keyframes expand {
+    from { width: 100px; }
+    to { width: 200px; }
+  }
+  #scroller1 {
+    scroll-timeline: timeline1;
+  }
+  #scroller2 {
+    scroll-timeline: timeline2;
+  }
+  #scroller3 {
+    scroll-timeline: timeline3;
+  }
+  #element {
+    width: 0px;
+    height: 20px;
+    animation-name: expand;
+    animation-duration: 1000s;
+    animation-timing-function: linear;
+    animation-timeline: timeline1;
+  }
+  /* Ensure stable expectations if feature is not supported */
+  @supports not (animation-timeline:foo) {
+    #element { animation-play-state: paused; }
+  }
+</style>
+<main>
+  <div id=scroller1><div></div></div>
+  <div id=scroller2><div></div></div>
+  <div id=scroller3><div></div></div>
+  <div id=scroller4><div></div></div>
+  <div id=container></div>
+</main>
+<script>
+  // Force layout of scrollers.
+  scroller1.offsetTop;
+  scroller2.offsetTop;
+  scroller3.offsetTop;
+  scroller4.offsetTop;
+
+  scroller1.scrollTop = 20;
+  scroller2.scrollTop = 40;
+  scroller3.scrollTop = 60;
+  scroller4.scrollTop = 80;
+
+  // Create #element in #container, run |func|, then clean up afterwards.
+  function test_animation_timeline(func, description) {
+    promise_test(async () => {
+      try {
+        let element = document.createElement('element');
+        element.setAttribute('id', 'element');
+        container.append(element);
+        await func();
+      } finally {
+        while (container.firstChild)
+          container.firstChild.remove();
+      }
+    }, description);
+  }
+
+  test_animation_timeline(async () => {
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element).width, '120px');
+    element.style = 'animation-timeline:timeline2';
+    assert_equals(getComputedStyle(element).width, '140px');
+  }, 'Changing animation-timeline changes the timeline (sanity check)');
+
+  test_animation_timeline(async () => {
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element).width, '120px');
+
+    // Set a (non-CSS) ScrollTimeline on the CSSAnimation.
+    let timeline4 = new ScrollTimeline({
+      source: scroller4,
+      scrollOffsets: [CSS.px(0), CSS.px(100)]
+    });
+
+    element.getAnimations()[0].timeline = timeline4;
+    assert_equals(getComputedStyle(element).width, '180px');
+
+    // Changing the animation-timeline property should have no effect.
+    element.style = 'animation-timeline:timeline2';
+    assert_equals(getComputedStyle(element).width, '180px');
+  }, 'animation-timeline ignored after setting timeline with JS (ScrollTimeline from JS)');
+
+  test_animation_timeline(async () => {
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element).width, '120px');
+    let animation = element.getAnimations()[0];
+    let timeline1 = animation.timeline;
+
+    element.style = 'animation-timeline:timeline2';
+    assert_equals(getComputedStyle(element).width, '140px');
+
+    animation.timeline = timeline1;
+    assert_equals(getComputedStyle(element).width, '120px');
+
+    // Should have no effect.
+    element.style = 'animation-timeline:timeline3';
+    assert_equals(getComputedStyle(element).width, '120px');
+  }, 'animation-timeline ignored after setting timeline with JS (ScrollTimeline from CSS)');
+
+  test_animation_timeline(async () => {
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element).width, '120px');
+    element.getAnimations()[0].timeline = document.timeline;
+
+    // (The animation continues from where the previous timeline left it).
+    assert_equals(getComputedStyle(element).width, '120px');
+
+    // Changing the animation-timeline property should have no effect.
+    element.style = 'animation-timeline:timeline2';
+    assert_equals(getComputedStyle(element).width, '120px');
+  }, 'animation-timeline ignored after setting timeline with JS (document timeline)');
+
+  test_animation_timeline(async () => {
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element).width, '120px');
+    element.getAnimations()[0].timeline = null;
+    assert_equals(getComputedStyle(element).width, '0px');
+
+    // Changing the animation-timeline property should have no effect.
+    element.style = 'animation-timeline:timeline2';
+    assert_equals(getComputedStyle(element).width, '0px');
+  }, 'animation-timeline ignored after setting timeline with JS (null)');
+
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/animation-timeline-in-keyframe.html b/testing/web-platform/tests/scroll-animations/css/animation-timeline-in-keyframe.html
new file mode 100644
index 0000000000..7548333139
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/animation-timeline-in-keyframe.html
@@ -0,0 +1,27 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#animation-timeline">
+<link rel="help" href="https://drafts.csswg.org/css-animations-1/#keyframes">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+</head>
+<style>
+  @keyframes test {
+    from { width: 100px; animation-timeline: foo; }
+    to { width: 100px; animation-timeline: foo; }
+  }
+  #target {
+    width: 50px;
+    animation-name: test;
+    animation-duration: 1s;
+    animation-play-state: paused;
+  }
+</style>
+<div id="target"></div>
+<script>
+test(() => {
+  let style = getComputedStyle(document.getElementById('target'));
+  // Checking 'width' verifies that the animation is applied at all.
+  assert_equals(style.width, '100px');
+  assert_equals(style.animationTimeline, 'auto');
+}, 'The animation-timeline property may not be used in keyframes');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/animation-timeline-multiple.html b/testing/web-platform/tests/scroll-animations/css/animation-timeline-multiple.html
new file mode 100644
index 0000000000..8ae37bf751
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/animation-timeline-multiple.html
@@ -0,0 +1,90 @@
+<!DOCTYPE html>
+<title>animation-timeline with multiple timelines</title>
+<link rel="help" src="https://drafts.csswg.org/css-animations-2/#animation-timeline">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<style>
+  main > div {
+    overflow: hidden;
+    width: 100px;
+    height: 100px;
+  }
+  main > div > div {
+    height: 200px;
+    width:  200px;
+  }
+
+  @keyframes top {
+    from { top: 100px; }
+    to { top: 200px; }
+  }
+  @keyframes bottom {
+    from { bottom: 100px; }
+    to { bottom: 200px; }
+  }
+  @keyframes left {
+    from { left: 100px; }
+    to { left: 200px; }
+  }
+  @keyframes right {
+    from { right: 100px; }
+    to { right: 200px; }
+  }
+
+  #top_scroller {
+    scroll-timeline: block top_timeline;
+  }
+  #bottom_scroller {
+    scroll-timeline: inline bottom_timeline;
+  }
+  #left_scroller {
+    scroll-timeline: block left_timeline;
+  }
+  #right_scroller {
+    scroll-timeline: inline right_timeline;
+  }
+
+  #element {
+    animation-name: top, bottom, left, right;
+    animation-duration: 10s;
+    animation-timing-function: linear;
+    animation-timeline: top_timeline, bottom_timeline, left_timeline, right_timeline;
+  }
+  /* Ensure stable expectations if feature is not supported */
+  @supports not (animation-timeline:foo) {
+    #element { animation-play-state: paused; }
+  }
+</style>
+<main>
+  <div id=top_scroller><div></div></div>
+  <div id=bottom_scroller><div></div></div>
+  <div id=left_scroller><div></div></div>
+  <div id=right_scroller><div></div></div>
+  <div id=element></div>
+</main>
+<script>
+  // Force layout of scrollers.
+  top_scroller.offsetTop;
+  bottom_scroller.offsetTop;
+  left_scroller.offsetTop;
+  right_scroller.offsetTop;
+
+  top_scroller.scrollTop = 20;
+  top_scroller.scrollLeft = 40;
+  bottom_scroller.scrollTop = 20;
+  bottom_scroller.scrollLeft = 40;
+  left_scroller.scrollTop = 60;
+  left_scroller.scrollLeft = 80;
+  right_scroller.scrollTop = 60;
+  right_scroller.scrollLeft = 80;
+
+  promise_test(async (t) => {
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element).top, '120px');
+    assert_equals(getComputedStyle(element).bottom, '140px');
+    assert_equals(getComputedStyle(element).left, '160px');
+    assert_equals(getComputedStyle(element).right, '180px');
+  }, 'animation-timeline works with multiple timelines');
+
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/animation-timeline-named-scroll-progress-timeline.tentative.html b/testing/web-platform/tests/scroll-animations/css/animation-timeline-named-scroll-progress-timeline.tentative.html
new file mode 100644
index 0000000000..9f55e2d2c9
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/animation-timeline-named-scroll-progress-timeline.tentative.html
@@ -0,0 +1,662 @@
+<!DOCTYPE html>
+<title>The animation-timeline: scroll-timeline-name</title>
+<meta name="viewport" content="width=device-width,initial-scale=1,minimum-scale=1">
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/rewrite#scroll-timelines-named">
+<link rel="help" src="https://github.com/w3c/csswg-drafts/issues/6674">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { translate: 50px; }
+    to { translate: 150px; }
+  }
+  #target {
+    width: 100px;
+    height: 100px;
+  }
+  .square {
+    width: 100px;
+    height: 100px;
+  }
+  .square-container {
+    width: 300px;
+    height: 300px;
+  }
+  .scroller {
+    overflow: scroll;
+  }
+  .content {
+    inline-size: 100%;
+    block-size: 100%;
+    padding-inline-end: 100px;
+    padding-block-end: 100px;
+  }
+</style>
+<body>
+<div id="log"></div>
+<script>
+"use strict";
+
+setup(assert_implements_animation_timeline);
+
+function createScroller(t, scrollerSizeClass) {
+  let scroller = document.createElement('div');
+  let className = scrollerSizeClass || 'square';
+  scroller.className = `scroller ${className}`;
+  let content = document.createElement('div');
+  content.className = 'content';
+
+  scroller.appendChild(content);
+
+  t.add_cleanup(function() {
+    content.remove();
+    scroller.remove();
+  });
+
+  return scroller;
+}
+
+function createTarget(t) {
+  let target = document.createElement('div');
+  target.id = 'target';
+
+  t.add_cleanup(function() {
+    target.remove();
+  });
+
+  return target;
+}
+
+function createScrollerAndTarget(t, scrollerSizeClass) {
+  return [createScroller(t, scrollerSizeClass), createTarget(t)];
+}
+
+// -------------------------
+// Test scroll-timeline-name
+// -------------------------
+
+promise_test(async t => {
+  let target = document.createElement('div');
+  target.id = 'target';
+  target.className = 'scroller';
+  let content = document.createElement('div');
+  content.className = 'content';
+
+  // <div id='target' class='scroller'>
+  //   <div id='content'></div>
+  // </div>
+  document.body.appendChild(target);
+  target.appendChild(content);
+
+  target.style.scrollTimelineName = 'timeline';
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline';
+
+  target.scrollTop = 50; // 50%, in [0, 100].
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, '100px');
+
+  content.remove();
+  target.remove();
+}, 'scroll-timeline-name is referenceable in animation-timeline on the ' +
+   'declaring element itself');
+
+promise_test(async t => {
+  let [parent, target] = createScrollerAndTarget(t, 'square-container');
+
+  // <div id='parent' class='scroller'>
+  //   <div id='target'></div>
+  //   <div id='content'></div>
+  // </div>
+  document.body.appendChild(parent);
+  parent.insertBefore(target, parent.firstElementChild);
+
+  parent.style.scrollTimelineName = 'timeline';
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline';
+
+  parent.scrollTop = 100; // 50%, in [0, 200].
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, '100px');
+}, "scroll-timeline-name is referenceable in animation-timeline on that " +
+   "element's descendants");
+
+promise_test(async t => {
+  let [sibling, target] = createScrollerAndTarget(t);
+
+  // <div id='sibling' class='scroller'> ... </div>
+  // <div id='target'></div>
+  document.body.appendChild(sibling);
+  document.body.appendChild(target);
+
+  sibling.style.scrollTimelineName = 'timeline';
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline';
+
+  sibling.scrollTop = 50; // 50%, in [0, 100].
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, '100px');
+}, "scroll-timeline-name is referenceable in animation-timeline on that " +
+   "element's following siblings");
+
+promise_test(async t => {
+  let [sibling, target] = createScrollerAndTarget(t);
+  let parent = document.createElement('div');
+
+  // <div id='sibling' class='scroller'> ... </div>
+  // <div id='parent'>
+  //   <div id='target'></div>
+  // </div>
+  document.body.appendChild(sibling);
+  document.body.appendChild(parent);
+  parent.appendChild(target);
+
+  sibling.style.scrollTimelineName = 'timeline';
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline';
+
+  sibling.scrollTop = 50; // 50%, in [0, 100].
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, '100px');
+
+  parent.remove();
+}, "scroll-timeline-name is referenceable in animation-timeline on that " +
+   "element's following siblings' descendants");
+
+// FIXME: We may use global scope for scroll-timeline-name.
+// See https://github.com/w3c/csswg-drafts/issues/7047
+promise_test(async t => {
+  let [sibling, target] = createScrollerAndTarget(t);
+
+  // <div id='target'></div>
+  // <div id='sibling' class='scroller'> ... </div>
+  document.body.appendChild(target);
+  document.body.appendChild(sibling);
+
+  sibling.style.scrollTimelineName = 'timeline';
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline';
+
+  sibling.scrollTop = 50; // 50%, in [0, 100].
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, '50px',
+    'Animation with unknown timeline name holds current time at zero');
+}, "scroll-timeline-name is not referenceable in animation-timeline on that " +
+   "element's previous siblings");
+
+promise_test(async t => {
+  let [sibling, target] = createScrollerAndTarget(t);
+  let parent = document.createElement('div');
+  parent.className = 'scroller square-container';
+  let content = document.createElement('div');
+  content.className = 'content';
+
+  // <div id='parent' class='scroller'>
+  //   <div id='sibling' class='scroller'> ... </div>
+  //   <div id='target'></div>
+  //   <div id='content'></div>
+  // </div>
+  document.body.appendChild(parent);
+  parent.appendChild(sibling);
+  parent.appendChild(target);
+  parent.appendChild(content);
+
+  parent.style.scrollTimelineName = 'timeline';
+  parent.style.scrollTimelineAxis = 'inline';
+  sibling.style.scrollTimelineName = 'timeline';
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline';
+
+  parent.scrollTop = 50; // 25%, in [0, 200].
+  sibling.scrollTop = 50; // 50%, in [0, 100].
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, '100px');
+
+  content.remove();
+  parent.remove();
+}, 'scroll-timeline-name is matched based on tree order, which considers ' +
+   'siblings closer than parents');
+
+promise_test(async t => {
+  let sibling = document.createElement('div');
+  sibling.className = 'square';
+  sibling.style.overflowX = 'clip'; // This makes overflow-y be clip as well.
+  let target = document.createElement('div');
+  target.id = 'target';
+
+  // <div id='sibling' style='overflow-x: clip'></div>
+  // <div id='target'></div>
+  document.body.appendChild(sibling);
+  document.body.appendChild(target);
+
+  sibling.style.scrollTimelineName = 'timeline';
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline';
+
+  sibling.scrollTop = 50; // 50%, in [0, 100].
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, 'none',
+    'Animation with an unresolved current time');
+
+  target.remove();
+  sibling.remove();
+}, 'scroll-timeline-name on an element which is not a scroll-container');
+
+promise_test(async t => {
+  let [sibling, target] = createScrollerAndTarget(t);
+  let main = document.createElement('div');
+  main.id = 'name';
+
+  // <div id='main'>
+  //   <div id='sibling' class='scroller'> ... </div>
+  //   <div id='target'></div>
+  // </div>
+  document.body.appendChild(main);
+  main.appendChild(sibling);
+  main.appendChild(target);
+
+  target.style.animation = 'anim 10s linear';
+  target.style.animationTimeline = 'timeline';
+  sibling.scrollTop = 50; // 50%, in [50, 150].
+  await waitForNextFrame();
+
+  // Unknown animation-timeline, current time held at zero.
+  assert_equals(getComputedStyle(target).translate, '50px');
+
+  // Ensure that #main (an ancestor of the scroller) needs style recalc.
+  main.style.background = 'lightgray';
+  sibling.style.scrollTimelineName = 'timeline';
+  await waitForCSSScrollTimelineStyle();
+  assert_equals(getComputedStyle(target).translate, '100px');
+
+  main.remove();
+}, 'scroll-timeline-name affects subsequent siblings when changed');
+
+promise_test(async t => {
+  let target = createTarget(t);
+
+  // <div id='target'></div>
+  document.body.appendChild(target);
+
+  target.style.animation = 'anim 10s linear';
+  target.style.animationTimeline = 'timeline';
+
+  // Unknown animation-timeline, current time held at zero.
+  assert_equals(getComputedStyle(target).translate, '50px');
+
+  let scroller = createScroller(t);
+  // <div class='scroller'> ... </div>
+  // <div id='target'></div>
+  document.body.insertBefore(scroller, target);
+  scroller.style.scrollTimelineName = 'timeline';
+
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, '50px');
+
+  // Ensure that time is not just held at zero.
+  scroller.scrollTop = 50; // 50%, in [50, 150].
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, '100px');
+}, 'scroll-timeline-name on inserted element affects subsequent siblings');
+
+promise_test(async t => {
+  let [scroller, target] = createScrollerAndTarget(t);
+
+  // <div class='scroller'> ... </div>
+  // <div id='target'></div>
+  document.body.appendChild(scroller);
+  document.body.appendChild(target);
+
+  scroller.scrollTop = 50; // 50%, in [50, 150].
+  await waitForNextFrame();
+
+  scroller.style.scrollTimelineName = 'timeline';
+  target.style.animation = 'anim 10s linear';
+  target.style.animationTimeline = 'timeline';
+  await waitForCSSScrollTimelineStyle();
+
+  assert_equals(getComputedStyle(target).translate, '100px');
+
+  // This effectively removes the CSS-created ScrollTimeline on this element,
+  // thus invoking "setting the timeline of an animation" [1] with a null-
+  // timeline on affected elements. This in turn causes the current time to
+  // become unresolved [2], ultimately resulting in no effect value.
+  //
+  // [1] https://drafts.csswg.org/web-animations-1/#setting-the-timeline
+  // [2] https://drafts.csswg.org/web-animations-1/#the-current-time-of-an-animation
+  scroller.remove();
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, 'none');
+}, 'scroll-timeline-name on removed element affects subsequent siblings');
+
+promise_test(async t => {
+  let [scroller, target] = createScrollerAndTarget(t);
+
+  // <div class='scroller' style='display:none'> ... </div>
+  // <div id='target'></div>
+  scroller.style.display = 'none';
+  document.body.appendChild(scroller);
+  document.body.appendChild(target);
+
+  scroller.style.scrollTimelineName = 'timeline';
+  target.style.animation = 'anim 10s linear';
+  target.style.animationTimeline = 'timeline';
+
+  // Unknown animation-timeline, current time held at zero.
+  assert_equals(getComputedStyle(target).translate, '50px');
+
+  scroller.style.display = 'block';
+  scroller.scrollTop = 50; // 50%, in [50, 150].
+  await waitForNextFrame();
+
+  assert_equals(getComputedStyle(target).translate, '100px');
+}, 'scroll-timeline-name on element leaving display:none affects subsequent siblings');
+
+promise_test(async t => {
+  let [scroller, target] = createScrollerAndTarget(t);
+
+  // <div class='scroller'> ... </div>
+  // <div id='target'></div>
+  document.body.appendChild(scroller);
+  document.body.appendChild(target);
+
+  scroller.scrollTop = 50; // 50%, in [50, 150].
+  await waitForNextFrame();
+
+  scroller.style.scrollTimelineName = 'timeline';
+  target.style.animation = 'anim 10s linear';
+  target.style.animationTimeline = 'timeline';
+  await waitForCSSScrollTimelineStyle();
+
+  assert_equals(getComputedStyle(target).translate, '100px');
+
+  // See comment in the test "scroll-timeline-name on removed element ..." for
+  // an explantation of this result. (Setting display:none is similar to
+  // removing the element).
+  scroller.style.display = 'none';
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, 'none');
+}, 'scroll-timeline-name on element becoming display:none affects subsequent siblings');
+
+promise_test(async t => {
+  let [scroller, target] = createScrollerAndTarget(t);
+
+  // <div id='scroller' class='scroller'> ... </div>
+  // <div id='target'></div>
+
+  document.body.appendChild(scroller);
+  document.body.appendChild(target);
+
+  scroller.style.scrollTimelineName = 'timeline';
+  scroller.style.display = 'none';
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline';
+
+  await waitForNextFrame();
+
+  const anim = target.getAnimations()[0];
+  assert_true(!!anim, 'Failed to create animation');
+  assert_equals(anim.timeline, null);
+  // Hold time of animation is zero.
+  assert_equals(getComputedStyle(target).translate, '50px');
+
+  scroller.style.display = 'block';
+  scroller.scrollTop = 50;
+  await waitForNextFrame();
+
+  assert_true(!!anim.timeline, 'Failed to create timeline');
+  assert_equals(getComputedStyle(target).translate, '100px');
+
+}, 'scroll-timeline-name on element not resolved until element becomes visible');
+
+promise_test(async t => {
+  let [scroller, target] = createScrollerAndTarget(t);
+
+  // <div id='scroller' class='scroller'> ... </div>
+  // <div id='target'></div>
+
+  document.body.appendChild(scroller);
+  document.body.appendChild(target);
+
+  scroller.style.scrollTimelineName = 'timeline-A';
+  scroller.scrollTop = 50;
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline-B';
+
+  await waitForNextFrame();
+
+  const anim = target.getAnimations()[0];
+  assert_true(!!anim, 'Failed to create animation');
+  assert_equals(anim.timeline, null);
+  // Hold time of animation is zero.
+  assert_equals(getComputedStyle(target).translate, '50px');
+
+  scroller.style.scrollTimelineName = 'timeline-B';
+  await waitForNextFrame();
+
+  assert_true(!!anim.timeline, 'Failed to create timeline');
+  assert_equals(getComputedStyle(target).translate, '100px');
+
+}, 'Change in scroll-timeline-name to match animation timeline updates animation.');
+
+promise_test(async t => {
+  let [scroller, target] = createScrollerAndTarget(t);
+
+  // <div id='scroller' class='scroller'> ... </div>
+  // <div id='target'></div>
+
+  document.body.appendChild(scroller);
+  document.body.appendChild(target);
+
+  scroller.style.scrollTimelineName = 'timeline-A';
+  scroller.scrollTop = 50;
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline-A';
+
+  await waitForNextFrame();
+
+  const anim = target.getAnimations()[0];
+  assert_true(!!anim, 'Failed to create animation');
+  assert_true(!!anim.timeline, 'Failed to create timeline');
+  assert_equals(getComputedStyle(target).translate, '100px');
+
+  scroller.style.scrollTimelineName = 'timeline-B';
+  await waitForNextFrame();
+
+  assert_equals(anim.timeline, null, 'Failed to remove timeline');
+  assert_equals(getComputedStyle(target).translate, 'none');
+
+}, 'Change in scroll-timeline-name to no longer match animation timeline updates animation.');
+
+promise_test(async t => {
+  let target = createTarget(t);
+  let scroller1 = createScroller(t);
+  let scroller2 = createScroller(t);
+
+  target.style.animation = 'anim 10s linear';
+  target.style.animationTimeline = 'timeline';
+  scroller1.style.scrollTimelineName = 'timeline';
+  scroller2.style.scrollTimelineName = 'timeline';
+  scroller1.id = 'A';
+  scroller2.id = 'B';
+
+  // <div class='scroller' id='A'> ... </div> (scroller1)
+  // <div class='scroller' id="B"> ... </div> (scroller2)
+  // <div id='target'></div>
+  document.body.appendChild(scroller1);
+  document.body.appendChild(scroller2);
+  document.body.append(target);
+
+  scroller1.scrollTop = 10; // 10%, in [50, 150].
+  scroller2.scrollTop = 50; // 50%, in [50, 150].
+  await waitForNextFrame();
+
+
+  // The named timeline lookup should select scroller2.
+  let anim = target.getAnimations()[0];
+  assert_true(!!anim, 'Failed to fetch animation');
+  assert_equals(anim.timeline.source.id, 'B');
+  assert_equals(getComputedStyle(target).translate, '100px');
+
+  scroller2.remove();
+
+  // Now it should select scroller1.
+  anim = target.getAnimations()[0];
+  assert_true(!!anim, 'Failed to fetch animation after update');
+  assert_true(!!anim.timeline, 'Animation no longer has a timeline');
+  assert_equals(anim.timeline.source.id, 'A', 'Timeline not updated');
+  assert_equals(getComputedStyle(target).translate, '60px');
+}, 'Timeline lookup finds next candidate when element is removed');
+
+promise_test(async t => {
+  let target = createTarget(t);
+  let scroller1 = createScroller(t);
+
+  target.style.animation = 'anim 10s linear';
+  target.style.animationTimeline = 'timeline';
+  scroller1.style.scrollTimelineName = 'timeline';
+  scroller1.id = 'A';
+
+  // <div class='scroller' id='A'> ... </div> (scroller1)
+  // <div id='target'></div>
+  document.body.appendChild(scroller1);
+  document.body.append(target);
+
+  scroller1.scrollTop = 10; // 10%, in [50, 150].
+
+  await waitForNextFrame();
+
+  const anim = target.getAnimations()[0];
+
+  assert_true(!!anim.timeline, 'Failed to retrieve animation');
+  assert_equals(anim.timeline.source.id, 'A');
+  assert_equals(getComputedStyle(target).translate, '60px');
+
+  await waitForNextFrame();
+
+  let scroller2 = createScroller(t);
+  scroller2.style.scrollTimelineName = 'timeline';
+  scroller2.id = 'B';
+
+  // <div class='scroller' id="A"> ... </div> (scroller1)
+  // <div class='scroller' id="B"> ... </div> (scroller2)
+  // <div id='target'></div>
+  document.body.insertBefore(scroller2, target);
+
+  scroller2.scrollTop = 50; // 50%, in [50, 150].
+
+  await waitForNextFrame();
+
+  // The timeline should be updated to scroller2.
+  assert_true(!!anim.timeline, 'Animation no longer has a timeline');
+  assert_equals(anim.timeline.source.id, 'B', 'Timeline not updated');
+  assert_equals(getComputedStyle(target).translate, '100px');
+}, 'Timeline lookup updates candidate when closer match available.');
+
+promise_test(async t => {
+  let target = createTarget(t);
+
+  // <div id='target'></div>
+  document.body.append(target);
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline';
+
+  await waitForNextFrame();
+
+  // Timeline initially cannot be resolved, resulting in a null
+  // timeline. The animation's hold time is zero.
+  let anim = document.getAnimations()[0];
+  assert_equals(getComputedStyle(target).translate, '50px');
+
+  await waitForNextFrame();
+
+  let scroller = createScroller(t);
+  scroller.style.scrollTimelineName = 'timeline';
+
+  // <div class='scroller'> ... </div> (scroller1)
+  // <div id='target'></div>
+  document.body.insertBefore(scroller, target);
+
+  scroller.scrollTop = 50; // 50%, in [50, 150].
+
+  await waitForNextFrame();
+
+  // The timeline should be updated to scroller.
+  assert_equals(getComputedStyle(target).translate, '100px');
+}, 'Timeline lookup updates candidate when match becomes available.');
+
+// -------------------------
+// Test scroll-timeline-axis
+// -------------------------
+
+promise_test(async t => {
+  let [scroller, target] = createScrollerAndTarget(t);
+  scroller.style.writingMode = 'vertical-lr';
+
+  document.body.appendChild(scroller);
+  document.body.appendChild(target);
+
+  scroller.style.scrollTimeline = 'timeline block';
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline';
+
+  scroller.scrollLeft = 50;
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, '100px');
+}, 'scroll-timeline-axis is block');
+
+promise_test(async t => {
+  let [scroller, target] = createScrollerAndTarget(t);
+  scroller.style.writingMode = 'vertical-lr';
+
+  document.body.appendChild(scroller);
+  document.body.appendChild(target);
+
+  scroller.style.scrollTimeline = 'timeline inline';
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline';
+
+  scroller.scrollTop = 50;
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, '100px');
+}, 'scroll-timeline-axis is inline');
+
+promise_test(async t => {
+  let [scroller, target] = createScrollerAndTarget(t);
+  scroller.style.writingMode = 'vertical-lr';
+
+  document.body.appendChild(scroller);
+  document.body.appendChild(target);
+
+  scroller.style.scrollTimeline = 'timeline horizontal';
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline';
+
+  scroller.scrollLeft = 50;
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, '100px');
+}, 'scroll-timeline-axis is horizontal');
+
+promise_test(async t => {
+  let [scroller, target] = createScrollerAndTarget(t);
+  scroller.style.writingMode = 'vertical-lr';
+
+  document.body.appendChild(scroller);
+  document.body.appendChild(target);
+
+  scroller.style.scrollTimeline = 'timeline vertical';
+  target.style.animation = "anim 10s linear";
+  target.style.animationTimeline = 'timeline';
+
+  scroller.scrollTop = 50;
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).translate, '100px');
+}, 'scroll-timeline-axis is vertical');
+
+// TODO: Add more tests which change scroll-timeline-axis property.
+// Those animations which use this timeline should be restyled properly.
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/css/animation-timeline-none.html b/testing/web-platform/tests/scroll-animations/css/animation-timeline-none.html
new file mode 100644
index 0000000000..a8e07a44d6
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/animation-timeline-none.html
@@ -0,0 +1,41 @@
+<!DOCTYPE html>
+<link rel="help" src="https://drafts.csswg.org/css-animations-2/#animation-timeline">
+<link rel="help" src="https://drafts.csswg.org/web-animations/#playing-an-animation-section">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<style>
+  @keyframes expand {
+    from { width: 100px; }
+    to { width: 200px; }
+  }
+
+  .test {
+    width: 0px;
+    animation-name: expand;
+    animation-duration: 1s;
+  }
+
+  #element_timeline_none {
+    animation-timeline: none;
+  }
+  #element_unknown_timeline {
+    animation-timeline: unknown_timeline;
+  }
+
+</style>
+<div class=test id=element_timeline_none></div>
+<div class=test id=element_unknown_timeline></div>
+<script>
+  promise_test(async (t) => {
+    assert_equals(getComputedStyle(element_timeline_none).width, '100px');
+    await waitForAnimationFrames(3);
+    assert_equals(getComputedStyle(element_timeline_none).width, '100px');
+  }, 'Animation with animation-timeline:none holds current time at zero');
+
+  promise_test(async (t) => {
+    assert_equals(getComputedStyle(element_unknown_timeline).width, '100px');
+    await waitForAnimationFrames(3);
+    assert_equals(getComputedStyle(element_unknown_timeline).width, '100px');
+  }, 'Animation with unknown timeline name holds current time at zero');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/animation-timeline-parsing.html b/testing/web-platform/tests/scroll-animations/css/animation-timeline-parsing.html
new file mode 100644
index 0000000000..1a30f11f85
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/animation-timeline-parsing.html
@@ -0,0 +1,88 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#animation-timeline">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/parsing-testcommon.js"></script>
+</head>
+<div id="target"></div>
+<script>
+test_valid_value('animation-timeline', 'initial');
+test_valid_value('animation-timeline', 'inherit');
+test_valid_value('animation-timeline', 'unset');
+test_valid_value('animation-timeline', 'revert');
+test_valid_value('animation-timeline', 'auto');
+test_valid_value('animation-timeline', 'none');
+test_valid_value('animation-timeline', 'auto, auto');
+test_valid_value('animation-timeline', 'none, none');
+test_valid_value('animation-timeline', 'auto, none');
+test_valid_value('animation-timeline', 'none, auto');
+test_valid_value('animation-timeline', '"test"', ["test", '"test"']);
+test_valid_value('animation-timeline', '"none"');
+test_valid_value('animation-timeline', '"auto"');
+test_valid_value('animation-timeline', '"initial"');
+test_valid_value('animation-timeline', '"inherit"');
+test_valid_value('animation-timeline', '"unset"');
+test_valid_value('animation-timeline', '"revert"');
+test_valid_value('animation-timeline', 'test');
+test_valid_value('animation-timeline', 'test1, test2');
+test_valid_value('animation-timeline', 'test1, "test2", none, test3, auto', ["test1, test2, none, test3, auto", 'test1, "test2", none, test3, auto']);
+
+test_invalid_value('animation-timeline', '10px');
+test_invalid_value('animation-timeline', 'auto auto');
+test_invalid_value('animation-timeline', 'none none');
+test_invalid_value('animation-timeline', 'foo bar');
+test_invalid_value('animation-timeline', '"foo" "bar"');
+test_invalid_value('animation-timeline', 'rgb(1, 2, 3)');
+test_invalid_value('animation-timeline', '#fefefe');
+
+// https://drafts.csswg.org/scroll-animations-1/#scroll-notation
+//
+// animation-timeline: scroll(<axis>? <scroller>?);
+// <axis> = block | inline | vertical | horizontal
+// <scroller> = root | nearest
+test_valid_value('animation-timeline', 'scroll()');
+test_valid_value('animation-timeline', 'scroll(block)', 'scroll()');
+test_valid_value('animation-timeline', 'scroll(inline)');
+test_valid_value('animation-timeline', 'scroll(horizontal)');
+test_valid_value('animation-timeline', 'scroll(vertical)');
+test_valid_value('animation-timeline', 'scroll(root)');
+test_valid_value('animation-timeline', 'scroll(nearest)', 'scroll()');
+test_valid_value('animation-timeline', 'scroll(inline nearest)', 'scroll(inline)');
+test_valid_value('animation-timeline', 'scroll(vertical root)');
+
+test_invalid_value('animation-timeline', 'scroll(root block)');
+test_invalid_value('animation-timeline', 'scroll(abc root)');
+test_invalid_value('animation-timeline', 'scroll(abc)');
+test_invalid_value('animation-timeline', 'scroll(vertical abc)');
+test_invalid_value('animation-timeline', 'scroll("string")');
+
+// https://drafts.csswg.org/scroll-animations-1/#view-notation
+test_valid_value('animation-timeline', 'view()');
+test_valid_value('animation-timeline', 'view(block)', 'view()');
+test_valid_value('animation-timeline', 'view(inline)');
+test_valid_value('animation-timeline', 'view(horizontal)');
+test_valid_value('animation-timeline', 'view(vertical)');
+test_valid_value('animation-timeline', 'view(vertical 1px 2px)');
+test_valid_value('animation-timeline', 'view(vertical 1px)');
+test_valid_value('animation-timeline', 'view(vertical auto)', 'view(vertical)');
+test_valid_value('animation-timeline', 'view(vertical auto auto)', 'view(vertical)');
+test_valid_value('animation-timeline', 'view(vertical auto 1px)');
+test_valid_value('animation-timeline', 'view(1px 2px vertical)', 'view(vertical 1px 2px)');
+test_valid_value('animation-timeline', 'view(1px vertical)', 'view(vertical 1px)');
+test_valid_value('animation-timeline', 'view(auto horizontal)', 'view(horizontal)');
+test_valid_value('animation-timeline', 'view(1px 2px)');
+test_valid_value('animation-timeline', 'view(1px)');
+test_valid_value('animation-timeline', 'view(1px 1px)', 'view(1px)');
+test_valid_value('animation-timeline', 'view(1px auto)');
+test_valid_value('animation-timeline', 'view(auto calc(1% + 1px))');
+test_valid_value('animation-timeline', 'view(auto)', 'view()');
+test_valid_value('animation-timeline', 'view(auto auto)', 'view()');
+
+test_invalid_value('animation-timeline', 'view(vertical 1px 2px 3px)');
+test_invalid_value('animation-timeline', 'view(1px vertical 3px)');
+test_invalid_value('animation-timeline', 'view(1px 2px 3px)');
+test_invalid_value('animation-timeline', 'view(abc block)');
+test_invalid_value('animation-timeline', 'view(abc)');
+test_invalid_value('animation-timeline', 'view(vertical abc)');
+test_invalid_value('animation-timeline', 'view("string")');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/animation-timeline-scroll-functional-notation.tentative.html b/testing/web-platform/tests/scroll-animations/css/animation-timeline-scroll-functional-notation.tentative.html
new file mode 100644
index 0000000000..e28e85245e
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/animation-timeline-scroll-functional-notation.tentative.html
@@ -0,0 +1,148 @@
+<!DOCTYPE html>
+<title>The animation-timeline: scroll() notation</title>
+<meta name="viewport" content="width=device-width,initial-scale=1,minimum-scale=1">
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/rewrite#scroll-notation">
+<link rel="help" src="https://github.com/w3c/csswg-drafts/issues/6674">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { translate: 50px; }
+    to { translate: 150px; }
+  }
+  html {
+    min-height: 100vh;
+    /* This makes the max scrollable ragne be 100px in root element */
+    padding-bottom: 100px;
+  }
+  #container {
+    width: 300px;
+    height: 300px;
+    overflow: scroll;
+  }
+  #target {
+    width: 100px;
+    /* This makes the max scrollable ragne be 100px in the block direction */
+    height: 100px;
+  }
+  /* large block content */
+  .block-content {
+    block-size: 100%;
+  }
+  /* large inline content */
+  .inline-content {
+    inline-size: 100%;
+    block-size: 5px;
+    /* This makes the max scrollable ragne be 100px in the inline direction */
+    padding-inline-end: 100px;
+  }
+</style>
+<body>
+<div id="log"></div>
+<script>
+"use strict";
+
+setup(assert_implements_animation_timeline);
+
+const root = document.scrollingElement;
+const createTargetWithStuff = function(t, contentClass) {
+  let container = document.createElement('div');
+  container.id = 'container';
+  let target = document.createElement('div');
+  target.id = 'target';
+  let content = document.createElement('div');
+  content.className = contentClass;
+
+  // <div id='container'>
+  //   <div id='target'></div>
+  //   <div class=contentClass></div>
+  // </div>
+  document.body.appendChild(container);
+  container.appendChild(target);
+  container.appendChild(content);
+
+  if (t && typeof t.add_cleanup === 'function') {
+    t.add_cleanup(() => {
+      content.remove();
+      target.remove();
+      container.remove();
+    });
+  }
+
+  return [container, target];
+};
+
+async function scrollLeft(element, value) {
+  element.scrollLeft = value;
+  await waitForNextFrame();
+}
+
+async function scrollTop(element, value) {
+  element.scrollTop = value;
+  await waitForNextFrame();
+}
+
+promise_test(async t => {
+  let [container, div] = createTargetWithStuff(t, 'block-content');
+  div.style.animation = "anim 10s linear";
+  div.style.animationTimeline = "scroll(nearest)";
+
+  await scrollTop(root, 50);
+  assert_equals(getComputedStyle(div).translate, '50px');
+
+  await scrollTop(container, 50);
+  assert_equals(getComputedStyle(div).translate, '100px');
+
+  await scrollTop(root, 0);
+}, 'animation-timeline: scroll(nearest)');
+
+promise_test(async t => {
+  let [container, div] = createTargetWithStuff(t, 'block-content');
+  div.style.animation = "anim 10s linear";
+  div.style.animationTimeline = "scroll(root)";
+
+  await scrollTop(container, 50);
+  assert_equals(getComputedStyle(div).translate, '50px');
+
+  await scrollTop(root, 50);
+  assert_equals(getComputedStyle(div).translate, '100px');
+
+  await scrollTop(root, 0);
+}, 'animation-timeline: scroll(root)');
+
+promise_test(async t => {
+  let [container, div] = createTargetWithStuff(t, 'inline-content');
+  div.style.animation = "anim 10s linear";
+  div.style.animationTimeline = "scroll(inline)";
+
+  await scrollLeft(container, 50);
+  assert_equals(getComputedStyle(div).translate, '100px');
+}, 'animation-timeline: scroll(inline)');
+
+promise_test(async t => {
+  let [container, div] = createTargetWithStuff(t, 'block-content');
+  container.style.writingMode = 'vertical-lr';
+  div.style.animation = "anim 10s linear";
+  div.style.animationTimeline = "scroll(horizontal)";
+
+  await scrollLeft(container, 50);
+  assert_equals(getComputedStyle(div).translate, '100px');
+}, 'animation-timeline: scroll(horizontal)');
+
+promise_test(async t => {
+  let [container, div] = createTargetWithStuff(t, 'inline-content');
+  container.style.writingMode = 'vertical-lr';
+  div.style.animation = "anim 10s linear";
+  div.style.animationTimeline = "scroll(vertical)";
+
+  await scrollTop(container, 50);
+  assert_equals(getComputedStyle(div).translate, '100px');
+}, 'animation-timeline: scroll(vertical)');
+
+// TODO: Add more tests which change the overflow property of the container for
+// scroll(nearest)
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/css/animation-timeline-view-functional-notation.tentative.html b/testing/web-platform/tests/scroll-animations/css/animation-timeline-view-functional-notation.tentative.html
new file mode 100644
index 0000000000..d93306819a
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/animation-timeline-view-functional-notation.tentative.html
@@ -0,0 +1,263 @@
+<!DOCTYPE html>
+<title>The animation-timeline: view() notation</title>
+<meta name="viewport" content="width=device-width,initial-scale=1,minimum-scale=1">
+<link rel="help" src="https://w3c.github.io/csswg-drafts/scroll-animations-1/#view-notation">
+<link rel="help" src="https://github.com/w3c/csswg-drafts/issues/7587">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes fade-in-out {
+    enter 0% { opacity: 0; }
+    enter 100% { opacity: 1; }
+    exit 0% { opacity: 1; }
+    exit 100% { opacity: 0; }
+  }
+  @keyframes fade-out {
+    exit 0% { opacity: 1; }
+    exit 100% { opacity: 0; }
+  }
+  @keyframes change-font-size {
+    exit 0% { font-size: 10px; }
+    exit 100% { font-size: 20px; }
+  }
+  #container {
+    width: 200px;
+    height: 200px;
+    overflow-y: scroll;
+    overflow-x: hidden;
+  }
+  .target {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+  .content {
+    width: 400px;
+    height: 400px;
+    background-color: blue;
+  }
+</style>
+
+<body>
+<script>
+"use strict";
+
+setup(assert_implements_animation_timeline);
+
+const createTargetWithStuff = function(t, divClasses) {
+  let container = document.createElement('div');
+  container.id = 'container';
+  document.body.appendChild(container);
+
+  // *** When testing inset
+  // <div id='container'>
+  //   <div class='content'></div>
+  //   <div class='target'></div>
+  //   <div class='content'></div>
+  // </div>
+  // *** When testing axis
+  // <div id='container'>
+  //   <div class='target'></div>
+  //   <div class='content'></div>
+  // </div>
+
+  let divs = [];
+  let target;
+  for(let className of divClasses) {
+    let div = document.createElement('div');
+    div.className = className;
+    container.appendChild(div);
+
+    divs.push(div);
+    if(className === 'target')
+      target = div;
+  }
+
+  if (t && typeof t.add_cleanup === 'function') {
+    t.add_cleanup(() => {
+      for(let div of divs)
+        div.remove();
+      container.remove();
+    });
+  }
+
+  return [container, target];
+};
+
+async function scrollLeft(element, value) {
+  element.scrollLeft = value;
+  await waitForNextFrame();
+}
+
+async function scrollTop(element, value) {
+  element.scrollTop = value;
+  await waitForNextFrame();
+}
+
+promise_test(async t => {
+  let [container, div] = createTargetWithStuff(t, ['content', 'target', 'content']);
+  div.style.animation = "fade-in-out 1s linear";
+  div.style.animationTimeline = "view()";
+
+  await scrollTop(container, 200);
+  assert_equals(getComputedStyle(div).opacity, '0', 'At enter 0%');
+  await scrollTop(container, 250);
+  assert_equals(getComputedStyle(div).opacity, '0.5', 'At enter 50%');
+  await scrollTop(container, 300);
+  assert_equals(getComputedStyle(div).opacity, '1', 'At enter 100%');
+
+  await scrollTop(container, 400);
+  assert_equals(getComputedStyle(div).opacity, '1', 'At exit 0%');
+  await scrollTop(container, 450);
+  assert_equals(getComputedStyle(div).opacity, '0.5', 'At exit 50%');
+  await scrollTop(container, 500);
+  assert_equals(getComputedStyle(div).opacity, '0', 'At exit 100%');
+}, 'animation-timeline: view()');
+
+promise_test(async t => {
+  let [container, div] = createTargetWithStuff(t, ['content', 'target', 'content']);
+  div.style.animation = "fade-in-out 1s linear";
+  div.style.animationTimeline = "view(50px)";
+
+  await scrollTop(container, 250);
+  assert_equals(getComputedStyle(div).opacity, '0', 'At enter 0%');
+  await scrollTop(container, 300);
+  assert_equals(getComputedStyle(div).opacity, '0.5', 'At enter 50%');
+
+  await scrollTop(container, 350);
+  assert_equals(getComputedStyle(div).opacity, '1', 'At enter 100% & exit 0%');
+
+  await scrollTop(container, 400);
+  assert_equals(getComputedStyle(div).opacity, '0.5', 'At exit 50%');
+  await scrollTop(container, 450);
+  assert_equals(getComputedStyle(div).opacity, '0', 'At exit 100%');
+}, 'animation-timeline: view(50px)');
+
+promise_test(async t => {
+  let [container, div] = createTargetWithStuff(t, ['content', 'target', 'content']);
+  div.style.animation = "fade-in-out 1s linear";
+  div.style.animationTimeline = "view(auto 50px)";
+
+  await scrollTop(container, 250);
+  assert_equals(getComputedStyle(div).opacity, '0', 'At enter 0%');
+  await scrollTop(container, 300);
+  assert_equals(getComputedStyle(div).opacity, '0.5', 'At enter 50%');
+  await scrollTop(container, 350);
+  assert_equals(getComputedStyle(div).opacity, '1', 'At enter 100%');
+
+  await scrollTop(container, 400);
+  assert_equals(getComputedStyle(div).opacity, '1', 'At exit 0%');
+  await scrollTop(container, 450);
+  assert_equals(getComputedStyle(div).opacity, '0.5', 'At exit 50%');
+  await scrollTop(container, 500);
+  assert_equals(getComputedStyle(div).opacity, '0', 'At exit 100%');
+}, 'animation-timeline: view(auto 50px)');
+
+promise_test(async t => {
+  let [container, div] = createTargetWithStuff(t, ['target', 'content']);
+  container.style.overflow = 'scroll';
+  div.style.animation = "fade-out 1s linear";
+  div.style.animationTimeline = "view(inline)";
+
+  await scrollLeft(container, 0);
+  assert_equals(getComputedStyle(div).opacity, '1', 'At exit 0%');
+  await scrollLeft(container, 50);
+  assert_equals(getComputedStyle(div).opacity, '0.5', 'At exit 50%');
+  await scrollLeft(container, 100);
+  assert_equals(getComputedStyle(div).opacity, '0', 'At exit 100%');
+}, 'animation-timeline: view(inline)');
+
+promise_test(async t => {
+  let [container, div] = createTargetWithStuff(t, ['target', 'content']);
+  container.style.overflow = 'scroll';
+  div.style.animation = "fade-out 1s linear";
+  div.style.animationTimeline = "view(horizontal)";
+
+  await scrollLeft(container, 0);
+  assert_equals(getComputedStyle(div).opacity, '1', 'At exit 0%');
+  await scrollLeft(container, 50);
+  assert_equals(getComputedStyle(div).opacity, '0.5', 'At exit 50%');
+  await scrollLeft(container, 100);
+  assert_equals(getComputedStyle(div).opacity, '0', 'At exit 100%');
+}, 'animation-timeline: view(horizontal)');
+
+promise_test(async t => {
+  let [container, div] = createTargetWithStuff(t, ['target', 'content']);
+  container.style.overflow = 'scroll';
+  div.style.animation = "fade-out 1s linear";
+  div.style.animationTimeline = "view(vertical)";
+
+  await scrollTop(container, 0);
+  assert_equals(getComputedStyle(div).opacity, '1', 'At exit 0%');
+  await scrollTop(container, 50);
+  assert_equals(getComputedStyle(div).opacity, '0.5', 'At exit 50%');
+  await scrollTop(container, 100);
+  assert_equals(getComputedStyle(div).opacity, '0', 'At exit 100%');
+}, 'animation-timeline: view(vertical)');
+
+promise_test(async t => {
+  let [container, div] = createTargetWithStuff(t, ['target', 'content']);
+  container.style.overflowY = 'hidden';
+  container.style.overflowX = 'scroll';
+  div.style.animation = "fade-out 1s linear";
+  div.style.animationTimeline = "view(horizontal 50px)";
+
+  await scrollLeft(container, 0);
+  assert_equals(getComputedStyle(div).opacity, '0.5', 'At exit 50%');
+  await scrollLeft(container, 50);
+  assert_equals(getComputedStyle(div).opacity, '0', 'At exit 100%');
+}, 'animation-timeline: view(horizontal 50px)');
+
+promise_test(async t => {
+  let [container, div] = createTargetWithStuff(t, ['target', 'content']);
+  container.style.overflow = 'scroll';
+  div.style.animation = "fade-out 1s linear, change-font-size 1s linear";
+  div.style.animationTimeline = "view(), view(inline)";
+
+  await scrollLeft(container, 0);
+  assert_equals(getComputedStyle(div).fontSize, '10px', 'At exit 0% inline');
+  await scrollLeft(container, 50);
+  assert_equals(getComputedStyle(div).fontSize, '15px', 'At exit 50% inline');
+  await scrollLeft(container, 100);
+  assert_equals(getComputedStyle(div).fontSize, '20px', 'At exit 100% inline');
+
+  await scrollLeft(container, 0);
+
+  await scrollTop(container, 0);
+  assert_equals(getComputedStyle(div).opacity, '1', 'At exit 0% block');
+  await scrollTop(container, 50);
+  assert_equals(getComputedStyle(div).opacity, '0.5', 'At exit 50% block');
+  await scrollTop(container, 100);
+  assert_equals(getComputedStyle(div).opacity, '0', 'At exit 100% block');
+
+  await scrollLeft(container, 50);
+  await scrollTop(container, 50);
+  assert_equals(getComputedStyle(div).fontSize, '15px', 'At exit 50% inline');
+  assert_equals(getComputedStyle(div).opacity, '0.5', 'At exit 50% block');
+}, 'animation-timeline: view(), view(inline)');
+
+promise_test(async t => {
+  let [container, div] = createTargetWithStuff(t, ['target', 'content']);
+  container.style.overflowY = 'hidden';
+  container.style.overflowX = 'scroll';
+  div.style.animation = "fade-out 1s linear";
+
+  div.style.animationTimeline = "view(inline)";
+  await scrollLeft(container, 0);
+  assert_equals(getComputedStyle(div).opacity, '1', 'At exit 0%');
+  await scrollLeft(container, 50);
+  assert_equals(getComputedStyle(div).opacity, '0.5', 'At exit 50%');
+  await scrollLeft(container, 100);
+  assert_equals(getComputedStyle(div).opacity, '0', 'At exit 100%');
+
+  div.style.animationTimeline = "view(inline 50px)";
+  await scrollLeft(container, 0);
+  assert_equals(getComputedStyle(div).opacity, '0.5', 'At exit 50%');
+  await scrollLeft(container, 50);
+  assert_equals(getComputedStyle(div).opacity, '0', 'At exit 100%');
+}, 'animation-timeline: view(inline) changes to view(inline 50px)');
+
+</script>
+</body>
\ No newline at end of file
diff --git a/testing/web-platform/tests/scroll-animations/css/get-animations-inactive-timeline.html b/testing/web-platform/tests/scroll-animations/css/get-animations-inactive-timeline.html
new file mode 100644
index 0000000000..10bf00fbbc
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/get-animations-inactive-timeline.html
@@ -0,0 +1,87 @@
+<!DOCTYPE html>
+<html>
+<meta charset="utf-8">
+<title>getAnimations for scroll-linked animations</title>
+<link rel="help"
+ href="https://www.w3.org/TR/web-animations-1/#animation-effect-phases-and-states">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes slide {
+    from { transform: translateX(100px);  }
+    to   { transform: translateX(100px); }
+  }
+
+  #container {
+    border:  10px solid lightgray;
+    overflow-x: scroll;
+    height:  200px;
+    width: 200px;
+    scroll-timeline-name:  timeline;
+  }
+  #spacer {
+    height:  200vh;
+  }
+  #target {
+    background-color:  green;
+    height:  100px;
+    width:  100px;
+    animation:  slide 1s linear;
+    animation-timeline: timeline;
+  }
+</style>
+<body>
+  <div id="container">
+    <div id="spacer"></div>
+    <div id="target"></div>
+  </div>
+</body>
+<script type="text/javascript">
+  setup(assert_implements_animation_timeline);
+
+  promise_test(async t => {
+    // Newly created timeline is inactive,
+    let animations = document.getAnimations();
+    assert_equals(animations.length, 1,
+                  'Single running animation');
+    assert_true(animations[0].timeline instanceof ScrollTimeline,
+                'Animation associated with a scroll timeline');
+    assert_equals(animations[0].timeline.currentTime, null,
+                  'Timeline is initially inactive');
+
+    // Canceled animation is no longer current.
+    const anim = animations[0];
+    animations[0].cancel();
+
+    assert_equals(
+        document.getAnimations().length, 0,
+        'A canceled animation is no longer returned by getAnimations');
+
+    // Replaying an animation makes it current.
+    anim.play();
+    assert_equals(
+        document.getAnimations().length, 1,
+        'A play-pending animation is return by getAnimations');
+
+    // Animation effect is still current even if the timeline's source element
+    // cannot be scrolled.
+    spacer.style = 'display: none';
+    t.add_cleanup(() => {
+      spacer.style = '';
+    });
+
+    animations = document.getAnimations();
+    assert_equals(
+        animations.length, 1,
+        'Running animation is included in getAnimations list even if ' +
+        'currentTime is null');
+    assert_true(animations[0].timeline  instanceof ScrollTimeline,
+                'Animation has timeline associated with an element that ' +
+                'cannot be scrolled');
+    assert_equals(animations[0].timeline.currentTime, null,
+                  'Inactive timeline when timeline\'s source element cannot ' +
+                  'be scrolled');
+  }, 'getAnimations includes inactive scroll-linked animations that have not ' +
+     'been canceled');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/named-range-keyframes-with-document-timeline.tentative.html b/testing/web-platform/tests/scroll-animations/css/named-range-keyframes-with-document-timeline.tentative.html
new file mode 100644
index 0000000000..a0094d3220
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/named-range-keyframes-with-document-timeline.tentative.html
@@ -0,0 +1,54 @@
+<!DOCTYPE html>
+<html>
+<meta charset="utf-8">
+<title>Named range keyframe offset when you have a document timeline</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes fade-in-animation {
+    from { opacity: 0 }
+
+    enter 0% { opacity: 0 }
+    enter 100% { opacity: 1 }
+    exit 0% { opacity: 1 }
+    exit 100% { opacity: 0 }
+
+    to { opacity: 1 }
+  }
+
+  #subject {
+    background-color: blue;
+    height: 200px;
+    width: 200px;
+    animation: linear both fade-in-animation;
+    animation-duration: 0.1s;
+    animation-play-state: paused;
+  }
+</style>
+<body onload="runTests()">
+  <div id="subject"></div>
+</body>
+
+<script type="text/javascript">
+  setup(assert_implements_animation_timeline);
+
+  function runTests() {
+    promise_test(async t => {
+      const anim = subject.getAnimations()[0];
+      anim.currentTime = -1;
+      assert_equals(getComputedStyle(subject).opacity, "0",
+              'unexpected value in the before phase');
+
+      anim.currentTime = 50;
+      assert_equals(getComputedStyle(subject).opacity, "0.5",
+              'unexpected value in the middle of the animation');
+
+      anim.currentTime = 100;
+      assert_equals(getComputedStyle(subject).opacity, "1",
+              'unexpected value in the after phase');
+    });
+  }
+</script>
+</html>
diff --git a/testing/web-platform/tests/scroll-animations/css/printing/animation-timeline-none-with-progress-print.tentative.html b/testing/web-platform/tests/scroll-animations/css/printing/animation-timeline-none-with-progress-print.tentative.html
new file mode 100644
index 0000000000..3939a1df48
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/printing/animation-timeline-none-with-progress-print.tentative.html
@@ -0,0 +1,56 @@
+<!DOCTYPE HTML>
+<html class="reftest-wait">
+<title>The animation-timeline:none with preserved progress for print</title>
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#animation-timeline">
+<meta name="assert" content="print correctly for an animation with animation-timeline:none with preserved progress">
+<link rel="match" href="animation-timeline-none-with-progress-ref.html">
+
+<style>
+  @keyframes anim {
+    from { transform: translateX(0px); }
+    to { transform: translateX(100px); }
+  }
+
+  #scroller {
+    scroll-timeline: timeline;
+    overflow: scroll;
+    width: 100px;
+    height: 100px;
+    scrollbar-width: none;
+  }
+
+  #contents {
+    height: 200px;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    animation: anim 1s linear timeline;
+  }
+</style>
+
+<div id="scroller">
+  <div id="contents"></div>
+</div>
+<div id="box"></div>
+
+<script>
+  window.addEventListener('load', function() {
+    const scroller = document.getElementById("scroller");
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+
+    window.requestAnimationFrame(() => {
+      let box = document.getElementById("box");
+      box.style.animationTimeline = "none";
+      getComputedStyle(box).marginLeft;
+
+      window.requestAnimationFrame(() => {
+        document.documentElement.classList.remove("reftest-wait");
+      });
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/printing/animation-timeline-none-with-progress-ref.html b/testing/web-platform/tests/scroll-animations/css/printing/animation-timeline-none-with-progress-ref.html
new file mode 100644
index 0000000000..09bcba2fd4
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/printing/animation-timeline-none-with-progress-ref.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<title>Reference for none animation-timeline</title>
+<style>
+  #scroller {
+    overflow: scroll;
+    width: 100px;
+    height: 100px;
+    scrollbar-width: none;
+  }
+
+  #contents {
+    height: 200px;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    transform: translateX(50px);
+  }
+</style>
+
+<div id="scroller">
+  <div id="contents"></div>
+</div>
+<div id="box"></div>
+
+<script>
+  window.addEventListener('load', function() {
+    const scroller = document.getElementById("scroller");
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+
+    document.documentElement.classList.remove("reftest-wait");
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-default-iframe-print.html b/testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-default-iframe-print.html
new file mode 100644
index 0000000000..d732ca141a
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-default-iframe-print.html
@@ -0,0 +1,65 @@
+<!DOCTYPE HTML>
+<html class="reftest-wait">
+<title>The default scroll() timeline in the iframe for print</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-notation">
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#animation-timeline">
+<meta name="assert" content="CSS animation correctly updates values when using the default scroll() timeline">
+<link rel="match" href="../scroll-timeline-default-iframe-ref.html">
+<meta name="fuzzy" content="25;100">
+
+<iframe id="target" width="400" height="400" srcdoc='
+  <html>
+  <style>
+    @keyframes update {
+      from { transform: translateY(0px); }
+      to { transform: translateY(200px); }
+    }
+    html {
+      min-height: 100%;
+      padding-bottom: 100px;
+    }
+    #box {
+      width: 100px;
+      height: 100px;
+      background-color: green;
+      animation: update 1s linear;
+      animation-timeline: scroll();
+    }
+    #covered {
+      width: 100px;
+      height: 100px;
+      background-color: red;
+    }
+
+    * {
+      margin-top: 0px;
+      margin-bottom: 0px;
+    }
+  </style>
+  <script>
+    window.addEventListener("load", function() {
+      const scroller = document.scrollingElement;
+
+      // Move the scroller to the halfway point.
+      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+      scroller.scrollTop = 0.5 * maxScroll;
+
+      window.requestAnimationFrame(() => {
+        window.parent.postMessage("ready", "*");
+      });
+    });
+  </script>
+  <body>
+    <div id="box"></div>
+    <div id="covered"></div>
+  </body>
+  </html>
+'></iframe>
+
+<script>
+  window.addEventListener("message", event => {
+    if (event.data == "ready") {
+      document.documentElement.classList.remove("reftest-wait");
+    }
+  }, false);
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-default-print.tentative.html b/testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-default-print.tentative.html
new file mode 100644
index 0000000000..975a59812e
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-default-print.tentative.html
@@ -0,0 +1,62 @@
+<!DOCTYPE HTML>
+<html class="reftest-wait">
+<title>The default scroll() timeline for print</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-notation">
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#animation-timeline">
+<meta name="assert" content="CSS animation correctly updates values when using the default scroll() timeline">
+<link rel="match" href="../scroll-timeline-default-ref.html">
+
+<style>
+  @keyframes update {
+    from { transform: translateY(0px); }
+    to { transform: translateY(200px); }
+  }
+
+  html {
+    min-height: 100%;
+    padding-bottom: 100px;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    animation: update 1s linear;
+    animation-timeline: scroll();
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  * {
+    margin-top: 0px;
+    margin-bottom: 0px;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+<script src="/web-animations/testcommon.js"></script>
+<script>
+  document.documentElement.addEventListener('TestRendered', async () => {
+    runTest();
+  }, { once: true });
+
+  async function runTest() {
+    const scroller = document.scrollingElement;
+
+    await waitForCompositorReady();
+
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+
+    await waitForNextFrame();
+    await waitForNextFrame();
+
+    document.documentElement.classList.remove("reftest-wait");
+  }
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-specified-scroller-print.html b/testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-specified-scroller-print.html
new file mode 100644
index 0000000000..7e5b6607b6
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-specified-scroller-print.html
@@ -0,0 +1,58 @@
+<!DOCTYPE HTML>
+<html class="reftest-wait">
+<title>A scroll timeline with a specified scroller for print</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-timelines">
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#animation-timeline">
+<meta name="assert" content="CSS animation correctly updates values when using a specified scroller">
+<link rel="match" href="scroll-timeline-specified-scroller-ref.html">
+
+<style>
+  @keyframes anim {
+    from { transform: translateX(0px); }
+    to { transform: translateX(100px); }
+  }
+
+  #scroller {
+    scroll-timeline: timeline;
+    overflow: scroll;
+    width: 100px;
+    height: 100px;
+    scrollbar-width: none;
+  }
+
+  #contents {
+    height: 200px;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    animation: anim 1s linear;
+    animation-timeline: timeline;
+  }
+
+  @supports not (animation-timeline:timeline) {
+    #box {
+      animation-play-state: paused;
+    }
+  }
+</style>
+
+<div id="scroller">
+  <div id="contents"></div>
+</div>
+<div id="box"></div>
+
+<script>
+  window.addEventListener('load', function() {
+    const scroller = document.getElementById("scroller");
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+
+    window.requestAnimationFrame(() => {
+      document.documentElement.classList.remove("reftest-wait");
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-specified-scroller-ref.html b/testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-specified-scroller-ref.html
new file mode 100644
index 0000000000..9074f2454b
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/printing/scroll-timeline-specified-scroller-ref.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<title>Reference for scroll timeline with a specified scroller</title>
+<style>
+  #scroller {
+    overflow: scroll;
+    width: 100px;
+    height: 100px;
+    scrollbar-width: none;
+  }
+
+  #contents {
+    height: 200px;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    transform: translateX(50px);
+  }
+</style>
+
+<div id="scroller">
+  <div id="contents"></div>
+</div>
+<div id="box"></div>
+
+<script>
+  window.addEventListener('load', function() {
+    const scroller = document.getElementById("scroller");
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+
+    document.documentElement.classList.remove("reftest-wait");
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/progress-based-animation-animation-longhand-properties.tentative.html b/testing/web-platform/tests/scroll-animations/css/progress-based-animation-animation-longhand-properties.tentative.html
new file mode 100644
index 0000000000..f4f9a669f3
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/progress-based-animation-animation-longhand-properties.tentative.html
@@ -0,0 +1,255 @@
+<!DOCTYPE html>
+<title>The various animation longhands with progress based animations</title>
+<link rel="help" src="https://drafts.csswg.org/css-animations-2">
+<link rel="help" src="https://github.com/w3c/csswg-drafts/issues/4862">
+<link rel="help" src="https://github.com/w3c/csswg-drafts/issues/6674">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { translate: 0px; }
+    to { translate: 100px; }
+  }
+  #container {
+    width: 300px;
+    height: 300px;
+    overflow: scroll;
+  }
+  #target {
+    width: 100px;
+    height: 100px;
+    translate: none;
+  }
+</style>
+<body>
+<div id="log"></div>
+<script>
+"use strict";
+
+setup(assert_implements_animation_timeline);
+
+const createTargetAndScroller = function(t) {
+  let container = document.createElement('div');
+  container.id = 'container';
+  let target = document.createElement('div');
+  target.id = 'target';
+  let content = document.createElement('div');
+  content.style.blockSize = '100%';
+
+  // The height of target is 100px and the content is 100%, so the scroll range
+  // is [0, 100].
+
+  // <div id='container'>
+  //   <div id='target'></div>
+  //   <div style='block-size: 100%;'></div>
+  // </div>
+  document.body.appendChild(container);
+  container.appendChild(target);
+  container.appendChild(content);
+
+  if (t && typeof t.add_cleanup === 'function') {
+    t.add_cleanup(() => {
+      content.remove();
+      target.remove();
+      container.remove();
+    });
+  }
+
+  return [target, container];
+};
+
+async function scrollTop(element, value) {
+  element.scrollTop = value;
+  await waitForNextFrame();
+}
+
+// ------------------------------
+// Test animation-duration
+// ------------------------------
+
+promise_test(async t => {
+  let [target, scroller] = createTargetAndScroller(t);
+  target.style.animation = '10s linear anim';
+  target.style.animationTimeline = 'scroll(nearest)';
+
+  await scrollTop(scroller, 25); // [0, 100].
+  assert_equals(getComputedStyle(target).translate, '25px');
+}, 'animation-duration');
+
+promise_test(async t => {
+  let [target, scroller] = createTargetAndScroller(t);
+  target.style.animation = '0s linear anim forwards';
+  target.style.animationTimeline = 'scroll(nearest)';
+
+  await scrollTop(scroller, 25); // [0, 100].
+  assert_equals(getComputedStyle(target).translate, '100px');
+}, 'animation-duration: 0s');
+
+
+// ------------------------------
+// Test animation-iteration-count
+// ------------------------------
+
+promise_test(async t => {
+  let [target, scroller] = createTargetAndScroller(t);
+  target.style.animation = '10s linear anim';
+  target.style.animationTimeline = 'scroll(nearest)';
+
+  await scrollTop(scroller, 25); // [0, 100].
+  assert_equals(getComputedStyle(target).translate, '25px');
+
+  // Let animation become 50% in the 1st iteration.
+  target.style.animationIterationCount = '2';
+  await waitForCSSScrollTimelineStyle();
+  assert_equals(getComputedStyle(target).translate, '50px');
+
+  // Let animation become 0% in the 2nd iteration.
+  target.style.animationIterationCount = '4';
+  await waitForCSSScrollTimelineStyle();
+  assert_equals(getComputedStyle(target).translate, '0px');
+}, 'animation-iteration-count');
+
+promise_test(async t => {
+  let [target, scroller] = createTargetAndScroller(t);
+  target.style.animation = '10s linear anim forwards';
+  target.style.animationTimeline = 'scroll(nearest)';
+  target.style.animationIterationCount = '0';
+
+  await scrollTop(scroller, 25); // [0, 100].
+  assert_equals(getComputedStyle(target).translate, '0px');
+}, 'animation-iteration-count: 0');
+
+promise_test(async t => {
+  let [target, scroller] = createTargetAndScroller(t);
+  target.style.animation = '10s linear anim forwards';
+  target.style.animationTimeline = 'scroll(nearest)';
+  target.style.animationIterationCount = 'infinite';
+
+  await scrollTop(scroller, 25); // [0, 100].
+  assert_equals(getComputedStyle(target).translate, '100px');
+}, 'animation-iteration-count: infinite');
+
+
+// ------------------------------
+// Test animation-direction
+// ------------------------------
+
+promise_test(async t => {
+  let [target, scroller] = createTargetAndScroller(t);
+  target.style.animation = '10s linear anim';
+  target.style.animationTimeline = 'scroll(nearest)';
+
+  await scrollTop(scroller, 25) // [0, 100].
+  assert_equals(getComputedStyle(target).translate, '25px');
+}, 'animation-direction: normal');
+
+promise_test(async t => {
+  let [target, scroller] = createTargetAndScroller(t);
+  target.style.animation = '10s linear anim';
+  target.style.animationTimeline = 'scroll(nearest)';
+  target.style.animationDirection = 'reverse';
+
+  await scrollTop(scroller, 25); // 25% in the reversing direction.
+  assert_equals(getComputedStyle(target).translate, '75px');
+}, 'animation-direction: reverse');
+
+promise_test(async t => {
+  let [target, scroller] = createTargetAndScroller(t);
+  target.style.animation = '10s linear anim';
+  target.style.animationTimeline = 'scroll(nearest)';
+  target.style.animationIterationCount = '2';
+  target.style.animationDirection = 'alternate';
+
+  await scrollTop(scroller, 10); // 20% in the 1st iteration.
+  assert_equals(getComputedStyle(target).translate, '20px');
+
+  await scrollTop(scroller, 60); // 20% in the 2nd iteration (reversing direction).
+  assert_equals(getComputedStyle(target).translate, '80px');
+}, 'animation-direction: alternate');
+
+promise_test(async t => {
+  let [target, scroller] = createTargetAndScroller(t);
+  target.style.animation = '10s linear anim';
+  target.style.animationTimeline = 'scroll(nearest)';
+  target.style.animationIterationCount = '2';
+  target.style.animationDirection = 'alternate-reverse';
+
+  await scrollTop(scroller, 10); // 20% in the 1st iteration (reversing direction).
+  assert_equals(getComputedStyle(target).translate, '80px');
+
+  await scrollTop(scroller, 60); // 20% in the 2nd iteration.
+  assert_equals(getComputedStyle(target).translate, '20px');
+}, 'animation-direction: alternate-reverse');
+
+
+// ------------------------------
+// Test animation-delay
+// ------------------------------
+
+promise_test(async t => {
+  let [target, scroller] = createTargetAndScroller(t);
+  target.style.animation = '10s linear anim';
+  target.style.animationTimeline = 'scroll(nearest)';
+
+  await scrollTop(scroller, 25); // [0, 100].
+  assert_equals(getComputedStyle(target).translate, '25px');
+
+  //    (start delay: 10s)    (duration: 10s)
+  //         before              active
+  // |--------------------|--------------------|
+  // 0px                 50px                100px (The scroller)
+  //                      0%                 100%  (The iteration progress)
+
+  // Let animation be in before phase.
+  target.style.animationDelay = '10s';
+  target.style.animationDelayStart = '10s'; // crbug.com/1375994
+  assert_equals(getComputedStyle(target).translate, 'none');
+
+  await scrollTop(scroller, 50); // The animation enters active phase.
+  assert_equals(getComputedStyle(target).translate, '0px');
+
+  await scrollTop(scroller, 75); // The ieration progress is 50%.
+  assert_equals(getComputedStyle(target).translate, '50px');
+}, 'animation-delay with a positive value');
+
+promise_test(async t => {
+  let [target, scroller] = createTargetAndScroller(t);
+  target.style.animation = '10s linear anim';
+  target.style.animationTimeline = 'scroll(nearest)';
+
+  //         active
+  // |--------------------|
+  // 0px                100px (The scroller)
+  // 50%                100%  (The iteration progress)
+
+  await scrollTop(scroller, 20); // [0, 100].
+  target.style.animationDelay = '-5s';
+  target.style.animationDelayStart = '-5s'; // crbug.com/1375994
+  await waitForCSSScrollTimelineStyle();
+  assert_equals(getComputedStyle(target).translate, '60px');
+}, 'animation-delay with a negative value');
+
+
+// ------------------------------
+// Test animation-fill-mode
+// ------------------------------
+
+promise_test(async t => {
+  let [target, scroller] = createTargetAndScroller(t);
+  target.style.animation = '10s linear anim';
+  target.style.animationTimeline = 'scroll(nearest)';
+  target.style.animationDelay = '10s';
+  target.style.animationDelayStart = '10s'; // crbug.com/1375994
+
+  await scrollTop(scroller, 25);
+  assert_equals(getComputedStyle(target).translate, 'none');
+
+  target.style.animationFillMode = 'backwards';
+  await waitForCSSScrollTimelineStyle();
+  assert_equals(getComputedStyle(target).translate, '0px');
+}, 'animation-fill-mode');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/css/progress-based-animation-timeline.html b/testing/web-platform/tests/scroll-animations/css/progress-based-animation-timeline.html
new file mode 100644
index 0000000000..2e910cbe51
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/progress-based-animation-timeline.html
@@ -0,0 +1,52 @@
+<!DOCTYPE html>
+<title>CSS Animation using progress based timeline</title>
+<link rel="help" src="https://drafts.csswg.org/css-animations-2/#animation-timeline">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<style>
+  main > div {
+    overflow: hidden;
+    width: 100px;
+    height: 100px;
+  }
+  main > div > div {
+    height: 200px;
+  }
+
+  @keyframes top {
+    from { top: 100px; }
+    to { top: 200px; }
+  }
+
+  #scroller1 {
+    scroll-timeline: top_timeline;
+  }
+
+  #element {
+    animation-name: top;
+    animation-duration: 10s;
+    animation-timing-function: linear;
+    animation-timeline: top_timeline;
+  }
+  /* Ensure stable expectations if feature is not supported */
+  @supports not (animation-timeline:foo) {
+    #element { animation-play-state: paused; }
+  }
+</style>
+<main>
+  <div id=scroller1><div></div></div>
+  <div id=element></div>
+</main>
+<script>
+  window.onload = async () => {
+    promise_test(async (t) => {
+      await waitForNextFrame();
+      const anim = document.getAnimations()[0];
+      await anim.ready;
+      scroller1.scrollTop = 20;
+      await waitForNextFrame();
+      assert_equals(getComputedStyle(element).top, '120px');
+    }, 'progress based animation timeline works');
+  };
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-axis-computed.tentative.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-axis-computed.tentative.html
new file mode 100644
index 0000000000..8e23968b8e
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-axis-computed.tentative.html
@@ -0,0 +1,35 @@
+<!DOCTYPE html>
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/6674">
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/rewrite#scroll-timeline-axis">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/computed-testcommon.js"></script>
+</head>
+<style>
+  #outer { scroll-timeline-axis: inline; }
+  #target { scroll-timeline-axis: vertical; }
+</style>
+<div id="outer">
+  <div id="target"></div>
+</div>
+<script>
+test_computed_value('scroll-timeline-axis', 'initial', 'block');
+test_computed_value('scroll-timeline-axis', 'inherit', 'inline');
+test_computed_value('scroll-timeline-axis', 'unset', 'block');
+test_computed_value('scroll-timeline-axis', 'revert', 'block');
+test_computed_value('scroll-timeline-axis', 'block');
+test_computed_value('scroll-timeline-axis', 'inline');
+test_computed_value('scroll-timeline-axis', 'vertical');
+test_computed_value('scroll-timeline-axis', 'horizontal');
+
+test(() => {
+  let style = getComputedStyle(document.getElementById('target'));
+  assert_not_equals(Array.from(style).indexOf('scroll-timeline-axis'), -1);
+}, 'The scroll-timeline-axis property shows up in CSSStyleDeclaration enumeration');
+
+test(() => {
+  let style = document.getElementById('target').style;
+  assert_not_equals(style.cssText.indexOf('scroll-timeline-axis'), -1);
+}, 'The scroll-timeline-axis property shows up in CSSStyleDeclaration.cssText');
+
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-axis-parsing.tentative.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-axis-parsing.tentative.html
new file mode 100644
index 0000000000..81be07e38f
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-axis-parsing.tentative.html
@@ -0,0 +1,27 @@
+<!DOCTYPE html>
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/6674">
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/rewrite#scroll-timeline-axis">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/parsing-testcommon.js"></script>
+</head>
+<div id="target"></div>
+
+<script>
+
+test_valid_value('scroll-timeline-axis', 'initial');
+test_valid_value('scroll-timeline-axis', 'inherit');
+test_valid_value('scroll-timeline-axis', 'unset');
+test_valid_value('scroll-timeline-axis', 'revert');
+
+test_valid_value('scroll-timeline-axis', 'block');
+test_valid_value('scroll-timeline-axis', 'inline');
+test_valid_value('scroll-timeline-axis', 'vertical');
+test_valid_value('scroll-timeline-axis', 'horizontal');
+
+test_invalid_value('scroll-timeline-axis', 'abc');
+test_invalid_value('scroll-timeline-axis', '10px');
+test_invalid_value('scroll-timeline-axis', 'auto');
+test_invalid_value('scroll-timeline-axis', 'none');
+
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-axis-writing-mode.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-axis-writing-mode.html
new file mode 100644
index 0000000000..90912bfc22
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-axis-writing-mode.html
@@ -0,0 +1,126 @@
+<!DOCTYPE html>
+<meta name="viewport" content="width=device-width,initial-scale=1,minimum-scale=1">
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#scroll-timeline-axis">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<style>
+  .scroller {
+    overflow: hidden;
+    width: 100px;
+    height: 100px;
+  }
+  .contents {
+    height: 200px;
+    width: 200px;
+  }
+  @keyframes expand {
+    from { width: 100px; }
+    to { width: 200px; }
+  }
+  #timeline_initial_axis {
+    scroll-timeline: timeline_initial_axis;
+  }
+  #timeline_vertical {
+    scroll-timeline: vertical timeline_vertical;
+  }
+  #timeline_horizontal {
+    scroll-timeline: horizontal timeline_horizontal;
+  }
+  #timeline_block_in_horizontal {
+    scroll-timeline: block timeline_block_in_horizontal;
+  }
+  #timeline_inline_in_horizontal {
+    scroll-timeline: inline timeline_inline_in_horizontal;
+  }
+  #timeline_block_in_vertical {
+    scroll-timeline: block timeline_block_in_vertical;
+    writing-mode: vertical-lr;
+  }
+  #timeline_inline_in_vertical {
+    scroll-timeline: inline timeline_inline_in_vertical;
+    writing-mode: vertical-lr;
+  }
+  #container > div {
+    width: 0px;
+    animation-name: expand;
+    animation-duration: 10s;
+    animation-timing-function: linear;
+  }
+  /* Ensure stable expectations if feature is not supported */
+  @supports not (animation-timeline:foo) {
+    #container > div { animation-play-state: paused; }
+  }
+  #element_initial_axis { animation-timeline: timeline_initial_axis; }
+  #element_vertical { animation-timeline: timeline_vertical; }
+  #element_horizontal { animation-timeline: timeline_horizontal; }
+  #element_block_in_horizontal { animation-timeline: timeline_block_in_horizontal; }
+  #element_inline_in_horizontal { animation-timeline: timeline_inline_in_horizontal; }
+  #element_block_in_vertical { animation-timeline: timeline_block_in_vertical; }
+  #element_inline_in_vertical { animation-timeline: timeline_inline_in_vertical; }
+</style>
+<div class=scroller id=timeline_initial_axis><div class=contents></div></div>
+<div class=scroller id=timeline_vertical><div class=contents></div></div>
+<div class=scroller id=timeline_horizontal><div class=contents></div></div>
+<div class=scroller id=timeline_block_in_horizontal><div class=contents></div></div>
+<div class=scroller id=timeline_inline_in_horizontal><div class=contents></div></div>
+<div class=scroller id=timeline_block_in_vertical><div class=contents></div></div>
+<div class=scroller id=timeline_inline_in_vertical><div class=contents></div></div>
+<div id=container>
+  <div id=element_initial_axis></div>
+  <div id=element_vertical></div>
+  <div id=element_horizontal></div>
+  <div id=element_block_in_horizontal></div>
+  <div id=element_inline_in_horizontal></div>
+  <div id=element_block_in_vertical></div>
+  <div id=element_inline_in_vertical></div>
+</div>
+<script>
+  // Animations linked to vertical scroll-timelines are at 75% progress.
+  timeline_initial_axis.scrollTop = 75;
+  timeline_vertical.scrollTop = 75;
+  timeline_block_in_horizontal.scrollTop = 75;
+  timeline_inline_in_vertical.scrollTop = 75;
+  // Animations linked to horizontal scroll-timelines are at 25% progress.
+  timeline_horizontal.scrollLeft = 25;
+  timeline_block_in_vertical.scrollLeft = 25;
+  timeline_inline_in_horizontal.scrollLeft = 25;
+
+  promise_test(async (t) => {
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element_initial_axis).width, '175px');
+  }, 'Initial axis');
+
+  promise_test(async (t) => {
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element_vertical).width, '175px');
+  }, 'Vertical axis');
+
+  promise_test(async (t) => {
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element_horizontal).width, '125px');
+  }, 'Horizontal axis');
+
+  promise_test(async (t) => {
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element_block_in_horizontal).width, '175px');
+  }, 'Block axis in horizontal writing-mode');
+
+  promise_test(async (t) => {
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element_inline_in_horizontal).width, '125px');
+  }, 'Inline axis in horizontal writing-mode');
+
+  promise_test(async (t) => {
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(timeline_block_in_vertical).writingMode, 'vertical-lr');
+    assert_equals(getComputedStyle(element_block_in_vertical).width, '125px');
+  }, 'Block axis in vertical writing-mode');
+
+  promise_test(async (t) => {
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(timeline_inline_in_vertical).writingMode, 'vertical-lr');
+    assert_equals(getComputedStyle(element_inline_in_vertical).width, '175px');
+  }, 'Inline axis in vertical writing-mode');
+
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-iframe-ref.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-iframe-ref.html
new file mode 100644
index 0000000000..1ab5646c8b
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-iframe-ref.html
@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<title>Reference for default scroll() timeline</title>
+<iframe width="400" height="400" srcdoc='
+  <html>
+  <style>
+    html {
+      min-height: 100%;
+      padding-bottom: 100px;
+    }
+
+    #box {
+      width: 100px;
+      height: 100px;
+      background-color: green;
+      transform: translateY(100px);
+    }
+
+    * {
+      margin-top: 0px;
+      margin-bottom: 0px;
+    }
+  </style>
+  <script>
+    window.addEventListener("load", function() {
+      // Move the scroller to halfway.
+      const scroller = document.scrollingElement;
+      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+      scroller.scrollTop = 0.5 * maxScroll;
+    });
+  </script>
+  <div id="box"></div>
+  </html>
+'></iframe>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-iframe.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-iframe.html
new file mode 100644
index 0000000000..dbcf5941a8
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-iframe.html
@@ -0,0 +1,73 @@
+<!DOCTYPE HTML>
+<html class="reftest-wait">
+<title>The default scroll() timeline in the iframe</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-notation">
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#animation-timeline">
+<meta name="assert" content="CSS animation correctly updates values when using the default scroll() timeline">
+<link rel="match" href="scroll-timeline-default-iframe-ref.html">
+
+<iframe id="target" width="400" height="400" srcdoc='
+  <html>
+  <style>
+    @keyframes update {
+      from { transform: translateY(0px); }
+      to { transform: translateY(200px); }
+    }
+    html {
+      min-height: 100%;
+      padding-bottom: 100px;
+    }
+    #box {
+      width: 100px;
+      height: 100px;
+      background-color: green;
+      animation: update 1s linear;
+      animation-timeline: scroll();
+    }
+    #covered {
+      width: 100px;
+      height: 100px;
+      background-color: red;
+    }
+
+    * {
+      margin-top: 0px;
+      margin-bottom: 0px;
+    }
+  </style>
+  <script src="/web-animations/testcommon.js"></script>
+  <script>
+    window.addEventListener("load", async function() {
+      const scroller = document.scrollingElement;
+
+      // Move the scroller to the halfway point.
+      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+      scroller.scrollTop = 0.5 * maxScroll;
+
+      await waitForCompositorReady();
+      await waitForNextFrame();
+      await waitForNextFrame();
+
+      window.parent.postMessage("success", "*");
+    });
+  </script>
+  <body>
+    <div id="box"></div>
+    <div id="covered"></div>
+  </body>
+  </html>
+'></iframe>
+<script src="/web-animations/testcommon.js"></script>
+<script>
+  async function finishTest() {
+    await waitForCompositorReady();
+    await waitForNextFrame();
+    await waitForNextFrame();
+    document.documentElement.classList.remove("reftest-wait");
+  }
+  window.addEventListener("message", event => {
+    if (event.data == "success") {
+      finishTest();
+    }
+  }, false);
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-quirks-mode.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-quirks-mode.html
new file mode 100644
index 0000000000..d2c28d86b6
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-quirks-mode.html
@@ -0,0 +1,63 @@
+<html class="reftest-wait">
+<title>The default scroll() timeline in quirks mode</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-notation">
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#animation-timeline">
+<meta name="assert" content="CSS animation correctly updates values when using the default scroll() timeline">
+<link rel="match" href="scroll-timeline-default-ref.html">
+
+<style>
+  @keyframes update {
+    from { transform: translateY(0px); }
+    to { transform: translateY(200px); }
+  }
+
+  html {
+    min-height: 100%;
+    padding-bottom: 100px;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    animation: update 1s linear;
+    animation-timeline: scroll();
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  * {
+    margin-top: 0px;
+    margin-bottom: 0px;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+<script src="/web-animations/testcommon.js"></script>
+<script>
+document.documentElement.addEventListener('TestRendered', async () => {
+  runTest();
+}, { once: true });
+
+async function runTest() {
+  const scroller = document.scrollingElement;
+
+  await waitForCompositorReady();
+
+  // Move the scroller to the halfway point. Then advance to the next frame
+  // to pick up the new timeline time.
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.5 * maxScroll;
+
+  await waitForNextFrame();
+  await waitForNextFrame();
+
+  document.documentElement.classList.remove("reftest-wait");
+}
+
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-ref.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-ref.html
new file mode 100644
index 0000000000..cb3b60e4bd
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-ref.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<title>Reference for default scroll() timeline</title>
+<style>
+  html {
+    min-height: 100%;
+    padding-bottom: 100px;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    transform: translateY(100px);
+  }
+
+  * {
+    margin-top: 0px;
+    margin-bottom: 0px;
+  }
+</style>
+
+<div id="box"></div>
+
+<script>
+  window.addEventListener('load', function() {
+    // Move the scroller to halfway.
+    const scroller = document.scrollingElement;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-writing-mode-rl-ref.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-writing-mode-rl-ref.html
new file mode 100644
index 0000000000..3c072829e6
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-writing-mode-rl-ref.html
@@ -0,0 +1,32 @@
+<!DOCTYPE html>
+<title>Reference for default scroll() timeline with vertical-rl</title>
+<meta name="viewport" content="width=device-width,initial-scale=1,minimum-scale=1">
+<style>
+  html {
+    min-block-size: 100%;
+    padding-block-end: 100px;
+    writing-mode: vertical-rl
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    transform: translateX(-100px);
+  }
+
+  * {
+    margin-block: 0px;
+  }
+</style>
+
+<div id="box"></div>
+
+<script>
+  window.addEventListener('load', function() {
+    // Move the scroller to halfway.
+    const scroller = document.scrollingElement;
+    const maxScroll = scroller.scrollWidth - scroller.clientWidth;
+    scroller.scrollLeft = -0.5 * maxScroll;
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-writing-mode-rl.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-writing-mode-rl.html
new file mode 100644
index 0000000000..27e6ec196b
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default-writing-mode-rl.html
@@ -0,0 +1,65 @@
+<!DOCTYPE HTML>
+<html class="reftest-wait">
+<title>The default scroll() timeline with writing-mode:vertical-rl</title>
+<meta name="viewport" content="width=device-width,initial-scale=1,minimum-scale=1">
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-notation">
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#animation-timeline">
+<meta name="assert" content="CSS animation correctly updates values when using
+                             the default scroll() timeline with writing-mode:vertical-rl">
+<link rel="match" href="scroll-timeline-default-writing-mode-rl-ref.html">
+
+<style>
+  @keyframes update {
+    from { transform: translateX(0px); }
+    to { transform: translateX(-200px); }
+  }
+
+  html {
+    min-block-size: 100%;
+    padding-block-end: 100px;
+    writing-mode: vertical-rl;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    animation: update 1s linear;
+    animation-timeline: scroll();
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  * {
+    margin-block: 0px;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+
+<script src="/web-animations/testcommon.js"></script>
+<script>
+  document.documentElement.addEventListener('TestRendered', async () => {
+    runTest();
+  }, { once: true });
+
+  async function runTest() {
+    const scroller = document.scrollingElement;
+
+    await waitForCompositorReady();
+
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollWidth - scroller.clientWidth;
+    scroller.scrollLeft = -0.5 * maxScroll;
+
+    await waitForNextFrame();
+    await waitForNextFrame();
+
+    document.documentElement.classList.remove("reftest-wait");
+  }
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default.html
new file mode 100644
index 0000000000..07eda33fd0
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-default.html
@@ -0,0 +1,63 @@
+<!DOCTYPE HTML>
+<html class="reftest-wait">
+<title>The default scroll() timeline</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-notation">
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#animation-timeline">
+<meta name="assert" content="CSS animation correctly updates values when using the default scroll() timeline">
+<link rel="match" href="scroll-timeline-default-ref.html">
+
+<style>
+  @keyframes update {
+    from { transform: translateY(0px); }
+    to { transform: translateY(200px); }
+  }
+
+  html {
+    min-height: 100%;
+    padding-bottom: 100px;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    animation: update 1s linear;
+    animation-timeline: scroll();
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  * {
+    margin-top: 0px;
+    margin-bottom: 0px;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+
+<script src="/web-animations/testcommon.js"></script>
+<script>
+  document.documentElement.addEventListener('TestRendered', async () => {
+    runTest();
+  }, { once: true });
+
+  async function runTest() {
+    const scroller = document.scrollingElement;
+
+    await waitForCompositorReady();
+
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+
+    await waitForNextFrame();
+    await waitForNextFrame();
+
+    document.documentElement.classList.remove("reftest-wait");
+  }
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-document-scroller-quirks.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-document-scroller-quirks.html
new file mode 100644
index 0000000000..809a658a15
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-document-scroller-quirks.html
@@ -0,0 +1,36 @@
+<!-- Quirks mode -->
+<title>Tests the document scroller in quirks mode</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-notation">
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1180575">
+<link rel="author" href="mailto:andruud@chromium.org">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="/css/css-animations/support/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { z-index: 100; }
+    to { z-index: 100; }
+  }
+  #element {
+    animation: anim forwards;
+    animation-timeline: scroll(root);
+  }
+  #spacer {
+    height:  200vh;
+  }
+</style>
+<div id=element></div>
+<div id=spacer></div>
+
+<script>
+'use strict';
+
+setup(assert_implements_animation_timeline);
+
+promise_test(async () => {
+  await waitForCSSScrollTimelineStyle();
+  assert_equals(getComputedStyle(element).zIndex, "100");
+});
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-dynamic.tentative.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-dynamic.tentative.html
new file mode 100644
index 0000000000..b0880a7cc5
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-dynamic.tentative.html
@@ -0,0 +1,267 @@
+<!DOCTYPE html>
+<meta name="viewport" content="width=device-width,initial-scale=1,minimum-scale=1">
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#scroll-timelines">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  main > div {
+    overflow: hidden;
+    width: 100px;
+    height: 100px;
+  }
+  main > div > div {
+    height: 200px;
+  }
+  @keyframes expand {
+    from { width: 100px; }
+    to { width: 200px; }
+  }
+  #element {
+    width: 0px;
+    height: 20px;
+    animation-name: expand;
+    /* Some of the tests in this file assume animations attached to the
+       DocumentTimeline are "stopped" without actually being paused.
+       Using 600s + steps(10, end) achieves this for one minute.*/
+    animation-duration: 600s;
+    animation-timing-function: steps(10, end);
+  }
+</style>
+<main>
+  <div id=scroller1><div></div></div>
+  <div id=scroller2><div></div></div>
+  <div id=container></div>
+</main>
+<script>
+  // Force layout of scrollers.
+  scroller1.offsetTop;
+  scroller2.offsetTop;
+
+  // Note the steps(10, end) timing function and height:100px. (10px scroll
+  // resolution).
+  scroller1.scrollTop = 20;
+  scroller2.scrollTop = 40;
+
+  function insertElement() {
+    let element = document.createElement('div');
+    element.id = 'element';
+    container.append(element);
+    return element;
+  }
+
+  // Runs a test with dynamically added/removed elements or CSS rules.
+  // Each test is instantiated twice: once for the initial style resolve where
+  // the DOM change was effectuated, and once after scrolling.
+  function dynamic_rule_test(func, description) {
+    // assert_width is an async function which verifies that the computed value
+    // of 'width' is as expected.
+    const instantiate = (assert_width, description) => {
+      promise_test(async (t) => {
+        try {
+          await func(t, assert_width);
+        } finally {
+          while (container.firstChild)
+            container.firstChild.remove();
+          scroller1.style = '';
+          scroller2.style = '';
+        }
+      }, description);
+    };
+
+    // Verify that the computed style is as expected after a full frame update
+    // following the rule change took place.
+    instantiate(async (element, expected) => {
+      await waitForCSSScrollTimelineStyle();
+      assert_equals(getComputedStyle(element).width, expected);
+    }, description + ' [immediate]');
+
+    // Verify that the computed style after scrolling a bit.
+    instantiate(async (element, expected) => {
+      scroller1.scrollTop = scroller1.scrollTop + 10;
+      scroller2.scrollTop = scroller2.scrollTop + 10;
+      await waitForNextFrame();
+      scroller1.scrollTop = scroller1.scrollTop - 10;
+      scroller2.scrollTop = scroller2.scrollTop - 10;
+      await waitForNextFrame();
+      assert_equals(getComputedStyle(element).width, expected);
+    }, description + ' [scroll]');
+  }
+
+  dynamic_rule_test(async (t, assert_width) => {
+    let element = insertElement();
+
+    // This element initially has a DocumentTimeline.
+    await assert_width(element, '100px');
+
+    // Switch to scroll timeline.
+    scroller1.style.scrollTimelineName = 'timeline';
+    element.style.animationTimeline = 'timeline';
+    await assert_width(element, '120px');
+
+    // Switching from ScrollTimeline -> DocumentTimeline should preserve
+    // current time.
+    scroller1.style = '';
+    element.style = '';
+    await assert_width(element, '120px');
+  }, 'Switching between document and scroll timelines');
+
+  dynamic_rule_test(async (t, assert_width) => {
+    let element = insertElement();
+
+    // Flush style and create the animation with play pending.
+    getComputedStyle(element).animation;
+
+    let anim = element.getAnimations()[0];
+    assert_true(anim.pending, "The animation is in play pending");
+
+    // Switch to scroll timeline for a pending animation.
+    scroller1.style.scrollTimelineName = 'timeline';
+    element.style.animationTimeline = 'timeline';
+
+    await anim.ready;
+    assert_false(anim.pending, "The animation is not pending");
+
+    await assert_width(element, '120px');
+  }, 'Switching pending animation from document to scroll timelines');
+
+  dynamic_rule_test(async (t, assert_width) => {
+    let element = insertElement();
+
+    // Note: #scroller1 is at 20%, and #scroller2 is at 40%.
+    scroller1.style.scrollTimelineName = 'timeline1';
+    scroller2.style.scrollTimelineName = 'timeline2';
+
+    await assert_width(element, '100px');
+
+    element.style.animationTimeline = 'timeline1';
+    await assert_width(element, '120px');
+
+    element.style.animationTimeline = 'timeline2';
+    await assert_width(element, '140px');
+
+    element.style.animationTimeline = 'timeline1';
+    await assert_width(element, '120px');
+
+    // Switching from ScrollTimeline -> DocumentTimeline should preserve
+    // current time.
+    element.style.animationTimeline = '';
+    await assert_width(element, '120px');
+
+  }, 'Changing computed value of animation-timeline changes effective timeline');
+
+  dynamic_rule_test(async (t, assert_width) => {
+    let element = insertElement();
+
+    scroller1.style.scrollTimelineName = 'timeline';
+
+    // DocumentTimeline applies by default.
+    await assert_width(element, '100px');
+
+    // DocumentTimeline -> none
+    element.style.animationTimeline = 'none';
+    await assert_width(element, '0px');
+
+    // none -> DocumentTimeline
+    element.style.animationTimeline = '';
+    await assert_width(element, '100px');
+
+    // DocumentTimeline -> ScrollTimeline
+    element.style.animationTimeline = 'timeline';
+    await assert_width(element, '120px');
+
+    // ScrollTimeline -> none
+    element.style.animationTimeline = 'none';
+    await assert_width(element, '120px');
+
+    // none -> ScrollTimeline
+    element.style.animationTimeline = 'timeline';
+    await assert_width(element, '120px');
+  }, 'Changing to/from animation-timeline:none');
+
+
+  dynamic_rule_test(async (t, assert_width) => {
+    let element = insertElement();
+
+    element.style.animationTimeline = 'timeline';
+
+    // Unknown animation-timeline, current time held at zero.
+    await assert_width(element, '100px');
+
+    scroller1.style.scrollTimelineName = 'timeline';
+    await assert_width(element, '120px');
+
+    scroller2.style.scrollTimelineName = 'timeline';
+    await assert_width(element, '140px');
+  }, 'Changing scroll-timeline on preceding elements affects target element');
+
+
+  dynamic_rule_test(async (t, assert_width) => {
+    let element = insertElement();
+
+    element.style.animationDirection = 'reverse';
+    element.style.animationTimeline = 'timeline';
+
+    // Unknown animation-timeline, current time held at zero.
+    await assert_width(element, '200px');
+
+    // Note: #scroller1 is at 20%.
+    scroller1.style.scrollTimelineName = 'timeline';
+    await assert_width(element, '180px');
+
+    // Note: #scroller2 is at 40%.
+    scroller2.style.scrollTimelineName = 'timeline';
+    await assert_width(element, '160px');
+
+    element.style.animationDirection = '';
+    await assert_width(element, '140px');
+  }, 'Reverse animation direction');
+
+  dynamic_rule_test(async (t, assert_width) => {
+    let element = insertElement();
+    element.style.animationTimeline = 'timeline';
+
+    // Unknown animation-timeline, current time held at zero.
+    await assert_width(element, '100px');
+
+    // Note: #scroller1 is at 20%.
+    scroller1.style.scrollTimelineName = 'timeline';
+    await assert_width(element, '120px');
+
+    element.style.animationPlayState = 'paused';
+
+    // We should still be at the same position after pausing.
+    await assert_width(element, '120px');
+
+    // Note: #scroller2 is at 40%.
+    scroller2.style.scrollTimelineName = 'timeline';
+
+    // Even when switching timelines, we should be at the same position until
+    // we unpause.
+    await assert_width(element, '120px');
+
+    // Unpausing should synchronize to the scroll position.
+    element.style.animationPlayState = '';
+    await assert_width(element, '140px');
+  }, 'Switching timelines while paused');
+
+  dynamic_rule_test(async (t, assert_width) => {
+    let element = insertElement();
+
+    // Note: #scroller1 is at 20%.
+    scroller1.style.scrollTimelineName = 'timeline';
+
+    await assert_width(element, '100px');
+
+    element.style.animationTimeline = 'timeline';
+    element.style.animationPlayState = 'paused';
+
+    // Pausing should happen before the timeline is modified. (Tentative).
+    // https://github.com/w3c/csswg-drafts/issues/5653
+    await assert_width(element, '100px');
+
+    element.style.animationPlayState = 'running';
+    await assert_width(element, '120px');
+  }, 'Switching timelines and pausing at the same time');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-frame-size-changed-ref.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-frame-size-changed-ref.html
new file mode 100644
index 0000000000..ea7628ac72
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-frame-size-changed-ref.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<title>Reference for the default scroll() timeline</title>
+<style>
+  html {
+    min-height: 100%;
+    padding-bottom: 50px;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    transform: translateY(100px);
+  }
+
+  * {
+    margin-top: 0px;
+    margin-bottom: 0px;
+  }
+</style>
+
+<div id="box"></div>
+
+<script>
+  window.addEventListener('load', function() {
+    // Move the scroller to halfway.
+    const scroller = document.scrollingElement;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-frame-size-changed.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-frame-size-changed.html
new file mode 100644
index 0000000000..fb0eb8aa17
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-frame-size-changed.html
@@ -0,0 +1,67 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<title>The default scroll() timeline when the frame size changed</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-notation">
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#animation-timeline">
+<meta name="assert" content="CSS animation correctly updates values when using
+                             the default scroll() timeline and update the
+                             frame size">
+<link rel="match" href="scroll-timeline-frame-size-changed-ref.html">
+
+<style>
+  @keyframes update {
+    from { transform: translateY(0px); }
+    to { transform: translateY(200px); }
+  }
+
+  html {
+    min-height: 100%;
+    padding-bottom: 100px;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    animation: update 1s linear;
+    animation-timeline: scroll();
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  * {
+    margin-top: 0px;
+    margin-bottom: 0px;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+
+<script src="/web-animations/testcommon.js"></script>
+<script>
+  document.documentElement.addEventListener('TestRendered', async () => {
+    runTest();
+  }, { once: true });
+
+  async function runTest() {
+    const scroller = document.scrollingElement;
+
+    await waitForCompositorReady();
+
+    // Move the scroller to the 25% point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.25 * maxScroll;
+    await waitForNextFrame();
+
+    // Update scroll range to make the current position become 50% point.
+    scroller.style.paddingBottom = "50px";
+    await waitForNextFrame();
+
+    document.documentElement.classList.remove("reftest-wait");
+  }
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-in-container-query.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-in-container-query.html
new file mode 100644
index 0000000000..d6a8ba8e60
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-in-container-query.html
@@ -0,0 +1,74 @@
+<!DOCTYPE html>
+<title>scroll-timeline and container queries</title>
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#scroll-timeline-shorthand">
+<link rel="help" src="https://drafts.csswg.org/css-contain-3/#container-queries">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  #outer {
+    height: 100px;
+    width: 150px;
+  }
+
+  #container {
+    container-type: size;
+  }
+
+  #scroller {
+    overflow: auto;
+    width: auto;
+    height: 100px;
+  }
+  #scroller > div {
+    height: 200px;
+  }
+
+  /* This does not apply initially. */
+  @container (width > 200px) {
+    #scroller {
+      scroll-timeline: timeline;
+    }
+  }
+
+  @keyframes expand {
+    from { background-color: rgb(100, 100, 100); }
+    to { background-color: rgb(200, 200, 200); }
+  }
+  #element {
+    height: 10px;
+    width: 10px;
+    animation: expand 10s linear;
+    animation-timeline: timeline;
+    background-color: rgb(0, 0, 0);
+  }
+</style>
+<div id=outer>
+  <div id=container>
+    <div id=scroller>
+      <div></div>
+    </div>
+    <div id=element>
+    </div>
+  </div>
+</div>
+<script>
+  setup(assert_implements_animation_timeline);
+
+  promise_test(async (t) => {
+    element.offsetTop;
+    scroller.scrollTop = 50;
+    await waitForNextFrame();
+    // Unknown timeline, time held at zero.
+    assert_equals(getComputedStyle(element).backgroundColor, 'rgb(100, 100, 100)');
+    // This causes the timeline to be created.
+    outer.style.width = '250px';
+    // Check value with getComputedStyle immediately, which is the unanimated
+    // value since the scroll timeline is inactive before the next frame.
+    assert_equals(getComputedStyle(element).backgroundColor, 'rgb(0, 0, 0)');
+    // Also check value after one frame.
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element).backgroundColor, 'rgb(150, 150, 150)');
+  }, 'Timeline appearing via container queries');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-inactive.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-inactive.html
new file mode 100644
index 0000000000..92165d7046
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-inactive.html
@@ -0,0 +1,51 @@
+<!DOCTYPE html>
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#scroll-timelines">
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#avoiding-cycles">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<style>
+  #scroller {
+    overflow: scroll;
+    width: 100px;
+    height: 100px;
+  }
+  #contents {
+    height: 200px;
+  }
+  @keyframes expand {
+    from { width: 100px; }
+    to { width: 200px; }
+  }
+  #element {
+    width: 0px;
+    animation: expand 10s linear paused;
+    animation-timeline: timeline;
+  }
+</style>
+<div id="container">
+  <div id=element></div>
+</div>
+<script>
+
+promise_test(async (t) => {
+  let div = document.createElement('div');
+  div.setAttribute('id', 'scroller');
+  div.style.scrollTimeline = 'timeline';
+  div.innerHTML = '<div id=contents></div>';
+  try {
+    container.insertBefore(div, element);
+
+    // The source has no layout box at the time the scroll timeline is created.
+    assert_equals(getComputedStyle(element).width, '0px');
+    scroller.offsetTop; // Ensure a layout box for the scroller.
+    // Wait for an update to the timeline state:
+    await waitForNextFrame();
+    // The timeline should now be active, and the animation should apply:
+    assert_equals(getComputedStyle(element).width, '100px');
+  } finally {
+    div.remove();
+  }
+}, 'Animation does not apply when timeline is initially inactive');
+
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-inline-orientation-ref.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-inline-orientation-ref.html
new file mode 100644
index 0000000000..7b87b1db39
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-inline-orientation-ref.html
@@ -0,0 +1,32 @@
+<!DOCTYPE html>
+<title>Reference for scroll timeline with inline orientation and root scroller</title>
+<meta name="viewport" content="width=device-width,initial-scale=1,minimum-scale=1">
+<style>
+  html {
+    min-width: 100%;
+    padding-right: 100px;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    transform: translateX(100px);
+  }
+
+  * {
+    margin-left: 0px;
+    margin-right: 0px;
+  }
+</style>
+
+<div id="box"></div>
+
+<script>
+  window.addEventListener('load', function() {
+    // Move the scroller to halfway.
+    const scroller = document.scrollingElement;
+    const maxScroll = scroller.scrollWidth - scroller.clientWidth;
+    scroller.scrollLeft = 0.5 * maxScroll;
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-inline-orientation.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-inline-orientation.html
new file mode 100644
index 0000000000..52b7427f2d
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-inline-orientation.html
@@ -0,0 +1,68 @@
+<!DOCTYPE HTML>
+<html class="reftest-wait">
+<title>Scroll timeline with inline orientation and root scroller</title>
+<meta name="viewport" content="width=device-width,initial-scale=1,minimum-scale=1">
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-notation">
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#descdef-scroll-timeline-orientation">
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#animation-timeline">
+<meta name="assert" content="CSS animation correctly updates values when using the inline orientation">
+<link rel="match" href="scroll-timeline-inline-orientation-ref.html">
+
+<style>
+  @keyframes update {
+    from { transform: translateX(0px); }
+    to { transform: translateX(200px); }
+  }
+
+  html {
+    min-width: 100%;
+    padding-right: 100px;
+    font-size: 0;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    animation: update 1s linear;
+    animation-timeline: scroll(inline root);
+    display: inline-block;
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+    display: inline-block;
+  }
+
+  * {
+    margin-left: 0px;
+    margin-right: 0px;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+
+<script src="/web-animations/testcommon.js"></script>
+<script>
+  document.documentElement.addEventListener('TestRendered', async () => {
+    runTest();
+  }, { once: true });
+
+  async function runTest() {
+    const scroller = document.scrollingElement;
+
+    await waitForCompositorReady();
+
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollWidth - scroller.clientWidth;
+    scroller.scrollLeft = 0.5 * maxScroll;
+
+    await waitForNextFrame();
+    await waitForNextFrame();
+
+    document.documentElement.classList.remove("reftest-wait");
+  }
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-multi-pass.tentative.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-multi-pass.tentative.html
new file mode 100644
index 0000000000..91668ada5b
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-multi-pass.tentative.html
@@ -0,0 +1,109 @@
+<!DOCTYPE html>
+<title>ScrollTimelines may trigger multiple style/layout passes</title>
+<link rel="help" src="https://github.com/w3c/csswg-drafts/issues/5261">
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#avoiding-cycles">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes expand_width {
+    from { width: 100px; }
+    to { width: 100px; }
+  }
+  @keyframes expand_height {
+    from { height: 100px; }
+    to { height: 100px; }
+  }
+  main {
+    height: 0px;
+    overflow: hidden;
+  }
+  .scroller {
+    height: 100px;
+    overflow: scroll;
+  }
+  .scroller > div {
+    height: 200px;
+  }
+  #element1 {
+    width: 1px;
+    animation: expand_width 10s;
+    animation-timeline: timeline1;
+  }
+  #element2 {
+    height: 1px;
+    animation: expand_height 10s;
+    animation-timeline: timeline2;
+  }
+</style>
+<main id=main>
+  <div id=element1></div>
+  <div>
+    <div id=element2></div>
+  </div>
+</main>
+<script>
+    setup(assert_implements_animation_timeline);
+
+    function insertScroller(timeline_name) {
+      let scroller = document.createElement('div');
+      scroller.classList.add('scroller');
+      scroller.style.scrollTimeline = timeline_name;
+      scroller.append(document.createElement('div'));
+      main.insertBefore(scroller, element1);
+    }
+
+    promise_test(async () => {
+      await waitForNextFrame();
+
+      let events1 = [];
+      let events2 = [];
+
+      insertScroller('timeline1');
+      // Even though the scroller was just inserted into the DOM, |timeline1|
+      // remains inactive until the next frame.
+      //
+      // https://drafts.csswg.org/scroll-animations-1/#avoiding-cycles
+      assert_equals(getComputedStyle(element1).width, '1px');
+      (new ResizeObserver(entries => {
+        events1.push(entries);
+        insertScroller('timeline2');
+        assert_equals(getComputedStyle(element2).height, '1px');
+      })).observe(element1);
+
+      (new ResizeObserver(entries => {
+        events2.push(entries);
+      })).observe(element2);
+
+      await waitForNextFrame();
+
+      // According to the basic rules of the spec [1], the timeline is
+      // inactive at the time the resize observer event was delivered, because
+      // #scroller1 did not have a layout box at the time style recalc for
+      // #element1 happened.
+      //
+      // However, an additional style/layout pass should take place
+      // (before resize observer deliveries) if we detect new ScrollTimelines
+      // in this situation, hence we ultimately do expect the animation to
+      // apply [2].
+      //
+      // [1] https://drafts.csswg.org/scroll-animations-1/#avoiding-cycles
+      // [2] https://github.com/w3c/csswg-drafts/issues/5261
+      assert_equals(events1.length, 1);
+      assert_equals(events1[0].length, 1);
+      assert_equals(events1[0][0].contentBoxSize.length, 1);
+      assert_equals(events1[0][0].contentBoxSize[0].inlineSize, 100);
+
+      // ScrollTimelines created during the ResizeObserver should remain
+      // inactive during the frame they're created, so the ResizeObserver
+      // event should not reflect the animated value.
+      assert_equals(events2.length, 1);
+      assert_equals(events2[0].length, 1);
+      assert_equals(events2[0][0].contentBoxSize.length, 1);
+      assert_equals(events2[0][0].contentBoxSize[0].blockSize, 1);
+
+      assert_equals(getComputedStyle(element1).width, '100px');
+      assert_equals(getComputedStyle(element2).height, '100px');
+    }, 'Multiple style/layout passes occur when necessary');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-name-computed.tentative.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-name-computed.tentative.html
new file mode 100644
index 0000000000..1acb964ef7
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-name-computed.tentative.html
@@ -0,0 +1,34 @@
+<!DOCTYPE html>
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/6674">
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/rewrite#scroll-timeline-name">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/computed-testcommon.js"></script>
+</head>
+<style>
+  #outer { scroll-timeline-name: foo; }
+  #target { scroll-timeline-name: bar; }
+</style>
+<div id="outer">
+  <div id="target"></div>
+</div>
+<script>
+test_computed_value('scroll-timeline-name', 'initial', 'none');
+test_computed_value('scroll-timeline-name', 'inherit', 'foo');
+test_computed_value('scroll-timeline-name', 'unset', 'none');
+test_computed_value('scroll-timeline-name', 'revert', 'none');
+test_computed_value('scroll-timeline-name', 'none');
+test_computed_value('scroll-timeline-name', 'test');
+test_computed_value('scroll-timeline-name', 'tEst');
+
+test(() => {
+  let style = getComputedStyle(document.getElementById('target'));
+  assert_not_equals(Array.from(style).indexOf('scroll-timeline-name'), -1);
+}, 'The scroll-timeline-name property shows up in CSSStyleDeclaration enumeration');
+
+test(() => {
+  let style = document.getElementById('target').style;
+  assert_not_equals(style.cssText.indexOf('scroll-timeline-name'), -1);
+}, 'The scroll-timeline-name property shows up in CSSStyleDeclaration.cssText');
+
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-name-parsing.tentative.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-name-parsing.tentative.html
new file mode 100644
index 0000000000..a8f2e31650
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-name-parsing.tentative.html
@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/6674">
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/rewrite#scroll-timeline-name">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/parsing-testcommon.js"></script>
+</head>
+<div id="target"></div>
+
+<script>
+
+test_valid_value('scroll-timeline-name', 'initial');
+test_valid_value('scroll-timeline-name', 'inherit');
+test_valid_value('scroll-timeline-name', 'unset');
+test_valid_value('scroll-timeline-name', 'revert');
+
+test_valid_value('scroll-timeline-name', 'none');
+test_valid_value('scroll-timeline-name', 'abc');
+test_valid_value('scroll-timeline-name', '  abc', 'abc');
+test_valid_value('scroll-timeline-name', 'aBc');
+test_valid_value('scroll-timeline-name', 'auto');
+
+test_invalid_value('scroll-timeline-name', 'default');
+test_invalid_value('scroll-timeline-name', '10px');
+test_invalid_value('scroll-timeline-name', 'foo bar');
+test_invalid_value('scroll-timeline-name', '"foo" "bar"');
+test_invalid_value('scroll-timeline-name', 'rgb(1, 2, 3)');
+test_invalid_value('scroll-timeline-name', '#fefefe');
+
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-name-tree-scoped.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-name-tree-scoped.html
new file mode 100644
index 0000000000..622a582cce
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-name-tree-scoped.html
@@ -0,0 +1,144 @@
+<!DOCTYPE html>
+<title>scroll-timelime-name and tree-scoped references</title>
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#scroll-timelines-named">
+<link rel="help" src="https://drafts.csswg.org/css-scoping-1/#shadow-names">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="/resources/declarative-shadow-dom-polyfill.js"></script>
+
+<main id=main></main>
+<script>
+  function inflate(t, template) {
+    t.add_cleanup(() => main.replaceChildren());
+    main.append(template.content.cloneNode(true));
+    main.offsetTop;
+  }
+
+  setup(() => {
+    polyfill_declarative_shadow_dom(document);
+  });
+</script>
+<style>
+  @keyframes anim {
+    from { z-index: 100; }
+    to { z-index: 100; }
+  }
+</style>
+
+<template id=scroll_timeline_host>
+  <style>
+    .target {
+      animation: anim 10s linear;
+      animation-timeline: timeline;
+    }
+    main > .scroller {
+      scroll-timeline: timeline horizontal;
+    }
+  </style>
+  <div class=scroller>
+    <div class=scroller>
+      <template shadowroot=open>
+        <style>
+          :host {
+            scroll-timeline: timeline vertical;
+          }
+        </style>
+        <slot></slot>
+      </template>
+      <div class=target></div>
+    </div>
+  </div>
+  <style>
+  </style>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, scroll_timeline_host);
+    let target = main.querySelector('.target');
+    assert_equals(target.getAnimations().length, 1);
+    let anim = target.getAnimations()[0];
+    assert_not_equals(anim.timeline, null);
+    assert_equals(anim.timeline.axis, 'horizontal');
+  }, 'Outer animation can not see scroll timeline defined by :host');
+</script>
+
+
+<template id=scroll_timeline_slotted>
+  <style>
+    .target {
+      animation: anim 10s linear;
+      animation-timeline: timeline;
+    }
+    .host {
+      scroll-timeline: timeline horizontal;
+    }
+  </style>
+  <div class=host>
+    <template shadowroot=open>
+      <style>
+        ::slotted(.scroller) {
+          scroll-timeline: timeline vertical;
+        }
+      </style>
+      <slot></slot>
+    </template>
+    <div class=scroller>
+      <div class=target></div>
+    </div>
+  </div>
+  <style>
+  </style>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, scroll_timeline_slotted);
+    let target = main.querySelector('.target');
+    assert_equals(target.getAnimations().length, 1);
+    let anim = target.getAnimations()[0];
+    assert_not_equals(anim.timeline, null);
+    assert_equals(anim.timeline.axis, 'horizontal');
+  }, 'Outer animation can not see scroll timeline defined by ::slotted');
+</script>
+
+
+<template id=scroll_timeline_part>
+  <style>
+    .host {
+      scroll-timeline: timeline vertical;
+    }
+    .host::part(foo) {
+      scroll-timeline: timeline horizontal;
+    }
+  </style>
+  <div class=host>
+    <template shadowroot=open>
+      <style>
+          /* Not using 'anim' at document scope, due to https://crbug.com/1334534 */
+          @keyframes anim2 {
+            from { z-index: 100; background-color: green; }
+            to { z-index: 100; background-color: green; }
+          }
+        .target {
+          animation: anim2 10s linear;
+          animation-timeline: timeline;
+        }
+      </style>
+      <div part=foo>
+        <div class=target></div>
+      </div>
+    </template>
+  </div>
+  <style>
+  </style>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, scroll_timeline_part);
+    let target = main.querySelector('.host').shadowRoot.querySelector('.target');
+    assert_equals(target.getAnimations().length, 1);
+    let anim = target.getAnimations()[0];
+    assert_not_equals(anim.timeline, null);
+    assert_equals(anim.timeline.axis, 'horizontal');
+  }, 'Inner animation can see scroll timeline defined by ::part');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-nearest-dirty.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-nearest-dirty.html
new file mode 100644
index 0000000000..1a79c9bb22
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-nearest-dirty.html
@@ -0,0 +1,42 @@
+<!DOCTYPE html>
+<title>Unrelated style mutation does not affect anonymous timeline</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-notation">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="/css/css-animations/support/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { z-index: 100; }
+    to { z-index: 100; }
+  }
+  #scroller {
+    overflow: auto;
+    width: 100px;
+    height: 100px;
+  }
+  #element {
+    animation: anim forwards;
+    animation-timeline: scroll();
+  }
+  #spacer {
+    height: 200px;
+  }
+</style>
+<div id=scroller>
+  <div id=element></div>
+  <div id=spacer></div>
+</div>
+
+<script>
+setup(assert_implements_animation_timeline);
+
+promise_test(async () => {
+  await waitForCSSScrollTimelineStyle();
+  assert_equals(getComputedStyle(element).zIndex, '100');
+  // Unrelated style mutation does not change the effect value:
+  element.style.color = 'green';
+  assert_equals(getComputedStyle(element).zIndex, '100');
+});
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-paused-animations.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-paused-animations.html
new file mode 100644
index 0000000000..54518a5e87
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-paused-animations.html
@@ -0,0 +1,95 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Scroll timeline with paused animations</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-notation">
+<link rel="help" href="https://drafts.csswg.org/css-animations/#animation-play-state">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="/css/css-animations/support/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { width: 100px; }
+    to { width: 200px; }
+  }
+
+  .fill-vh {
+    width: 100px;
+    height: 100vh;
+  }
+</style>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+setup(assert_implements_animation_timeline);
+
+async function resetScrollPosition() {
+  // Reset to 0 so we don't affect following tests.
+  document.scrollingElement.scrollTop = 0;
+  return waitForNextFrame();
+}
+
+promise_test(async t => {
+  const div = addDiv(t, { style: 'width: 50px; height: 100px;' });
+  const filling = addDiv(t, { class: 'fill-vh' });
+  const scroller = document.scrollingElement;
+  t.add_cleanup(resetScrollPosition);
+
+  div.style.animation = 'anim 100s linear paused';
+  div.style.animationTimeline = 'scroll(root)';
+  await waitForCSSScrollTimelineStyle();
+
+  const anim = div.getAnimations()[0];
+  await anim.ready;
+  assert_percents_equal(anim.currentTime, 0, 'timeline time reset');
+  assert_equals(getComputedStyle(div).width, '100px');
+
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = maxScroll;
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(div).width, '100px');
+
+}, 'Test that the scroll animation is paused');
+
+promise_test(async t => {
+  const div = addDiv(t, { style: 'width: 50px; height: 100px;' });
+  const filling = addDiv(t, { class: 'fill-vh' });
+  const scroller = document.scrollingElement;
+  await waitForNextFrame();
+
+  div.style.animation = 'anim 100s linear forwards';
+  div.style.animationTimeline = 'scroll(root)';
+  await waitForCSSScrollTimelineStyle();
+
+  const anim = div.getAnimations()[0];
+  await anim.ready;
+  assert_percents_equal(anim.currentTime, 0, 'timeline time reset');
+  assert_equals(getComputedStyle(div).width, '100px');
+
+  await waitForNextFrame();
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = maxScroll;
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(div).width, '200px');
+
+  div.style.animationPlayState = 'paused';
+  assert_equals(anim.playState, 'paused');
+  assert_equals(getComputedStyle(div).width, '200px',
+                'Current time preserved when pause-pending.');
+  assert_true(anim.pending,
+              'Pending state after changing animationPlayState');
+  await anim.ready;
+  assert_equals(getComputedStyle(div).width, '200px',
+                'Current time preserved when paused.');
+  assert_percents_equal(anim.timeline.currentTime, 100);
+  document.scrollingElement.scrollTop = 0;
+  await waitForNextFrame();
+  assert_percents_equal(anim.timeline.currentTime, 0);
+  assert_equals(getComputedStyle(div).width, '200px');
+}, 'Test that the scroll animation is paused by updating animation-play-state');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-responsiveness-from-endpoint.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-responsiveness-from-endpoint.html
new file mode 100644
index 0000000000..71d3699077
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-responsiveness-from-endpoint.html
@@ -0,0 +1,62 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Root-scrolling timeline with animation moving from end point</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-notation">
+<link rel="help" href="https://drafts.csswg.org/web-animations/#update-an-animations-finished-state">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/css-animations/support/testcommon.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+
+<style>
+  @keyframes anim {
+    from { width: 100px; }
+    to { width: 200px; }
+  }
+
+  .fill-vh {
+    width: 100px;
+    height: 100vh;
+  }
+</style>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+setup(assert_implements_animation_timeline);
+
+promise_test(async t => {
+  const div = addDiv(t, { style: 'width: 50px; height: 100px;' });
+  const filling = addDiv(t, { class: 'fill-vh' });
+  const scroller = document.scrollingElement;
+  scroller.scrollTop = 0;
+  await waitForNextFrame();
+
+  div.style.animation = 'anim 100s linear';
+  div.style.animationTimeline = 'scroll(root)';
+  await waitForCSSScrollTimelineStyle();
+
+  const anim = div.getAnimations()[0];
+  await anim.ready;
+  assert_percents_equal(anim.timeline.currentTime, 0,
+                        'Timeline time when animation is ready');
+  assert_equals(getComputedStyle(div).width, '100px',
+                'Width at animation start');
+
+  await waitForNextFrame();
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = maxScroll;
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(div).width, '200px',
+                'Width at scroll limit');
+
+  document.scrollingElement.scrollTop = 0;
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(div).width, '100px',
+                'Width after reset to scroll top');
+}, 'Test that the scroll animation is still responsive after moving from 100%');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-root-dirty.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-root-dirty.html
new file mode 100644
index 0000000000..1c0b73ab45
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-root-dirty.html
@@ -0,0 +1,35 @@
+<!DOCTYPE html>
+<title>Unrelated style mutation does not affect anonymous timeline (root)</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-notation">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="/css/css-animations/support/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { z-index: 100; }
+    to { z-index: 100; }
+  }
+  #element {
+    animation: anim forwards;
+    animation-timeline: scroll(root);
+  }
+  #spacer {
+    height: 200vh;
+  }
+</style>
+<div id=element></div>
+<div id=spacer></div>
+
+<script>
+setup(assert_implements_animation_timeline);
+
+promise_test(async () => {
+  await waitForCSSScrollTimelineStyle();
+  assert_equals(getComputedStyle(element).zIndex, '100');
+  // Unrelated style mutation does not change the effect value:
+  element.style.color = 'green';
+  assert_equals(getComputedStyle(element).zIndex, '100');
+});
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-sampling.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-sampling.html
new file mode 100644
index 0000000000..1fe354edfd
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-sampling.html
@@ -0,0 +1,46 @@
+<!DOCTYPE html>
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#avoiding-cycles">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<style>
+  #scroller {
+    overflow: hidden;
+    width: 100px;
+    height: 100px;
+    scroll-timeline: timeline;
+  }
+  #contents {
+    height: 200px;
+  }
+  @keyframes expand {
+    from { width: 100px; }
+    to { width: 200px; }
+  }
+  #element {
+    width: 0px;
+    animation: expand 10s linear;
+    animation-timeline: timeline;
+  }
+  /* Ensure stable expectations if feature is not supported */
+  @supports not (animation-timeline:foo) {
+    #element { animation-play-state: paused; }
+  }
+</style>
+<div id=scroller>
+  <div id=contents></div>
+</div>
+<div id=element></div>
+<script>
+  promise_test(async (t) => {
+    // The scroll timeline is initially inactive until the first frame.
+    assert_equals(getComputedStyle(element).width, '0px');
+    await waitForNextFrame();
+    scroller.scrollTop = 50;
+    // Scrolling position should not yet be reflected in the animation,
+    // since the new scroll position has not yet been sampled.
+    assert_equals(getComputedStyle(element).width, '100px');
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element).width, '150px');
+  }, 'Scroll position is sampled once per frame');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-shorthand.tentative.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-shorthand.tentative.html
new file mode 100644
index 0000000000..b340ff34ff
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-shorthand.tentative.html
@@ -0,0 +1,41 @@
+<!DOCTYPE html>
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/6674">
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/rewrite#scroll-timeline-axis">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/computed-testcommon.js"></script>
+<script src="/css/support/parsing-testcommon.js"></script>
+<script src="/css/support/shorthand-testcommon.js"></script>
+<div id="target"></div>
+<script>
+test_valid_value('scroll-timeline', 'block none', 'block');
+test_valid_value('scroll-timeline', 'none inline', 'inline');
+test_valid_value('scroll-timeline', 'horizontal abc');
+test_valid_value('scroll-timeline', 'inline abc');
+test_valid_value('scroll-timeline', 'inline aBc');
+test_valid_value('scroll-timeline', 'vertical none', 'vertical');
+test_valid_value('scroll-timeline', 'inline inline');
+test_valid_value('scroll-timeline', 'abc');
+test_valid_value('scroll-timeline', 'inline');
+
+test_invalid_value('scroll-timeline', '');
+test_invalid_value('scroll-timeline', 'abc abc');
+
+test_computed_value('scroll-timeline', 'block none', 'block');
+test_computed_value('scroll-timeline', 'abc inline', 'inline abc');
+test_computed_value('scroll-timeline', 'none vertical', 'vertical');
+test_computed_value('scroll-timeline', 'horizontal abc');
+test_computed_value('scroll-timeline', 'vertical vertical');
+test_computed_value('scroll-timeline', 'abc');
+
+test_shorthand_value('scroll-timeline', 'vertical abc',
+{
+  'scroll-timeline-axis': 'vertical',
+  'scroll-timeline-name': 'abc',
+});
+test_shorthand_value('scroll-timeline', 'horizontal inline',
+{
+  'scroll-timeline-axis': 'horizontal',
+  'scroll-timeline-name': 'inline',
+});
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-sibling-gcs.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-sibling-gcs.html
new file mode 100644
index 0000000000..6062e795f4
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-sibling-gcs.html
@@ -0,0 +1,52 @@
+<!DOCTYPE html>
+<title>scroll-timeline and container queries</title>
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#scroll-timeline-shorthand">
+<link rel="help" src="https://drafts.csswg.org/css-contain-3/#container-queries">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  #scroller {
+    overflow: auto;
+    width: auto;
+    height: 100px;
+  }
+  #scroller > div {
+    height: 200px;
+  }
+
+  @keyframes anim {
+    from { background-color: rgb(100, 100, 100); }
+    to { background-color: rgb(200, 200, 200); }
+  }
+  #element {
+    height: 10px;
+    width: 10px;
+    animation: anim 10s linear;
+    animation-timeline: timeline;
+    background-color: rgb(0, 0, 0);
+  }
+</style>
+<div>
+  <div id=scroller>
+    <div></div>
+  </div>
+  <div>
+    <div id=element></div>
+  </div>
+</div>
+<script>
+  setup(assert_implements_animation_timeline);
+
+  promise_test(async (t) => {
+    element.offsetTop;
+    scroller.scrollTop = 50;
+    await waitForNextFrame();
+    // Unknown timeline, time held at zero.
+    assert_equals(getComputedStyle(element).backgroundColor, 'rgb(100, 100, 100)');
+    scroller.style.scrollTimeline = 'timeline';
+    await waitForCSSScrollTimelineStyle();
+    assert_equals(getComputedStyle(element).backgroundColor, 'rgb(150, 150, 150)');
+  }, 'Timelines appearing on preceding siblings are visible to getComputedStyle');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/support/testcommon.js b/testing/web-platform/tests/scroll-animations/css/support/testcommon.js
new file mode 100644
index 0000000000..66bc27bb10
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/support/testcommon.js
@@ -0,0 +1,19 @@
+'use strict';
+
+/**
+ * Returns a Promise that is resolved after a CSS scroll timeline is created (as
+ * the result of a style change) and a snapshot has been taken, so that the
+ * animation style is correctly reflected by getComputedStyle().
+ * Technically, this only takes a full frame update. We implement this as two
+ * requestAnimationFrame callbacks because the result will be available at the
+ * beginning of the second frame.
+ */
+async function waitForCSSScrollTimelineStyle() {
+  await waitForNextFrame();
+  await waitForNextFrame();
+}
+
+function assert_implements_animation_timeline() {
+  assert_implements(CSS.supports('animation-timeline:foo'),
+      'animation-timeline not supported');
+}
diff --git a/testing/web-platform/tests/scroll-animations/css/timeline-range-name-offset-in-keyframes.tentative.html b/testing/web-platform/tests/scroll-animations/css/timeline-range-name-offset-in-keyframes.tentative.html
new file mode 100644
index 0000000000..993046c5f2
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/timeline-range-name-offset-in-keyframes.tentative.html
@@ -0,0 +1,94 @@
+<!DOCTYPE html>
+<html>
+<meta charset="utf-8">
+<title>Timeline offset in Animation Keyframes</title>
+<link rel="help" href="https://w3c.github.io/csswg-drafts/scroll-animations-1/#named-range-keyframes">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes fade-in-out-animation {
+    enter 0%, exit 100% { opacity: 0 }
+    enter 100%, exit 0% { opacity: 1 }
+  }
+
+  #subject {
+    background-color:  rgba(0, 0, 255);
+    height: 200px;
+    width: 200px;
+    view-timeline-name: foo;
+    animation: linear 1s both fade-in-out-animation;
+    animation-timeline: foo;
+  }
+
+  #container {
+    border:  5px solid black;
+    height:  400px;
+    width:  400px;
+    overflow-y:  scroll;
+    resize: both;
+  }
+
+  .spacer {
+    height: 600px;
+    width: 100%;
+  }
+</style>
+<body onload="runTests()">
+  <div id="container">
+    <div class="spacer"></div>
+    <div id="subject"></div>
+    <div class="spacer"></div>
+  </div>
+</body>
+
+<script type="text/javascript">
+  setup(assert_implements_animation_timeline);
+
+  function runTests() {
+    promise_test(async t => {
+      // scrollTop=200 to 400 is the enter range
+      container.scrollTop = 200;
+      await waitForNextFrame();
+      assert_equals(getComputedStyle(subject).opacity, '0', 'Effect at enter 0%');
+
+      container.scrollTop = 300;
+      await waitForNextFrame();
+      assert_equals(getComputedStyle(subject).opacity, '0.5', 'Effect at enter 50%');
+
+      container.scrollTop = 400;
+      await waitForNextFrame();
+      assert_equals(getComputedStyle(subject).opacity, '1', 'Effect at enter 100%');
+
+      // scrollTop=600-800 is the exit range
+      container.scrollTop = 600;
+      await waitForNextFrame();
+      assert_equals(getComputedStyle(subject).opacity, '1', 'Effect at exit 0%');
+
+      container.scrollTop = 700;
+      await waitForNextFrame();
+      assert_equals(getComputedStyle(subject).opacity, '0.5', 'Effect at exit 50%');
+
+      container.scrollTop = 800;
+      await waitForNextFrame();
+      assert_equals(getComputedStyle(subject).opacity, '0', 'Effect at exit 100%');
+
+      // First change scrollTop so that you are at enter 100%, then resize the container in a way
+      // that scrollTop is the same, but now the animation is at enter 50% and check opacity.
+      // After changing the height of container, scrollTop=300-500 is the enter range
+      container.scrollTop = 400;
+      await waitForNextFrame();
+      assert_equals(getComputedStyle(subject).opacity, '1', 'Effect at enter 100%');
+      container.style.height = '300px';
+      await waitForNextFrame();
+      assert_equals(getComputedStyle(subject).opacity, '0.5', 'Effect at enter 50%');
+
+      // After changing the height of container, scrollTop=600-800 is still the exit range
+      container.scrollTop = 700;
+      await waitForNextFrame();
+      assert_equals(getComputedStyle(subject).opacity, '0.5', 'Effect at exit 50%');
+    });
+  }
+</script>
+</html>
diff --git a/testing/web-platform/tests/scroll-animations/css/view-timeline-animation.html b/testing/web-platform/tests/scroll-animations/css/view-timeline-animation.html
new file mode 100644
index 0000000000..b816bb6897
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/view-timeline-animation.html
@@ -0,0 +1,219 @@
+<!DOCTYPE html>
+<title>Animations using view-timeline</title>
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#view-timelines-named">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { z-index: 0; }
+    to { z-index: 100; }
+  }
+  .vertical-scroller {
+    overflow: auto;
+    width: 100px;
+    height: 100px;
+  }
+  .vertical-scroller > div {
+    height: 50px;
+    z-index: -1;
+  }
+  .horizontal-scroller {
+    overflow: auto;
+    width: 100px;
+    height: 100px;
+    writing-mode: vertical-lr;
+  }
+  .horizontal-scroller > div {
+    width: 50px;
+    z-index: -1;
+  }
+</style>
+<main id=main></main>
+<script>
+  setup(assert_implements_animation_timeline);
+
+  function inflate(t, template) {
+    t.add_cleanup(() => main.replaceChildren());
+    main.append(template.content.cloneNode(true));
+  }
+  async function scrollTop(e, value) {
+    e.scrollTop = value;
+    await waitForNextFrame();
+  }
+  async function scrollLeft(e, value) {
+    e.scrollLeft = value;
+    await waitForNextFrame();
+  }
+</script>
+
+<template id=default_view_timeline>
+  <style>
+    #target {
+      view-timeline: t1;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=vertical-scroller>
+    <div></div>           <!-- [0px,   50px]  -->
+    <div></div>           <!-- [50px,  100px] -->
+    <div></div>           <!-- [100px, 150px] -->
+    <div id=target></div> <!-- [150px, 200px] -->
+    <div></div>           <!-- [200px, 250px] -->
+    <div></div>           <!-- [250px, 300px] -->
+    <div></div>           <!-- [300px, 350px] -->
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, default_view_timeline);
+    assert_equals(getComputedStyle(target).zIndex, '-1');
+    await scrollTop(scroller, 25);
+    assert_equals(getComputedStyle(target).zIndex, '-1');
+    await scrollTop(scroller, 50); // 0% (enter 0%)
+    assert_equals(getComputedStyle(target).zIndex, '0');
+    await scrollTop(scroller, 125); // 50%
+    assert_equals(getComputedStyle(target).zIndex, '50');
+    await scrollTop(scroller, 200); // 100% (exit 100%)
+    assert_equals(getComputedStyle(target).zIndex, '100');
+    await scrollTop(scroller, 225);
+    assert_equals(getComputedStyle(target).zIndex, '-1');
+  }, 'Default view-timeline');
+</script>
+
+<template id=horizontal_timeline>
+  <style>
+    #target {
+      view-timeline: t1 horizontal;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=horizontal-scroller>
+    <div></div>           <!-- [0px,   50px]  -->
+    <div></div>           <!-- [50px,  100px] -->
+    <div></div>           <!-- [100px, 150px] -->
+    <div id=target></div> <!-- [150px, 200px] -->
+    <div></div>           <!-- [200px, 250px] -->
+    <div></div>           <!-- [250px, 300px] -->
+    <div></div>           <!-- [300px, 350px] -->
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, horizontal_timeline);
+    assert_equals(getComputedStyle(target).zIndex, '-1');
+    await scrollLeft(scroller, 25);
+    assert_equals(getComputedStyle(target).zIndex, '-1');
+    await scrollLeft(scroller, 50); // 0% (enter 0%)
+    assert_equals(getComputedStyle(target).zIndex, '0');
+    await scrollLeft(scroller, 125); // 50%
+    assert_equals(getComputedStyle(target).zIndex, '50');
+    await scrollLeft(scroller, 200); // 100% (exit 100%)
+    assert_equals(getComputedStyle(target).zIndex, '100');
+    await scrollLeft(scroller, 225);
+    assert_equals(getComputedStyle(target).zIndex, '-1');
+  }, 'Horizontal view-timeline');
+</script>
+
+<template id=multiple_timelines>
+  <style>
+    #timelines {
+      view-timeline: tv vertical, th horizontal;
+      background-color: red;
+    }
+    #scroller {
+      width: 100px;
+      height: 100px;
+      overflow: hidden;
+      display: grid;
+      grid-template-columns: 50px 50px 50px 50px 50px 50px 50px;
+      grid-template-row: 50px 50px 50px 50px 50px 50px 50px;
+    }
+    #scroller > div {
+      z-index: -1;
+      width: 50px;
+      height: 50px;
+    }
+    #target_v {
+      animation: anim 1s linear;
+      animation-timeline: tv;
+    }
+    #target_h {
+      animation: anim 1s linear;
+      animation-timeline: th;
+    }
+  </style>
+  <div id=scroller>
+    <!-- Created dynamically -->
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, multiple_timelines);
+
+    // Create a 350px x 350px grid (7x7 items of 50x50px each), with the
+    // timelines at item [3,3], an element attached to the horizontal timeline
+    // at [4,3], and an element attached to the vertical timeline at [3,4].
+
+    // x  x  x  x  x  x  x
+    // x  x  x  x  x  x  x
+    // x  x  x  x  x  x  x
+    // x  x  x  T  H  x  x
+    // x  x  x  V  x  x  x
+    // x  x  x  x  x  x  x
+    // x  x  x  x  x  x  x
+    // x  x  x  x  x  x  x
+
+    let grid_size = 7;
+    for (let i = 0; i < (grid_size*grid_size); ++i) {
+      let div = document.createElement('div');
+      if (i == (3 * grid_size + 3))
+        div.id = 'timelines';
+      if (i == (3 * grid_size + 4))
+        div.id = 'target_h';
+      if (i == (4 * grid_size + 3))
+        div.id = 'target_v';
+      scroller.append(div);
+    }
+
+    assert_equals(getComputedStyle(target_v).zIndex, '-1');
+    assert_equals(getComputedStyle(target_h).zIndex, '-1');
+
+    // First scroll vertically.
+    await scrollTop(scroller, 25);
+    assert_equals(getComputedStyle(target_v).zIndex, '-1');
+    assert_equals(getComputedStyle(target_h).zIndex, '-1');
+    await scrollTop(scroller, 50); // 0% (enter 0%)
+    assert_equals(getComputedStyle(target_v).zIndex, '0');
+    assert_equals(getComputedStyle(target_h).zIndex, '-1');
+    await scrollTop(scroller, 125); // 50%
+    assert_equals(getComputedStyle(target_v).zIndex, '50');
+    assert_equals(getComputedStyle(target_h).zIndex, '-1');
+    await scrollTop(scroller, 200); // 100% (exit 100%)
+    assert_equals(getComputedStyle(target_v).zIndex, '100');
+    assert_equals(getComputedStyle(target_h).zIndex, '-1');
+    await scrollTop(scroller, 225);
+    assert_equals(getComputedStyle(target_v).zIndex, '-1');
+    assert_equals(getComputedStyle(target_h).zIndex, '-1');
+
+    // Then horizontally.
+    await scrollLeft(scroller, 25);
+    assert_equals(getComputedStyle(target_v).zIndex, '-1');
+    assert_equals(getComputedStyle(target_h).zIndex, '-1');
+    await scrollLeft(scroller, 50); // 0% (enter 0%)
+    assert_equals(getComputedStyle(target_v).zIndex, '-1');
+    assert_equals(getComputedStyle(target_h).zIndex, '0');
+    await scrollLeft(scroller, 125); // 50%
+    assert_equals(getComputedStyle(target_v).zIndex, '-1');
+    assert_equals(getComputedStyle(target_h).zIndex, '50');
+    await scrollLeft(scroller, 200); // 100% (exit 100%)
+    assert_equals(getComputedStyle(target_v).zIndex, '-1');
+    assert_equals(getComputedStyle(target_h).zIndex, '100');
+    await scrollLeft(scroller, 225);
+    assert_equals(getComputedStyle(target_v).zIndex, '-1');
+    assert_equals(getComputedStyle(target_h).zIndex, '-1');
+  }, 'Multiple view-timelines on the same element');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/view-timeline-axis-computed.html b/testing/web-platform/tests/scroll-animations/css/view-timeline-axis-computed.html
new file mode 100644
index 0000000000..f4649dab04
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/view-timeline-axis-computed.html
@@ -0,0 +1,37 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#view-timeline-axis">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/computed-testcommon.js"></script>
+</head>
+<style>
+  #outer { view-timeline-axis: block, inline; }
+  #target { view-timeline-axis: vertical; }
+</style>
+<div id=outer>
+  <div id=target></div>
+</div>
+<script>
+test_computed_value('view-timeline-axis', 'initial', 'block');
+test_computed_value('view-timeline-axis', 'inherit', 'block, inline');
+test_computed_value('view-timeline-axis', 'unset', 'block');
+test_computed_value('view-timeline-axis', 'revert', 'block');
+test_computed_value('view-timeline-axis', 'block');
+test_computed_value('view-timeline-axis', 'inline');
+test_computed_value('view-timeline-axis', 'vertical');
+test_computed_value('view-timeline-axis', 'horizontal');
+test_computed_value('view-timeline-axis', 'block, inline');
+test_computed_value('view-timeline-axis', 'inline, block');
+test_computed_value('view-timeline-axis', 'block, vertical, horizontal, inline');
+test_computed_value('view-timeline-axis', 'inline, inline, inline, inline');
+
+test(() => {
+  let style = getComputedStyle(document.getElementById('target'));
+  assert_not_equals(Array.from(style).indexOf('view-timeline-axis'), -1);
+}, 'The view-timeline-axis property shows up in CSSStyleDeclaration enumeration');
+
+test(() => {
+  let style = document.getElementById('target').style;
+  assert_not_equals(style.cssText.indexOf('view-timeline-axis'), -1);
+}, 'The view-timeline-axis property shows up in CSSStyleDeclaration.cssText');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/view-timeline-axis-parsing.html b/testing/web-platform/tests/scroll-animations/css/view-timeline-axis-parsing.html
new file mode 100644
index 0000000000..ffcc36c320
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/view-timeline-axis-parsing.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#view-timeline-axis">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/parsing-testcommon.js"></script>
+</head>
+<div id="target"></div>
+<script>
+test_valid_value('view-timeline-axis', 'initial');
+test_valid_value('view-timeline-axis', 'inherit');
+test_valid_value('view-timeline-axis', 'unset');
+test_valid_value('view-timeline-axis', 'revert');
+
+test_valid_value('view-timeline-axis', 'block');
+test_valid_value('view-timeline-axis', 'inline');
+test_valid_value('view-timeline-axis', 'vertical');
+test_valid_value('view-timeline-axis', 'horizontal');
+test_valid_value('view-timeline-axis', 'block, inline');
+test_valid_value('view-timeline-axis', 'inline, block');
+test_valid_value('view-timeline-axis', 'block, vertical, horizontal, inline');
+test_valid_value('view-timeline-axis', 'inline, inline, inline, inline');
+
+test_invalid_value('view-timeline-axis', 'abc');
+test_invalid_value('view-timeline-axis', '10px');
+test_invalid_value('view-timeline-axis', 'auto');
+test_invalid_value('view-timeline-axis', 'none');
+test_invalid_value('view-timeline-axis', 'block inline');
+test_invalid_value('view-timeline-axis', 'block / inline');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/view-timeline-delay-animation.html b/testing/web-platform/tests/scroll-animations/css/view-timeline-delay-animation.html
new file mode 100644
index 0000000000..dfb0e59f5d
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/view-timeline-delay-animation.html
@@ -0,0 +1,144 @@
+<!DOCTYPE html>
+<title>Animations using named timeline ranges</title>
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#named-timeline-range">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { z-index: 0; background-color: skyblue;}
+    to { z-index: 100; background-color: coral; }
+  }
+  #scroller {
+    border:  10px solid lightgray;
+    overflow-y: scroll;
+    width: 200px;
+    height: 200px;
+  }
+  #target {
+    margin: 800px 0px;
+    width: 100px;
+    height: 100px;
+    z-index: -1;
+    background-color: green;
+  }
+</style>
+<main id=main>
+</main>
+<template>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  setup(assert_implements_animation_timeline);
+
+  function inflate(t, template) {
+    t.add_cleanup(() => main.replaceChildren());
+    main.append(template.content.cloneNode(true));
+  }
+  async function scrollTop(e, value) {
+    e.scrollTop = value;
+    await waitForNextFrame();
+  }
+  async function waitForAnimationReady(target) {
+    await waitForNextFrame();
+    await Promise.all(target.getAnimations().map(x => x.promise));
+  }
+  async function assertValueAt(scroller, target, args) {
+    await waitForAnimationReady(target);
+    await scrollTop(scroller, args.scrollTop);
+    assert_equals(getComputedStyle(target).zIndex, args.expected.toString());
+  }
+  function test_animation_delay(options) {
+    promise_test(async (t) => {
+      inflate(t, document.querySelector('template'));
+      let scroller = main.querySelector('#scroller');
+      let target = main.querySelector('#target');
+
+      target.style.viewTimeline = 't1 block';
+      // TODO(crbug.com/1375998): Create the timeline in a separate frame to
+      // work around a bug.
+      await waitForNextFrame();
+
+      target.style.animation = 'anim 10s linear';
+      target.style.animationTimeline = 't1';
+      target.style.animationDelayStart = options.startDelay;
+      target.style.animationDelayEnd = options.endDelay;
+
+      // Accommodates floating point precision errors at the endpoints.
+      target.style.animationFillMode = 'both';
+
+      // 0%
+      await assertValueAt(scroller, target,
+          { scrollTop: options.rangeStart, expected: 0 });
+      // 50%
+      await assertValueAt(scroller, target,
+          { scrollTop: (options.rangeStart + options.rangeEnd) / 2, expected: 50 });
+      // 100%
+      await assertValueAt(scroller, target,
+          { scrollTop: options.rangeEnd, expected: 100 });
+
+      // Test before/after phases (need to clear the fill mode for that).
+      target.style.animationFillMode = 'initial';
+      await assertValueAt(scroller, target,
+          { scrollTop: options.rangeStart - 10, expected: -1 });
+      await assertValueAt(scroller, target,
+          { scrollTop: options.rangeEnd + 10, expected: -1 });
+      // Check 50% again without fill mode.
+      await assertValueAt(scroller, target,
+          { scrollTop: (options.rangeStart + options.rangeEnd) / 2, expected: 50 });
+
+    }, `Animation with delays [${options.startDelay}, ${options.endDelay}]`);
+  }
+
+  test_animation_delay({
+    startDelay: 'initial',
+    endDelay: 'initial',
+    rangeStart: 600,
+    rangeEnd: 900
+  });
+
+  test_animation_delay({
+    startDelay: 'cover 0%',
+    endDelay: 'cover 100%',
+    rangeStart: 600,
+    rangeEnd: 900
+  });
+
+  test_animation_delay({
+    startDelay: 'contain 0%',
+    endDelay: 'contain 100%',
+    rangeStart: 700,
+    rangeEnd: 800
+  });
+
+  test_animation_delay({
+    startDelay: 'enter 0%',
+    endDelay: 'enter 100%',
+    rangeStart: 600,
+    rangeEnd: 700
+  });
+
+  test_animation_delay({
+    startDelay: 'exit 0%',
+    endDelay: 'exit 100%',
+    rangeStart: 800,
+    rangeEnd: 900
+  });
+
+  test_animation_delay({
+    startDelay: 'contain -50%',
+    endDelay: 'enter 200%',
+    rangeStart: 650,
+    rangeEnd: 800
+  });
+
+  test_animation_delay({
+    startDelay: 'enter 0%',
+    endDelay: 'exit 100%',
+    rangeStart: 600,
+    rangeEnd: 900
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/view-timeline-dynamic.html b/testing/web-platform/tests/scroll-animations/css/view-timeline-dynamic.html
new file mode 100644
index 0000000000..74da8850d4
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/view-timeline-dynamic.html
@@ -0,0 +1,183 @@
+<!DOCTYPE html>
+<title>Changes to view-timeline are reflected in dependent elements</title>
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#view-timeline-shorthand">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { z-index: 0; }
+    to { z-index: 100; }
+  }
+  .scroller {
+    overflow: hidden;
+    width: 100px;
+    height: 100px;
+  }
+  .scroller > div {
+    height: 100px;
+  }
+  #target {
+    height: 0px;
+    z-index: -1;
+  }
+</style>
+<main id=main></main>
+<script>
+  setup(assert_implements_animation_timeline);
+
+  function inflate(t, template) {
+    t.add_cleanup(() => main.replaceChildren());
+    main.append(template.content.cloneNode(true));
+    main.offsetTop;
+  }
+  async function scrollTop(e, value) {
+    e.scrollTop = value;
+    await waitForNextFrame();
+  }
+  async function scrollLeft(e, value) {
+    e.scrollLeft = value;
+    await waitForNextFrame();
+  }
+</script>
+
+<template id=dynamic_view_timeline_name>
+  <style>
+    .timeline {
+      view-timeline-name: t1;
+    }
+    #target {
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=scroller>
+    <div id=div75></div>
+    <div id=div25></div>
+    <div id=div_before></div>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, dynamic_view_timeline_name);
+
+    await scrollTop(scroller, 50);
+
+    // scrollTop=50 is 75% for div75.
+    div75.classList.add('timeline');
+    await waitForCSSScrollTimelineStyle();
+    assert_equals(getComputedStyle(target).zIndex, '75');
+
+    // scrollTop=50 is 25% for div25.
+    div25.classList.add('timeline');
+    await waitForCSSScrollTimelineStyle();
+    assert_equals(getComputedStyle(target).zIndex, '25');
+
+    // scrollTop=50 is before the timeline start for div_before.
+    div_before.classList.add('timeline');
+    await waitForCSSScrollTimelineStyle();
+    assert_equals(getComputedStyle(target).zIndex, '-1');
+    // Scroll to 25% (for div_before) to verify that we're linked to that
+    // timeline.
+    await scrollTop(scroller, 150);
+    assert_equals(getComputedStyle(target).zIndex, '25');
+
+    // Now we should be back to div25's timeline, although with the new
+    // scrollTop=150, it's actually at 75%.
+    div_before.classList.remove('timeline');
+    await waitForCSSScrollTimelineStyle();
+    assert_equals(getComputedStyle(target).zIndex, '75');
+  }, 'Dynamically changing view-timeline-name');
+</script>
+
+<template id=dynamic_view_timeline_axis>
+  <style>
+    #timeline {
+      width: 100px;
+      height: 100px;
+      margin: 100px;
+      view-timeline-name: t1;
+    }
+    #target {
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=scroller>
+    <div id=timeline style="background: red;"></div>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, dynamic_view_timeline_axis);
+
+    await scrollTop(scroller, 50); // 25% (vertical)
+    await scrollLeft(scroller, 20); // 10% (horizontal)
+
+    assert_equals(getComputedStyle(target).zIndex, '25');
+    timeline.style.viewTimelineAxis = 'horizontal';
+    await waitForCSSScrollTimelineStyle();
+    assert_equals(getComputedStyle(target).zIndex, '10');
+  }, 'Dynamically changing view-timeline-axis');
+</script>
+
+<template id=dynamic_view_timeline_inset>
+  <style>
+    #timeline {
+      width: 100px;
+      height: 100px;
+      margin: 100px;
+      view-timeline-name: t1;
+    }
+    #target {
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=scroller>
+    <div id=timeline style="background: red;"></div>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, dynamic_view_timeline_inset);
+
+    await scrollTop(scroller, 50); // 25% (without inset).
+
+    assert_equals(getComputedStyle(target).zIndex, '25');
+    timeline.style.viewTimelineInset = '0px 50px';
+    await waitForCSSScrollTimelineStyle();
+    assert_equals(getComputedStyle(target).zIndex, '0');
+  }, 'Dynamically changing view-timeline-inset');
+</script>
+
+<template id=timeline_display_none>
+  <style>
+    #timeline {
+      view-timeline-name: t1;
+    }
+    #target {
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=scroller>
+    <div></div>
+    <div id=timeline></div>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, timeline_display_none);
+
+    await scrollTop(scroller, 50);
+    assert_equals(getComputedStyle(target).zIndex, '25');
+    timeline.style.display = 'none';
+    assert_equals(getComputedStyle(target).zIndex, '-1');
+  }, 'Element with view-timeline becoming display:none');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/view-timeline-inset-animation.html b/testing/web-platform/tests/scroll-animations/css/view-timeline-inset-animation.html
new file mode 100644
index 0000000000..a95086b62b
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/view-timeline-inset-animation.html
@@ -0,0 +1,768 @@
+<!DOCTYPE html>
+<title>Animations using view-timeline-inset</title>
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#propdef-view-timeline-inset">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { z-index: 0; }
+    to { z-index: 100; }
+  }
+  #scroller {
+    overflow: hidden;
+    width: 80px;
+    height: 100px;
+  }
+  #target {
+    margin: 150px;
+    width: 50px;
+    height: 50px;
+    z-index: -1;
+  }
+</style>
+<main id=main></main>
+<script>
+  setup(assert_implements_animation_timeline);
+
+  function inflate(t, template) {
+    t.add_cleanup(() => main.replaceChildren());
+    main.append(template.content.cloneNode(true));
+  }
+  async function scrollTop(e, value) {
+    e.scrollTop = value;
+    await waitForNextFrame();
+  }
+  async function scrollLeft(e, value) {
+    e.scrollLeft = value;
+    await waitForNextFrame();
+  }
+  async function assertValueAt(scroller, target, args) {
+    if (args.scrollTop !== undefined)
+      await scrollTop(scroller, args.scrollTop);
+    if (args.scrollLeft !== undefined)
+      await scrollLeft(scroller, args.scrollLeft);
+    assert_equals(getComputedStyle(target).zIndex, args.expected.toString());
+  }
+</script>
+
+<!--
+  Explanation of scroll positions
+  ===============================
+
+  Please note the following:
+
+   - The scroller has a width x height of 80x100px.
+   - The content is 50x50px with a 150px margin on all sides.
+     In other words, the size of the scroller content is 200x200px.
+
+  This means that, for vertical direction scrolling, assuming no insets:
+
+   - The start offset is 50px (scroller height + 50px is 150px, which consumes
+     exactly the margin of the content).
+   - The end offset is 200px (this is where the bottom edge of the scroller has
+     just cleared the content).
+   - The halfway point is (50px + 200px) / 2 = 125px.
+
+  For horizontal direction scrolling, assuming no insets:
+
+   - The start offset is 70px (scroller width + 70px is 150px, which consumes
+     exactly the margin of the content).
+   - The end offset is 200px (this is where the left edge of the scroller has
+     just cleared the content).
+   - The halfway point is (70px + 200px) / 2 = 135px.
+
+  The start and end insets will adjust the start and end offsets accordingly,
+  and the expectations in this file explicitly write out those adjustments.
+  For example, if the start offset is normally 50px, but there's an inset of
+  10px, we'll expect 50px + 10px rather than 60px.
+
+  Halfway-point expectations write out the adjustment from the "normal"
+  halfway-point, e.g. for start-inset:10px and end-inset:20px, we expect
+  "125px + 5px" since (20-10)/2 == 5.
+
+  Finally, note that for right-to-left and bottom-to-top scrolling directions
+  scroll offsets go the in the negative direction. This is why some expectations
+  negate all the offsets.
+-->
+
+<template id=test_one_value>
+  <style>
+    #target {
+      view-timeline: t1;
+      view-timeline-inset: 10px;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=vertical>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_one_value);
+    await assertValueAt(scroller, target, { scrollTop:50, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:50 + 10, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:125 + 0, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:200, expected:-1 });
+  }, 'view-timeline-inset with one value');
+</script>
+
+<template id=test_two_values>
+  <style>
+    #target {
+      view-timeline: t1;
+      view-timeline-inset: 10px 20px;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=vertical>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_two_values);
+    await assertValueAt(scroller, target, { scrollTop:50, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:50 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:125 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:200, expected:-1 });
+  }, 'view-timeline-inset with two values');
+</script>
+
+<template id=test_em_values>
+  <style>
+    #target {
+      font-size: 10px;
+      view-timeline: t1;
+      view-timeline-inset: 10px 2em;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=vertical>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_em_values);
+    await assertValueAt(scroller, target, { scrollTop:50, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:50 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:125 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:200, expected:-1 });
+  }, 'view-timeline-inset with em values');
+</script>
+
+<template id=test_percentage_values>
+  <style>
+    #target {
+      font-size: 10px;
+      view-timeline: t1;
+      view-timeline-inset: calc(5px + max(1%, 5%)) 20%;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=vertical>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_percentage_values);
+    await assertValueAt(scroller, target, { scrollTop:50, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:50 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:125 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:200, expected:-1 });
+  }, 'view-timeline-inset with percentage values');
+</script>
+
+<template id=test_outset>
+  <style>
+    #target {
+      view-timeline: t1;
+      view-timeline-inset: -10px -20px;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=vertical>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_outset);
+    await assertValueAt(scroller, target, { scrollTop:20, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:50 - 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:125 - 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:200 + 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:220, expected:-1 });
+  }, 'view-timeline-inset with negative values');
+</script>
+
+<template id=test_horizontal>
+  <style>
+    #target {
+      view-timeline: t1 horizontal;
+      view-timeline-inset: 10px 20px;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_horizontal);
+    await assertValueAt(scroller, target, { scrollLeft:20, expected:-1 });
+    await assertValueAt(scroller, target, { scrollLeft:70 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollLeft:135 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollLeft:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollLeft:200, expected:-1 });
+  }, 'view-timeline-inset with horizontal scroller');
+</script>
+
+<template id=test_block>
+  <style>
+    #target {
+      view-timeline: t1 block;
+      view-timeline-inset: 10px 20px;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_block);
+    await assertValueAt(scroller, target, { scrollTop:50, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:50 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:125 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:200, expected:-1 });
+  }, 'view-timeline-inset with block scroller');
+</script>
+
+<template id=test_inline>
+  <style>
+    #target {
+      view-timeline: t1 inline;
+      view-timeline-inset: 10px 20px;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_inline);
+    await assertValueAt(scroller, target, { scrollLeft:20, expected:-1 });
+    await assertValueAt(scroller, target, { scrollLeft:70 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollLeft:135 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollLeft:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollLeft:200, expected:-1 });
+  }, 'view-timeline-inset with inline scroller');
+</script>
+
+<template id=test_auto_block>
+  <style>
+    #scroller {
+      scroll-padding-block: 10px 20px;
+    }
+    #target {
+      view-timeline: t1 block;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_block);
+    await assertValueAt(scroller, target, { scrollTop:50, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:50 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:125 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:200, expected:-1 });
+  }, 'view-timeline-inset:auto, block');
+</script>
+
+<template id=test_auto_block_vertical_lr>
+  <style>
+    #scroller {
+      scroll-padding-block: 10px 20px;
+      writing-mode: vertical-lr;
+    }
+    #target {
+      view-timeline: t1 block;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_block_vertical_lr);
+    await assertValueAt(scroller, target, { scrollLeft:20, expected:-1 });
+    await assertValueAt(scroller, target, { scrollLeft:70 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollLeft:135 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollLeft:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollLeft:200, expected:-1 });
+  }, 'view-timeline-inset:auto, block, vertical-lr');
+</script>
+
+<template id=test_auto_block_vertical_rl>
+  <style>
+    #scroller {
+      scroll-padding-block: 10px 20px;
+      writing-mode: vertical-rl;
+    }
+    #target {
+      view-timeline: t1 block;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_block_vertical_rl);
+    // Note: this represents horizontal scrolling from right to left.
+    await assertValueAt(scroller, target, { scrollLeft:-20, expected:-1 });
+    await assertValueAt(scroller, target, { scrollLeft:-(70 + 20), expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollLeft:-(135 + 5), expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollLeft:-(200 - 10), expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollLeft:-200, expected:-1 });
+  }, 'view-timeline-inset:auto, block, vertical-rl');
+</script>
+
+<template id=test_auto_inline>
+  <style>
+    #scroller {
+      scroll-padding-inline: 10px 20px;
+    }
+    #target {
+      view-timeline: t1 inline;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_inline);
+    await assertValueAt(scroller, target, { scrollLeft:20, expected:-1 });
+    await assertValueAt(scroller, target, { scrollLeft:70 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollLeft:135 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollLeft:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollLeft:200, expected:-1 });
+  }, 'view-timeline-inset:auto, inline');
+</script>
+
+<template id=test_auto_inline_vertical_rl>
+  <style>
+    #scroller {
+      scroll-padding-inline: 10px 20px;
+      writing-mode: vertical-rl;
+    }
+    #target {
+      view-timeline: t1 inline;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_inline_vertical_rl);
+    await assertValueAt(scroller, target, { scrollTop:50, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:50 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:125 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:200, expected:-1 });
+  }, 'view-timeline-inset:auto, inline, vertical-rl');
+</script>
+
+<template id=test_auto_inline_vertical_lr>
+  <style>
+    #scroller {
+      scroll-padding-inline: 10px 20px;
+      writing-mode: vertical-lr;
+    }
+    #target {
+      view-timeline: t1 inline;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_inline_vertical_lr);
+    await assertValueAt(scroller, target, { scrollTop:50, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:50 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:125 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:200, expected:-1 });
+  }, 'view-timeline-inset:auto, inline, vertical-lr');
+</script>
+
+<template id=test_auto_inline_rtl>
+  <style>
+    #scroller {
+      scroll-padding-inline: 10px 20px;
+      direction: rtl;
+    }
+    #target {
+      view-timeline: t1 inline;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_inline_rtl);
+    await assertValueAt(scroller, target, { scrollLeft:-20, expected:-1 });
+    await assertValueAt(scroller, target, { scrollLeft:-(70 + 20), expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollLeft:-(135 + 5), expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollLeft:-(200 - 10), expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollLeft:-200, expected:-1 });
+  }, 'view-timeline-inset:auto, inline, rtl');
+</script>
+
+<template id=test_auto_inline_vertical_rl_rtl>
+  <style>
+    #scroller {
+      scroll-padding-inline: 10px 20px;
+      writing-mode: vertical-rl;
+      direction: rtl;
+    }
+    #target {
+      view-timeline: t1 inline;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_inline_vertical_rl_rtl);
+    await assertValueAt(scroller, target, { scrollTop:-50, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:-(50 + 20), expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:-(125 + 5), expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:-(200 - 10), expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:-200, expected:-1 });
+  }, 'view-timeline-inset:auto, inline, vertical-rl, rtl');
+</script>
+
+<template id=test_auto_inline_vertical_lr_rtl>
+  <style>
+    #scroller {
+      scroll-padding-inline: 10px 20px;
+      writing-mode: vertical-lr;
+      direction: rtl;
+    }
+    #target {
+      view-timeline: t1 inline;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_inline_vertical_lr_rtl);
+    await assertValueAt(scroller, target, { scrollTop:-50, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:-(50 + 20), expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:-(125 + 5), expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:-(200 - 10), expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:-200, expected:-1 });
+  }, 'view-timeline-inset:auto, inline, vertical-lr, rtl');
+</script>
+
+<template id=test_auto_vertical>
+  <style>
+    #scroller {
+      scroll-padding-block: 10px 20px;
+    }
+    #target {
+      view-timeline: t1 vertical;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_vertical);
+    await assertValueAt(scroller, target, { scrollTop:50, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:50 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:125 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:200, expected:-1 });
+  }, 'view-timeline-inset:auto, vertical');
+</script>
+
+<template id=test_auto_vertical_vertical_rl>
+  <style>
+    #scroller {
+      scroll-padding-inline: 10px 20px;
+      writing-mode: vertical-rl;
+    }
+    #target {
+      view-timeline: t1 vertical;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_vertical_vertical_rl);
+    await assertValueAt(scroller, target, { scrollTop:50, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:50 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:125 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:200, expected:-1 });
+  }, 'view-timeline-inset:auto, vertical, vertical-rl');
+</script>
+
+<template id=test_auto_vertical_vertical_rl_rtl>
+  <style>
+    #scroller {
+      scroll-padding-inline: 10px 20px;
+      writing-mode: vertical-rl;
+      direction: rtl;
+    }
+    #target {
+      view-timeline: t1 vertical;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_vertical_vertical_rl_rtl);
+    await assertValueAt(scroller, target, { scrollTop:-50, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:-(50 + 20), expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:-(125 + 5), expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:-(200 - 10), expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:-200, expected:-1 });
+  }, 'view-timeline-inset:auto, vertical, vertical-rl, rtl');
+</script>
+
+<template id=test_auto_horizontal>
+  <style>
+    #scroller {
+      scroll-padding-inline: 10px 20px;
+    }
+    #target {
+      view-timeline: t1 horizontal;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_horizontal);
+    await assertValueAt(scroller, target, { scrollLeft:20, expected:-1 });
+    await assertValueAt(scroller, target, { scrollLeft:70 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollLeft:135 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollLeft:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollLeft:200, expected:-1 });
+  }, 'view-timeline-inset:auto, horizontal');
+</script>
+
+<template id=test_auto_horizontal_rtl>
+  <style>
+    #scroller {
+      scroll-padding-inline: 10px 20px;
+      direction: rtl;
+    }
+    #target {
+      view-timeline: t1 horizontal;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_horizontal_rtl);
+    await assertValueAt(scroller, target, { scrollLeft:-20, expected:-1 });
+    await assertValueAt(scroller, target, { scrollLeft:-(70 + 20), expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollLeft:-(135 + 5), expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollLeft:-(200 - 10), expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollLeft:-200, expected:-1 });
+  }, 'view-timeline-inset:auto, horizontal, rtl');
+</script>
+
+<template id=test_auto_horizontal_vertical_lr>
+  <style>
+    #scroller {
+      scroll-padding-block: 10px 20px;
+      writing-mode: vertical-lr;
+    }
+    #target {
+      view-timeline: t1 horizontal;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_horizontal_vertical_lr);
+    await assertValueAt(scroller, target, { scrollLeft:20, expected:-1 });
+    await assertValueAt(scroller, target, { scrollLeft:70 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollLeft:135 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollLeft:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollLeft:200, expected:-1 });
+  }, 'view-timeline-inset:auto, horizontal, vertical-lr');
+</script>
+
+<template id=test_auto_horizontal_vertical_rl>
+  <style>
+    #scroller {
+      scroll-padding-block: 10px 20px;
+      writing-mode: vertical-rl;
+    }
+    #target {
+      view-timeline: t1 horizontal;
+      view-timeline-inset: auto auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_horizontal_vertical_rl);
+    await assertValueAt(scroller, target, { scrollLeft:-20, expected:-1 });
+    await assertValueAt(scroller, target, { scrollLeft:-(70 + 20), expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollLeft:-(135 + 5), expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollLeft:-(200 - 10), expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollLeft:-200, expected:-1 });
+  }, 'view-timeline-inset:auto, horizontal, vertical-rl');
+</script>
+
+<template id=test_auto_mix>
+  <style>
+    #scroller {
+      font-size: 10px;
+      scroll-padding-block: 50px calc(10% + 1em);
+    }
+    #target {
+      view-timeline: t1;
+      view-timeline-inset: 10% auto;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, test_auto_mix);
+    // Note: 10% of scroller height 100px is 10px, and 1em with font-size:10px
+    // is also 10px. Hence we expect the end inset specified as calc(10% + 1em)
+    // to be 20px.
+    await assertValueAt(scroller, target, { scrollTop:50, expected:-1 });
+    await assertValueAt(scroller, target, { scrollTop:50 + 20, expected:0 }); // 0%
+    await assertValueAt(scroller, target, { scrollTop:125 + 5, expected:50 }); // 50%
+    await assertValueAt(scroller, target, { scrollTop:200 - 10, expected:100 }); // 100%
+    await assertValueAt(scroller, target, { scrollTop:200, expected:-1 });
+  }, 'view-timeline-inset:auto, mix');
+</script>
+
+<!--
+  TODO: How to test view-timeline:auto + scroll-padding:auto? The UA may
+  in theory use any value in that case.
+
+  https://drafts.csswg.org/css-scroll-snap-1/#valdef-scroll-padding-auto
+-->
diff --git a/testing/web-platform/tests/scroll-animations/css/view-timeline-inset-computed.html b/testing/web-platform/tests/scroll-animations/css/view-timeline-inset-computed.html
new file mode 100644
index 0000000000..77683a4678
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/view-timeline-inset-computed.html
@@ -0,0 +1,41 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#view-timeline-inset">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/7243">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/computed-testcommon.js"></script>
+<style>
+  #outer { font-size:10px; }
+  #outer { view-timeline-inset: 1px 2px, auto 3px; }
+  #target { view-timeline-inset: 42px; }
+</style>
+<div id=outer>
+  <div id=target></div>
+</div>
+<script>
+test_computed_value('view-timeline-inset', 'initial', '0px');
+test_computed_value('view-timeline-inset', 'inherit', '1px 2px, auto 3px');
+test_computed_value('view-timeline-inset', 'unset', '0px');
+test_computed_value('view-timeline-inset', 'revert', '0px');
+test_computed_value('view-timeline-inset', '1px');
+test_computed_value('view-timeline-inset', '1%');
+test_computed_value('view-timeline-inset', 'calc(1% + 1px)');
+test_computed_value('view-timeline-inset', '1px 2px');
+test_computed_value('view-timeline-inset', '1px 2em', '1px 20px');
+test_computed_value('view-timeline-inset', 'calc(1px + 1em) 2px', '11px 2px');
+test_computed_value('view-timeline-inset', '1px 2px, 3px 4px');
+test_computed_value('view-timeline-inset', '1px auto, auto 4px');
+test_computed_value('view-timeline-inset', '1px, 2px, 3px');
+test_computed_value('view-timeline-inset', '1px 1px, 2px 3px', '1px, 2px 3px');
+test_computed_value('view-timeline-inset', 'auto auto, auto auto', 'auto, auto');
+
+test(() => {
+  let style = getComputedStyle(document.getElementById('target'));
+  assert_not_equals(Array.from(style).indexOf('view-timeline-inset'), -1);
+}, 'The view-timeline-inset property shows up in CSSStyleDeclaration enumeration');
+
+test(() => {
+  let style = document.getElementById('target').style;
+  assert_not_equals(style.cssText.indexOf('view-timeline-inset'), -1);
+}, 'The view-timeline-inset property shows up in CSSStyleDeclaration.cssText');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/view-timeline-inset-parsing.html b/testing/web-platform/tests/scroll-animations/css/view-timeline-inset-parsing.html
new file mode 100644
index 0000000000..d502b13593
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/view-timeline-inset-parsing.html
@@ -0,0 +1,34 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#view-timeline-inset">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/7243">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/parsing-testcommon.js"></script>
+<div id="target"></div>
+<script>
+test_valid_value('view-timeline-inset', 'initial');
+test_valid_value('view-timeline-inset', 'inherit');
+test_valid_value('view-timeline-inset', 'unset');
+test_valid_value('view-timeline-inset', 'revert');
+
+test_valid_value('view-timeline-inset', '1px');
+test_valid_value('view-timeline-inset', '1px 2px');
+test_valid_value('view-timeline-inset', '1px 2em');
+test_valid_value('view-timeline-inset', 'calc(1em + 1px) 2px');
+test_valid_value('view-timeline-inset', '1px 2px, 3px 4px');
+test_valid_value('view-timeline-inset', '1px auto, auto 4px');
+test_valid_value('view-timeline-inset', '1px, 2px, 3px');
+test_valid_value('view-timeline-inset', '1px 1px, 2px 3px', '1px, 2px 3px');
+test_valid_value('view-timeline-inset', 'auto auto, auto auto', 'auto, auto');
+
+test_invalid_value('view-timeline-inset', 'none');
+test_invalid_value('view-timeline-inset', 'foo bar');
+test_invalid_value('view-timeline-inset', '"foo" "bar"');
+test_invalid_value('view-timeline-inset', 'rgb(1, 2, 3)');
+test_invalid_value('view-timeline-inset', '#fefefe');
+test_invalid_value('view-timeline-inset', '1px 2px 3px');
+test_invalid_value('view-timeline-inset', '1px 2px auto');
+test_invalid_value('view-timeline-inset', 'auto 2px 3px');
+test_invalid_value('view-timeline-inset', 'auto auto auto');
+test_invalid_value('view-timeline-inset', '1px / 2px');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/view-timeline-lookup.html b/testing/web-platform/tests/scroll-animations/css/view-timeline-lookup.html
new file mode 100644
index 0000000000..c1797c7ba1
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/view-timeline-lookup.html
@@ -0,0 +1,301 @@
+<!DOCTYPE html>
+<title>Named view-timeline lookup</title>
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#view-timelines-named">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { z-index: 0; }
+    to { z-index: 100; }
+  }
+  .scroller {
+    overflow: auto;
+    width: 100px;
+    height: 100px;
+  }
+  .scroller > div {
+    height: 25px;
+    z-index: -1;
+  }
+</style>
+<main id=main></main>
+<script>
+  function inflate(t, template) {
+    t.add_cleanup(() => main.replaceChildren());
+    main.append(template.content.cloneNode(true));
+    main.offsetTop;
+  }
+</script>
+
+<template id=timeline_self>
+  <style>
+    #target {
+      height: 0px;
+      view-timeline: t1;
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=scroller>
+    <div id=target></div>
+    <div></div>
+    <div></div>
+    <div></div>
+    <div></div>
+    <div></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, timeline_self);
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(target).zIndex, '100');
+  }, 'view-timeline on self');
+</script>
+
+<template id=timeline_preceding_sibling>
+  <style>
+    #timeline {
+      height: 0px;
+      view-timeline: t1;
+    }
+    #target {
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=scroller>
+    <div></div>
+    <div id=timeline></div>
+    <div></div>
+    <div></div>
+    <div id=target></div>
+    <div></div>
+    <div></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, timeline_preceding_sibling);
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(target).zIndex, '75');
+  }, 'view-timeline on preceding sibling');
+</script>
+
+<template id=timeline_ancestor>
+  <style>
+    #timeline {
+      height: 0px;
+      view-timeline: t1;
+    }
+    #target {
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=scroller>
+    <div></div>
+    <div></div>
+    <div></div>
+    <div id=timeline>
+      <div>
+        <div id=target></div>
+      </div>
+    </div>
+    <div></div>
+    <div></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, timeline_ancestor);
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(target).zIndex, '25');
+  }, 'view-timeline on ancestor');
+</script>
+
+<template id=timeline_ancestor_sibling>
+  <style>
+    #timeline {
+      height: 0px;
+      view-timeline: t1;
+    }
+    #target {
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=scroller>
+    <div></div>
+    <div id=timeline></div>
+    <div></div>
+    <div>
+      <div>
+        <div id=target></div>
+      </div>
+    </div>
+    <div></div>
+    <div></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, timeline_ancestor_sibling);
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(target).zIndex, '75');
+  }, 'view-timeline on ancestor sibling');
+</script>
+
+<template id=timeline_ancestor_sibling_closest>
+  <style>
+    #timeline1, #timeline2 {
+      height: 0px;
+      view-timeline: t1;
+    }
+    #target {
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=scroller>
+    <div></div>
+    <div id=timeline1></div>
+    <div></div>
+    <div id=timeline2></div>
+    <div>
+      <div>
+        <div id=target></div>
+      </div>
+    </div>
+    <div></div>
+    <div></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, timeline_ancestor_sibling_closest);
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(target).zIndex, '50');
+  }, 'view-timeline on ancestor sibling, closest wins');
+</script>
+
+<template id=timeline_ancestor_sibling_skips_nonmatching>
+  <style>
+    #timeline1 {
+      height: 0px;
+      view-timeline: t1;
+    }
+    #timeline2 {
+      height: 0px;
+      view-timeline: t2;
+    }
+    #target {
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=scroller>
+    <div></div>
+    <div id=timeline1></div>
+    <div></div>
+    <div id=timeline2></div>
+    <div>
+      <div>
+        <div id=target></div>
+      </div>
+    </div>
+    <div></div>
+    <div></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, timeline_ancestor_sibling_skips_nonmatching);
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(target).zIndex, '75');
+  }, 'view-timeline on ancestor sibling, skips nonmatching names');
+</script>
+
+<template id=timeline_ancestor_closer_scroll_timeline_wins>
+  <style>
+    #timeline {
+      height: 0px;
+      view-timeline: t1;
+    }
+    #scroll {
+      overflow: auto;
+      scroll-timeline: t1;
+    }
+    #scroll > div {
+      height: 50px;
+    }
+    #target {
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=scroller>
+    <div></div>
+    <div id=timeline></div>
+    <div></div>
+    <div id=scroll>
+      <div></div>
+    </div>
+    <div>
+      <div>
+        <div id=target></div>
+      </div>
+    </div>
+    <div></div>
+    <div></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, timeline_ancestor_closer_scroll_timeline_wins);
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(target).zIndex, '0');
+  }, 'view-timeline on ancestor sibling, closer scroll-timeline wins');
+</script>
+
+
+<template id=timeline_ancestor_view_timeline_wins_on_same_element>
+  <style>
+    #timelines {
+      height: 0px;
+      view-timeline: t1;
+      scroll-timeline: t1;
+      overflow: auto;
+    }
+    #timelines > div {
+      height: 50px;
+    }
+    #target {
+      animation: anim 1s linear;
+      animation-timeline: t1;
+    }
+  </style>
+  <div id=scroller class=scroller>
+    <div></div>
+    <div id=timelines>
+      <div></div>
+    </div>
+    <div></div>
+    <div>
+      <div>
+        <div id=target></div>
+      </div>
+    </div>
+    <div></div>
+    <div></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, timeline_ancestor_view_timeline_wins_on_same_element);
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(target).zIndex, '75');
+  }, 'view-timeline on ancestor sibling, view-timeline wins on same element');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/view-timeline-name-computed.html b/testing/web-platform/tests/scroll-animations/css/view-timeline-name-computed.html
new file mode 100644
index 0000000000..5657dc7817
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/view-timeline-name-computed.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#view-timeline-name">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/computed-testcommon.js"></script>
+</head>
+<style>
+  #outer { view-timeline-name: foo, bar; }
+  #target { view-timeline-name: faz; }
+</style>
+<div id=outer>
+  <div id=target></div>
+</div>
+<script>
+test_computed_value('view-timeline-name', 'initial', 'none');
+test_computed_value('view-timeline-name', 'inherit', 'foo, bar');
+test_computed_value('view-timeline-name', 'unset', 'none');
+test_computed_value('view-timeline-name', 'revert', 'none');
+test_computed_value('view-timeline-name', 'none');
+test_computed_value('view-timeline-name', 'foo');
+test_computed_value('view-timeline-name', 'foo, bar');
+test_computed_value('view-timeline-name', 'bar, foo');
+test_computed_value('view-timeline-name', 'a, b, c, D, e');
+test_computed_value('view-timeline-name', 'none, none');
+test_computed_value('view-timeline-name', 'a, b, c, none, d, e');
+
+test(() => {
+  let style = getComputedStyle(document.getElementById('target'));
+  assert_not_equals(Array.from(style).indexOf('view-timeline-name'), -1);
+}, 'The view-timeline-name property shows up in CSSStyleDeclaration enumeration');
+
+test(() => {
+  let style = document.getElementById('target').style;
+  assert_not_equals(style.cssText.indexOf('view-timeline-name'), -1);
+}, 'The view-timeline-name property shows up in CSSStyleDeclaration.cssText');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/view-timeline-name-parsing.html b/testing/web-platform/tests/scroll-animations/css/view-timeline-name-parsing.html
new file mode 100644
index 0000000000..3878d5c583
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/view-timeline-name-parsing.html
@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#view-timeline-name">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/parsing-testcommon.js"></script>
+<div id="target"></div>
+<script>
+test_valid_value('view-timeline-name', 'initial');
+test_valid_value('view-timeline-name', 'inherit');
+test_valid_value('view-timeline-name', 'unset');
+test_valid_value('view-timeline-name', 'revert');
+
+test_valid_value('view-timeline-name', 'none');
+test_valid_value('view-timeline-name', 'abc');
+test_valid_value('view-timeline-name', '  abc', 'abc');
+test_valid_value('view-timeline-name', 'abc ', 'abc');
+test_valid_value('view-timeline-name', 'aBc');
+test_valid_value('view-timeline-name', 'foo, bar');
+test_valid_value('view-timeline-name', 'bar, foo');
+test_valid_value('view-timeline-name', 'none, none');
+test_valid_value('view-timeline-name', 'a, none, b');
+test_valid_value('view-timeline-name', 'auto');
+
+test_invalid_value('view-timeline-name', 'default');
+test_invalid_value('view-timeline-name', '10px');
+test_invalid_value('view-timeline-name', 'foo bar');
+test_invalid_value('view-timeline-name', '"foo" "bar"');
+test_invalid_value('view-timeline-name', 'rgb(1, 2, 3)');
+test_invalid_value('view-timeline-name', '#fefefe');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/view-timeline-name-tree-scoped.html b/testing/web-platform/tests/scroll-animations/css/view-timeline-name-tree-scoped.html
new file mode 100644
index 0000000000..c216c345ed
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/view-timeline-name-tree-scoped.html
@@ -0,0 +1,145 @@
+<!DOCTYPE html>
+<title>view-timelime-name and tree-scoped references</title>
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#view-timelines-named">
+<link rel="help" src="https://drafts.csswg.org/css-scoping-1/#shadow-names">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="/resources/declarative-shadow-dom-polyfill.js"></script>
+
+<main id=main></main>
+<script>
+  function inflate(t, template) {
+    t.add_cleanup(() => main.replaceChildren());
+    main.append(template.content.cloneNode(true));
+    main.offsetTop;
+  }
+
+  setup(() => {
+    polyfill_declarative_shadow_dom(document);
+  });
+</script>
+<style>
+  @keyframes anim {
+    from { z-index: 100; }
+    to { z-index: 100; }
+  }
+</style>
+
+
+<template id=view_timeline_host>
+  <style>
+    .target {
+      animation: anim 10s linear;
+      animation-timeline: timeline;
+    }
+    .scroller > div {
+      view-timeline: timeline horizontal;
+    }
+  </style>
+  <div class=scroller>
+    <div>
+      <div class=target>
+        <template shadowroot=open>
+          <style>
+            :host {
+              view-timeline: timeline vertical;
+            }
+          </style>
+        </template>
+      </div>
+    </div>
+  </div>
+  <style>
+  </style>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, view_timeline_host);
+    let target = main.querySelector('.target');
+    assert_equals(target.getAnimations().length, 1);
+    let anim = target.getAnimations()[0];
+    assert_not_equals(anim.timeline, null);
+    assert_equals(anim.timeline.axis, 'horizontal');
+  }, 'Outer animation can not see view timeline defined by :host');
+</script>
+
+
+<template id=view_timeline_slotted>
+  <style>
+    .target {
+      animation: anim 10s linear;
+      animation-timeline: timeline;
+    }
+    .host {
+      view-timeline: timeline horizontal;
+    }
+  </style>
+  <div class=scroller>
+    <div class=host>
+      <template shadowroot=open>
+        <style>
+          ::slotted(.target) {
+            view-timeline: timeline vertical;
+          }
+        </style>
+        <slot></slot>
+      </template>
+      <div class=target></div>
+    </div>
+  </div>
+  <style>
+  </style>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, view_timeline_slotted);
+    let target = main.querySelector('.target');
+    assert_equals(target.getAnimations().length, 1);
+    let anim = target.getAnimations()[0];
+    assert_not_equals(anim.timeline, null);
+    assert_equals(anim.timeline.axis, 'horizontal');
+  }, 'Outer animation can not see view timeline defined by ::slotted');
+</script>
+
+
+<template id=view_timeline_part>
+  <style>
+    .host {
+      view-timeline: timeline vertical;
+    }
+    .host::part(foo) {
+      view-timeline: timeline horizontal;
+    }
+  </style>
+  <div class=host>
+    <template shadowroot=open>
+      <style>
+          /* Not using 'anim' at document scope, due to https://crbug.com/1334534 */
+          @keyframes anim2 {
+            from { z-index: 100; }
+            to { z-index: 100; }
+          }
+        .target {
+          animation: anim2 10s linear;
+          animation-timeline: timeline;
+        }
+      </style>
+      <div part=foo>
+        <div class=target></div>
+      </div>
+    </template>
+  </div>
+  <style>
+  </style>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, view_timeline_part);
+    let target = main.querySelector('.host').shadowRoot.querySelector('.target');
+    assert_equals(target.getAnimations().length, 1);
+    let anim = target.getAnimations()[0];
+    assert_not_equals(anim.timeline, null);
+    assert_equals(anim.timeline.axis, 'horizontal');
+  }, 'Inner animation can see view timeline defined by ::part');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/view-timeline-shorthand.tentative.html b/testing/web-platform/tests/scroll-animations/css/view-timeline-shorthand.tentative.html
new file mode 100644
index 0000000000..f3efa88c92
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/view-timeline-shorthand.tentative.html
@@ -0,0 +1,104 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#view-timeline-shorthand">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/7627">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/pull/7694">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/computed-testcommon.js"></script>
+<script src="/css/support/parsing-testcommon.js"></script>
+<script src="/css/support/shorthand-testcommon.js"></script>
+<div id="target"></div>
+<script>
+test_valid_value('view-timeline', 'abcd');
+test_valid_value('view-timeline', 'none block', 'none');
+test_valid_value('view-timeline', 'none inline');
+
+// view-timeline-name: inline/block/horizontal/vertical.
+test_valid_value('view-timeline', 'inline block', 'inline');
+test_valid_value('view-timeline', 'block block', 'block');
+test_valid_value('view-timeline', 'vertical block', 'vertical');
+test_valid_value('view-timeline', 'horizontal block', 'horizontal');
+
+test_valid_value('view-timeline', 'a, b, c');
+test_valid_value('view-timeline', 'a inline, b block, c vertical', 'a inline, b, c vertical');
+test_valid_value('view-timeline', 'auto');
+
+test_invalid_value('view-timeline', 'abc abc');
+test_invalid_value('view-timeline', 'block none');
+test_invalid_value('view-timeline', 'none none');
+test_invalid_value('view-timeline', 'default');
+test_invalid_value('view-timeline', ',');
+test_invalid_value('view-timeline', ',,block,,');
+
+test_computed_value('view-timeline', 'abcd');
+test_computed_value('view-timeline', 'none block', 'none');
+test_computed_value('view-timeline', 'none inline');
+test_computed_value('view-timeline', 'inline block', 'inline');
+test_computed_value('view-timeline', 'block block', 'block');
+test_computed_value('view-timeline', 'vertical block', 'vertical');
+test_computed_value('view-timeline', 'horizontal block', 'horizontal');
+test_computed_value('view-timeline', 'a, b, c');
+test_computed_value('view-timeline', 'a inline, b block, c vertical', 'a inline, b, c vertical');
+
+test_shorthand_value('view-timeline', 'abc vertical',
+{
+  'view-timeline-name': 'abc',
+  'view-timeline-axis': 'vertical',
+});
+test_shorthand_value('view-timeline', 'abc vertical, def',
+{
+  'view-timeline-name': 'abc, def',
+  'view-timeline-axis': 'vertical, block',
+});
+test_shorthand_value('view-timeline', 'abc, def',
+{
+  'view-timeline-name': 'abc, def',
+  'view-timeline-axis': 'block, block',
+});
+test_shorthand_value('view-timeline', 'inline horizontal',
+{
+  'view-timeline-name': 'inline',
+  'view-timeline-axis': 'horizontal',
+});
+
+function test_shorthand_contraction(shorthand, longhands, expected) {
+  let longhands_fmt = Object.entries(longhands).map((e) => `${e[0]}:${e[1]}`).join(';');
+  test((t) => {
+    t.add_cleanup(() => {
+      for (let shorthand of Object.keys(longhands))
+        target.style.removeProperty(shorthand);
+    });
+    for (let [shorthand, value] of Object.entries(longhands))
+      target.style.setProperty(shorthand, value);
+    assert_equals(target.style.getPropertyValue(shorthand), expected, 'Declared value');
+    assert_equals(getComputedStyle(target).getPropertyValue(shorthand), expected, 'Computed value');
+  }, `Shorthand contraction of ${longhands_fmt}`);
+}
+
+test_shorthand_contraction('view-timeline', {
+  'view-timeline-name': 'abc',
+  'view-timeline-axis': 'inline',
+}, 'abc inline');
+
+test_shorthand_contraction('view-timeline', {
+  'view-timeline-name': 'a, b',
+  'view-timeline-axis': 'inline, block',
+}, 'a inline, b');
+
+test_shorthand_contraction('view-timeline', {
+  'view-timeline-name': 'none, none',
+  'view-timeline-axis': 'block, block',
+}, 'none, none');
+
+// Longhands with different lengths:
+
+test_shorthand_contraction('view-timeline', {
+  'view-timeline-name': 'a, b, c',
+  'view-timeline-axis': 'inline, inline',
+}, '');
+
+test_shorthand_contraction('view-timeline', {
+  'view-timeline-name': 'a, b',
+  'view-timeline-axis': 'inline, inline, inline',
+}, '');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/css/view-timeline-used-values.html b/testing/web-platform/tests/scroll-animations/css/view-timeline-used-values.html
new file mode 100644
index 0000000000..6627eeb998
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/view-timeline-used-values.html
@@ -0,0 +1,104 @@
+<!DOCTYPE html>
+<title>Used values of view-timeline properties</title>
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#view-timeline-axis">
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#view-timeline-name">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { z-index: 0; }
+    to { z-index: 100; }
+  }
+  .scroller {
+    overflow: hidden;
+    width: 100px;
+    height: 100px;
+  }
+  .scroller > div {
+    width: 300px;
+    height: 300px;
+    z-index: -1;
+  }
+</style>
+<main id=main></main>
+<script>
+  setup(assert_implements_animation_timeline);
+
+  function inflate(t, template) {
+    t.add_cleanup(() => main.replaceChildren());
+    main.append(template.content.cloneNode(true));
+  }
+  async function scrollTop(e, value) {
+    e.scrollTop = value;
+    await waitForNextFrame();
+  }
+  async function scrollLeft(e, value) {
+    e.scrollLeft = value;
+    await waitForNextFrame();
+  }
+</script>
+
+<template id=omitted_axis>
+  <style>
+    #target {
+      view-timeline-name: t1, t2; /* Two items */
+      view-timeline-axis: inline; /* One item */
+      animation: anim 1s linear;
+      animation-timeline: t2;
+    }
+  </style>
+  <div id=scroller class=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, omitted_axis);
+    assert_equals(getComputedStyle(target).zIndex, '-1');
+
+    // enter 0% is at scrollTop/Left = -100
+    // exit 100% is at scrollTop/Left = 300
+    // This means that at scrollTop/Left=0, the animation is at 25%.
+
+    await scrollTop(scroller, 0);
+    await scrollLeft(scroller, 0);
+    assert_equals(getComputedStyle(target).zIndex, '25');
+
+    // The timeline should be inline-axis:
+    await scrollTop(scroller, 100); // 50%
+    await scrollLeft(scroller, 40); // 35%
+    assert_equals(getComputedStyle(target).zIndex, '35');
+  }, 'Use the last value from view-timeline-axis if omitted');
+</script>
+
+<template id=omitted_inset>
+  <style>
+    #target {
+      view-timeline-name: t1, t2; /* Two items */
+      view-timeline-inset: 100px; /* One item */
+      animation: anim 1s linear;
+      animation-timeline: t2;
+    }
+  </style>
+  <div id=scroller class=scroller>
+    <div id=target></div>
+  </div>
+</template>
+<script>
+  promise_test(async (t) => {
+    inflate(t, omitted_inset);
+    assert_equals(getComputedStyle(target).zIndex, '-1');
+
+    // 0% is normally at at scrollTop = -100
+    // 100% is normally at scrollTop/Left = 300
+    // However, we have a 100px inset in both ends, which makes the
+    // range [0, 200].
+
+    await scrollTop(scroller, 0);
+    assert_equals(getComputedStyle(target).zIndex, '0');
+    await scrollTop(scroller, 100); // 50%
+    assert_equals(getComputedStyle(target).zIndex, '50');
+  }, 'Use the last value from view-timeline-inset if omitted');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-ref.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-ref.html
new file mode 100644
index 0000000000..9158715321
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-ref.html
@@ -0,0 +1,45 @@
+<!DOCTYPE html>
+<title>Reference for Web Animation with scroll timeline tests</title>
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    transform: translate(0, 100px);
+    opacity: 0.5;
+    will-change: transform; /* force compositing */
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  #scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+    will-change: transform; /* force compositing */
+  }
+
+  #contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+<div id="scroller">
+  <div id="contents"></div>
+</div>
+
+<script>
+  window.addEventListener('load', function() {
+    // Move the scroller to halfway.
+    const scroller = document.getElementById("scroller");
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-animatable-interface.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-animatable-interface.html
new file mode 100644
index 0000000000..b04aaf2d33
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-animatable-interface.html
@@ -0,0 +1,66 @@
+<html class="reftest-wait">
+<title>Scroll-linked animation with Animatable interface</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations/">
+<meta name="assert" content="ScrollTimeline should work with animatable
+interface">
+<link rel="match" href="animation-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  #scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+    will-change: transform;
+    /* force compositing */
+  }
+
+  #contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+<div id="scroller">
+  <div id="contents"><p>Scrolling Contents</p></div>
+</div>
+
+<script>
+  const scroller = document.getElementById('scroller');
+  const scroll_timeline = new ScrollTimeline({source: scroller});
+  const box = document.getElementById('box');
+  const animation = box.animate(
+    [
+      { transform: 'translateY(0)', opacity: 1 },
+      { transform: 'translateY(200px)', opacity: 0 }
+    ], {
+      timeline: scroll_timeline
+    }
+  );
+
+  animation.ready.then(() => {
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+
+    waitForAnimationFrames(2).then(_ => {
+      takeScreenshot();
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-delay-crash.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-delay-crash.html
new file mode 100644
index 0000000000..9d821f9e20
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-delay-crash.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/web-animations-1/#the-effecttiming-dictionaries">
+<style>
+.scroller {
+  overflow: auto;
+  height: 100px;
+  width: 100px;
+  will-change: transform;
+}
+
+.contents {
+  height: 1000px;
+  width: 100%;
+}
+</style>
+<div class="scroller">
+  <div class="contents"></div>
+</div>
+<script>
+  // Test passes if it does not crash.
+  // Scroll timeline animations are progress-based and not compatible with
+  // delays specified in milliseconds.
+  const timeline = new ScrollTimeline();
+  const options = {
+    timeline: timeline,
+    endDelay: 200
+  };
+  const keyframes = { opacity: [0, 1]};
+  const element = document.querySelector('.contents');
+  element.animate(keyframes, options);
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-display-none.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-display-none.html
new file mode 100644
index 0000000000..a62916833c
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-display-none.html
@@ -0,0 +1,75 @@
+<html class="reftest-wait">
+<title>Scroll timeline with Web Animation and transition from display:none to display:block</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations/">
+<meta name="assert" content="Scroll timeline should properly handle going from display:none to display:block">
+<link rel="match" href="animation-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  #scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+    will-change: transform; /* force compositing */
+  }
+
+  .removed {
+    display: none;
+  }
+
+  #contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+<div id="scroller">
+  <div id="contents"><p>Scrolling Contents</p></div>
+</div>
+
+<script>
+  const box = document.getElementById('box');
+  const effect = new KeyframeEffect(box,
+    [
+    { transform: 'translateY(0)', opacity: 1 },
+    { transform: 'translateY(200px)', opacity: 0 }
+    ], {
+      duration: 1000,
+    }
+  );
+
+  const scroller = document.getElementById('scroller');
+  scroller.classList.add('removed');
+  const timeline = new ScrollTimeline(
+      { source: scroller, orientation: 'block' });
+  const animation = new Animation(effect, timeline);
+  animation.play();
+
+  waitForAnimationFrames(2).then(_ => {
+    scroller.classList.remove('removed');
+    animation.ready.then(() => {
+      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+      scroller.scrollTop = 0.5 * maxScroll;
+
+      waitForAnimationFrames(2).then(_ => {
+        takeScreenshot();
+      });
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-offsets-crash.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-offsets-crash.html
new file mode 100644
index 0000000000..d4d1a55214
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-offsets-crash.html
@@ -0,0 +1,32 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scrolltimeline-interface">
+<style>
+.scroller {
+  overflow: auto;
+  height: 100px;
+  width: 100px;
+  will-change: transform;
+}
+
+.contents {
+  height: 1000px;
+  width: 100%;
+}
+</style>
+<div class="scroller">
+  <div class="contents"></div>
+</div>
+<script>
+  // Test passes if it does not crash.
+  // Scroll timeline animations are progress-based and not compatible with
+  // delays specified in milliseconds.
+  const scroller = document.querySelector('.scroller');
+  const animation = new Animation();
+  const timeline = animation.timeline;
+  const duration = timeline.duration;
+  const options = {
+    source: scroller,
+    scrollOffsets: [new CSSMathInvert(duration)]
+  };
+  const scroll_timeline = new ScrollTimeline(options);
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-overflow-hidden-ref.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-overflow-hidden-ref.html
new file mode 100644
index 0000000000..c045f1a1c9
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-overflow-hidden-ref.html
@@ -0,0 +1,45 @@
+<!DOCTYPE html>
+<title>Scroll timeline with Web Animation using a scroller with overflow hidden</title>
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    transform: translate(0, 100px);
+    opacity: 0.5;
+    will-change: transform; /* force compositing */
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  #scroller {
+    overflow: hidden;
+    height: 100px;
+    width: 100px;
+    will-change: transform; /* force compositing */
+  }
+
+  #contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+<div id="scroller">
+  <div id="contents"></div>
+</div>
+
+<script>
+  window.addEventListener('load', function() {
+    // Move the scroller to halfway.
+    const scroller = document.getElementById("scroller");
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-overflow-hidden.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-overflow-hidden.html
new file mode 100644
index 0000000000..bc7611d05a
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-overflow-hidden.html
@@ -0,0 +1,64 @@
+<html class="reftest-wait">
+<title>Scroll timeline with Web Animation using a scroller with overflow hidden</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations/">
+<meta name="assert" content="Web animation correctly updates values when using a overflow: hidden on the scroller being used as the source for the ScrollTimeline">
+<link rel="match" href="animation-with-overflow-hidden-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  #scroller {
+    overflow: hidden;
+    height: 100px;
+    width: 100px;
+  }
+
+  #contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+<div id="scroller">
+  <div id="contents"></div>
+</div>
+
+<script>
+  const box = document.getElementById('box');
+  const effect = new KeyframeEffect(box,
+    [
+      {transform: 'translateY(0)', opacity: 1},
+      {transform: 'translateY(200px)', opacity: 0}
+    ]
+  );
+
+  const scroller = document.getElementById('scroller');
+  const timeline = new ScrollTimeline(
+      { source: scroller, orientation: 'block' });
+  const animation = new Animation(effect, timeline);
+  animation.play();
+
+  animation.ready.then(() => {
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+    waitForAnimationFrames(2).then(_ => {
+      takeScreenshot();
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-root-scroller-ref.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-root-scroller-ref.html
new file mode 100644
index 0000000000..58435be631
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-root-scroller-ref.html
@@ -0,0 +1,37 @@
+<!DOCTYPE html>
+<title>Reference for Scroll timeline with Web Animation using the root scroller</title>
+<style>
+  html {
+    min-height: 100%;
+    min-width: 100%;
+    padding-bottom: 100px;
+    padding-right: 100px;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    transform: translate(0, 100px);
+    opacity: 0.5;
+    will-change: transform; /* force compositing */
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"><p>Covered Contents</p></div>
+
+<script>
+  window.addEventListener('load', function() {
+    // Move the scroller to halfway.
+    const scroller = document.scrollingElement;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-root-scroller.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-root-scroller.html
new file mode 100644
index 0000000000..6ba1a22445
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-root-scroller.html
@@ -0,0 +1,60 @@
+<html class="reftest-wait">
+<title>Scroll timeline with Web Animation using the root scroller</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations/">
+<meta name="assert" content="Web animation correctly updates values when using the root scroller as the source for the ScrollTimeline">
+<link rel="match" href="animation-with-root-scroller-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+
+<style>
+  html {
+    min-height: 100%;
+    min-width: 100%;
+    padding-bottom: 100px;
+    padding-right: 100px;
+  }
+
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"><p>Covered Contents</p></div>
+
+<script>
+  const box = document.getElementById('box');
+  const effect = new KeyframeEffect(box,
+    [
+      {transform: 'translateY(0)', opacity: 1},
+      {transform: 'translateY(200px)', opacity: 0}
+    ], {
+      duration: 1000,
+    }
+  );
+
+  const scroller = document.scrollingElement;
+  const timeline = new ScrollTimeline(
+      { source: scroller, orientation: 'block' });
+  const animation = new Animation(effect, timeline);
+  animation.play();
+
+  animation.ready.then(() => {
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+
+    waitForAnimationFrames(2).then(_ => {
+      takeScreenshot();
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-transform.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-transform.html
new file mode 100644
index 0000000000..f741cc634d
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/animation-with-transform.html
@@ -0,0 +1,68 @@
+<html class="reftest-wait">
+<title>Basic use of scroll timeline with Web Animation</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations/">
+<meta name="assert" content="Should be able to use the scroll timeline to drive the animation timing">
+<link rel="match" href="animation-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  #scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+    will-change: transform; /* force compositing */
+  }
+
+  #contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+<div id="scroller">
+  <div id="contents"><p>Scrolling Contents</p></div>
+</div>
+
+<script>
+  const box = document.getElementById('box');
+  const effect = new KeyframeEffect(box,
+    [
+    { transform: 'translateY(0)', opacity: 1},
+    { transform: 'translateY(200px)', opacity: 0}
+    ], {
+      duration: 1000,
+    }
+  );
+
+  const scroller = document.getElementById('scroller');
+  const timeline = new ScrollTimeline(
+      { source: scroller, orientation: 'block' });
+  const animation = new Animation(effect, timeline);
+  animation.play();
+
+  animation.ready.then(() => {
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+
+    waitForAnimationFrames(2).then(_ => {
+      takeScreenshot();
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/cancel-animation.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/cancel-animation.html
new file mode 100644
index 0000000000..7daf76a7a5
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/cancel-animation.html
@@ -0,0 +1,214 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Canceling an animation</title>
+<link rel="help"
+    href="https://drafts.csswg.org/web-animations/#canceling-an-animation-section">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+.scroller {
+  overflow: auto;
+  height: 100px;
+  width: 100px;
+  will-change: transform;
+}
+
+.contents {
+  height: 1000px;
+  width: 100%;
+}
+</style>
+<body>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.cancel();
+
+  assert_equals(animation.startTime, null,
+                'The start time of a canceled animation should be unresolved');
+  assert_equals(animation.currentTime, null,
+                'The hold time of a canceled animation should be unresolved');
+}, 'Canceling an animation should cause its start time and hold time to be'
+   + ' unresolved');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  const retPromise = animation.ready.then(() => {
+    assert_unreached('ready promise was fulfilled');
+  }).catch(err => {
+    assert_equals(err.name, 'AbortError',
+                  'ready promise is rejected with AbortError');
+  });
+
+  animation.cancel();
+
+  return retPromise;
+}, 'A play-pending ready promise should be rejected when the animation is'
+   + ' canceled');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  // Make it pause-pending
+  animation.pause();
+
+  // We need to store the original ready promise since cancel() will
+  // replace it
+  const originalPromise = animation.ready;
+  animation.cancel();
+
+  await promise_rejects_dom(t, 'AbortError', originalPromise,
+                            'Cancel should abort ready promise');
+}, 'A pause-pending ready promise should be rejected when the animation is'
+   + ' canceled');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.cancel();
+  const promiseResult = await animation.ready;
+  assert_equals(promiseResult, animation);
+}, 'When an animation is canceled, it should create a resolved Promise');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  const promise = animation.ready;
+  animation.cancel();
+  assert_not_equals(animation.ready, promise);
+  promise_rejects_dom(t, 'AbortError', promise,
+                      'Cancel should abort ready promise');
+}, 'The ready promise should be replaced when the animation is canceled');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  assert_equals(animation.playState, 'idle',
+                'The animation should be initially idle');
+
+  animation.finished.then(t.step_func(() => {
+    assert_unreached('Finished promise should not resolve');
+  }), t.step_func(() => {
+    assert_unreached('Finished promise should not reject');
+  }));
+
+  animation.cancel();
+
+  return waitForAnimationFrames(3);
+}, 'The finished promise should NOT be rejected if the animation is already'
+   + ' idle');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  assert_equals(animation.playState, 'idle',
+                'The animation should be initially idle');
+
+  animation.oncancel = t.step_func(() => {
+    assert_unreached('Cancel event should not be fired');
+  });
+
+  animation.cancel();
+
+  return waitForAnimationFrames(3);
+}, 'The cancel event should NOT be fired if the animation is already idle');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.effect.target.remove();
+
+  const eventWatcher = new EventWatcher(t, animation, 'cancel');
+
+  await animation.ready;
+  animation.cancel();
+
+  await eventWatcher.wait_for('cancel');
+
+  assert_equals(animation.effect.target.parentNode, null,
+      'cancel event should be fired for the animation on an orphaned element');
+}, 'Canceling an animation should fire cancel event on orphaned element');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  // Make the scroll timeline inactive.
+  scroller.style.overflow = 'visible';
+  scroller.scrollTop;
+  await waitForNextFrame();
+  assert_equals(animation.timeline.currentTime, null,
+                'Sanity check the timeline is inactive.');
+  animation.cancel();
+  assert_equals(animation.startTime, null,
+                'The start time of a canceled animation should be unresolved');
+  assert_equals(animation.currentTime, null,
+              'The current time of a canceled animation should be unresolved');
+}, 'Canceling an animation with inactive timeline should cause its start time'
+   + ' and hold time to be unresolved');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  // Make the scroll timeline inactive.
+  scroller.style.overflow = 'visible';
+  scroller.scrollTop;
+  await waitForNextFrame();
+  assert_equals(animation.timeline.currentTime, null,
+                'Sanity check the timeline is inactive.');
+
+  const eventWatcher = new EventWatcher(t, animation, 'cancel');
+  animation.cancel();
+  const cancelEvent = await eventWatcher.wait_for('cancel');
+
+  assert_equals(cancelEvent.currentTime, null,
+      'event.currentTime should be unresolved when the timeline is inactive.');
+  assert_equals(cancelEvent.timelineTime, null,
+      'event.timelineTime should be unresolved when the timeline is inactive');
+}, 'oncancel event is fired when the timeline is inactive.');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/constructor-no-document.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/constructor-no-document.html
new file mode 100644
index 0000000000..d2cc590bc7
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/constructor-no-document.html
@@ -0,0 +1,19 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>ScrollTimeline constructor - no document</title>
+<link rel="help" href="https://wicg.github.io/scroll-animations/#scrolltimeline-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<script>
+'use strict';
+
+test(function() {
+  document.documentElement.remove();
+  assert_equals(document.scrollingElement, null);
+
+  const timeline = new ScrollTimeline();
+  assert_equals(timeline.source, null);
+  assert_equals(timeline.currentTime, null);
+}, 'The source can be null if the document.scrollingElement does not exist');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/constructor.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/constructor.html
new file mode 100644
index 0000000000..8e211efa11
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/constructor.html
@@ -0,0 +1,95 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>ScrollTimeline constructor</title>
+  <link rel="help" href="https://wicg.github.io/scroll-animations/#scrolltimeline-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<style>
+.scroller {
+  height: 100px;
+  width: 100px;
+  overflow: scroll;
+}
+
+.content {
+  height: 500px;
+  width: 500px;
+}
+</style>
+
+<div class="scroller">
+  <div class="content"></div>
+</div>
+
+<script>
+'use strict';
+
+function formatOffset(v) {
+  if (typeof(v) == 'object')
+    return `${v.constructor.name}(${v.toString()})`;
+  return `'${v.toString()}'`;
+}
+
+function assert_offsets_equal(a, b) {
+  assert_equals(formatOffset(a), formatOffset(b));
+}
+
+// source
+
+test(t => {
+  const scroller = document.querySelector('.scroller');
+  assert_equals(
+      new ScrollTimeline({source: scroller}).source, scroller);
+}, 'A ScrollTimeline can be created with a source');
+
+test(t => {
+  const div = document.createElement('div');
+  assert_equals(new ScrollTimeline({source: div}).source, div);
+}, 'A ScrollTimeline can be created with a non-scrolling source');
+
+test(t => {
+  assert_equals(new ScrollTimeline({source: null}).source, null);
+}, 'A ScrollTimeline created with a null source should have no source');
+
+test(t => {
+  assert_equals(new ScrollTimeline().source, document.scrollingElement);
+}, 'A ScrollTimeline created without a source should use the ' +
+   'document.scrollingElement');
+
+// axis
+
+test(t => {
+  assert_equals(new ScrollTimeline().axis, 'block');
+}, 'A ScrollTimeline created with the default axis should default to ' +
+   `'block'`);
+
+const gValidAxisValues = [
+  'block',
+  'inline',
+  'horizontal',
+  'vertical',
+];
+
+for (let axis of gValidAxisValues) {
+  test(function() {
+    const scrollTimeline =
+        new ScrollTimeline({axis: axis});
+    assert_equals(scrollTimeline.axis, axis);
+  }, `'${axis}' is a valid axis value`);
+}
+
+test(t => {
+  let constructorFunc = function() {
+    new ScrollTimeline({axis: 'nonsense'})
+  };
+  assert_throws_js(TypeError, constructorFunc);
+
+  // 'auto' for axis was previously in the spec, but was removed. Make
+  // sure that implementations do not support it.
+  constructorFunc = function() {
+    new ScrollTimeline({axis: 'auto'})
+  };
+  assert_throws_js(TypeError, constructorFunc);
+}, 'Creating a ScrollTimeline with an invalid axis value should throw');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/current-time-nan.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/current-time-nan.html
new file mode 100644
index 0000000000..440b1f413e
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/current-time-nan.html
@@ -0,0 +1,80 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>ScrollTimeline current time algorithm - NaN cases</title>
+<link rel="help" href="https://wicg.github.io/scroll-animations/#current-time-algorithm">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<style>
+.scroller {
+  height: 100px;
+  width: 100px;
+  overflow: auto;
+}
+
+.content {
+  height: 500px;
+  width: 500px;
+}
+</style>
+
+<div id='inlineScroller' class='scroller' style='display: inline;'>
+  <div class='content'></div>
+</div>
+<script>
+'use strict';
+
+test(function() {
+  const scroller = document.querySelector('#inlineScroller');
+  const scrollTimeline = new ScrollTimeline(
+      { source: scroller, orientation: 'block' });
+
+  assert_equals(scrollTimeline.currentTime, null);
+}, 'currentTime should be null for a display: inline source');
+</script>
+
+<div id='displayNoneScroller' class='scroller' style='display: none;'>
+  <div class='content'></div>
+</div>
+<script>
+test(function() {
+  const scroller = document.querySelector('#displayNoneScroller');
+  const scrollTimeline = new ScrollTimeline(
+      { source: scroller, orientation: 'block' });
+
+  assert_equals(scrollTimeline.currentTime, null);
+}, 'currentTime should be null for a display: none source');
+</script>
+
+<script>
+test(function() {
+  const scroller = document.createElement('div');
+  const content = document.createElement('div');
+
+  scroller.style.overflow = 'auto';
+  scroller.style.height = '100px';
+  scroller.style.width = '100px';
+  content.style.height = '250px';
+  content.style.width = '250px';
+
+  scroller.appendChild(content);
+
+  const scrollTimeline = new ScrollTimeline(
+      { source: scroller, orientation: 'block' });
+
+  assert_equals(scrollTimeline.currentTime, null);
+}, 'currentTime should be null for an unattached source');
+</script>
+
+<div id='notAScroller' class='scroller' style='overflow: visible;'>
+  <div class='content'></div>
+</div>
+<script>
+test(function() {
+  const scroller = document.querySelector('#notAScroller');
+  const scrollTimeline = new ScrollTimeline(
+      { source: scroller, orientation: 'block' });
+
+  assert_equals(scrollTimeline.currentTime, null);
+}, 'currentTime should be null when the source is not a scroller');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/current-time-root-scroller.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/current-time-root-scroller.html
new file mode 100644
index 0000000000..be1d62bec5
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/current-time-root-scroller.html
@@ -0,0 +1,49 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>ScrollTimeline current time algorithm - root scroller</title>
+<link rel="help" href="https://wicg.github.io/scroll-animations/#current-time-algorithm">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="./testcommon.js"></script>
+
+<style>
+html {
+  /* Ensure the document is scrollable. */
+  min-height: 100%;
+  min-width: 100%;
+  padding-bottom: 100px;
+  padding-right: 100px;
+}
+</style>
+
+<script>
+promise_test(async t => {
+  const scroller = document.scrollingElement;
+  // Allow layout to finish, otherwise the scroller isn't set up by the time
+  // we check the currentTime of the scroll timeline.
+  await waitForNextFrame();
+
+  const blockScrollTimeline = new ScrollTimeline(
+      { source: scroller, axis: 'block' });
+  const inlineScrollTimeline = new ScrollTimeline(
+      { source: scroller, axis: 'inline' });
+
+  // Wait for new animation frame which allows the timeline to fully initialize
+  await waitForNextFrame();
+
+  // Unscrolled, both timelines should read a currentTime of 0.
+  assert_percents_equal(blockScrollTimeline.currentTime, 0);
+  assert_percents_equal(inlineScrollTimeline.currentTime, 0);
+
+  // Now do some scrolling and make sure that the ScrollTimelines update.
+  scroller.scrollTop = 50;
+  scroller.scrollLeft = 75;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  assert_percents_equal(blockScrollTimeline.currentTime, 50);
+  assert_percents_equal(inlineScrollTimeline.currentTime, 75);
+}, 'currentTime calculates the correct time for a document.scrollingElement source');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/current-time-writing-modes.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/current-time-writing-modes.html
new file mode 100644
index 0000000000..d18abcd48f
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/current-time-writing-modes.html
@@ -0,0 +1,148 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>ScrollTimeline current time algorithm - interaction with writing modes</title>
+<link rel="help" href="https://wicg.github.io/scroll-animations/#current-time-algorithm">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="./testcommon.js"></script>
+
+<body></body>
+
+<script>
+'use strict';
+
+promise_test(async t => {
+  const scrollerOverrides = new Map([['direction', 'rtl']]);
+  const scroller = setupScrollTimelineTest(scrollerOverrides);
+  const verticalScrollRange = scroller.scrollHeight - scroller.clientHeight;
+  const horizontalScrollRange = scroller.scrollWidth - scroller.clientWidth;
+
+  const blockScrollTimeline = new ScrollTimeline(
+      {source: scroller, axis: 'block'});
+  const inlineScrollTimeline = new ScrollTimeline(
+      {source: scroller, axis: 'inline'});
+  const horizontalScrollTimeline = new ScrollTimeline(
+      {source: scroller, axis: 'horizontal'});
+  const verticalScrollTimeline = new ScrollTimeline(
+      {source: scroller, axis: 'vertical'});
+
+  // Unscrolled, all timelines should read a current time of 0 even though the
+  // X-axis will have started at the right hand side for rtl.
+  assert_percents_equal(blockScrollTimeline.currentTime, 0,
+                        'Unscrolled block timeline');
+  assert_percents_equal(inlineScrollTimeline.currentTime, 0,
+                        'Unscrolled inline timeline');
+  assert_percents_equal(horizontalScrollTimeline.currentTime, 0,
+                        'Unscrolled horizontal timeline');
+  assert_percents_equal(verticalScrollTimeline.currentTime, 0,
+                        'Unscrolled vertical timeline');
+
+  // The offset in the inline/horizontal direction should be inverted. The
+  // block/vertical direction should be unaffected.
+  scroller.scrollTop = 0.1 * verticalScrollRange;
+  scroller.scrollLeft = -0.8 * horizontalScrollRange;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  assert_percents_equal(blockScrollTimeline.currentTime, 10,
+                        'Scrolled block timeline');
+  assert_percents_equal(inlineScrollTimeline.currentTime, 80,
+                        'Scrolled inline timeline');
+  assert_percents_equal(horizontalScrollTimeline.currentTime, 80,
+                        'Scrolled horizontal timeline');
+  assert_percents_equal(verticalScrollTimeline.currentTime, 10,
+                        'Scrolled vertical timeline');
+}, 'currentTime handles direction: rtl correctly');
+
+promise_test(async t => {
+  const scrollerOverrides = new Map([['writing-mode', 'vertical-rl']]);
+  const scroller = setupScrollTimelineTest(scrollerOverrides);
+  const verticalScrollRange = scroller.scrollHeight - scroller.clientHeight;
+  const horizontalScrollRange = scroller.scrollWidth - scroller.clientWidth;
+
+  const blockScrollTimeline = new ScrollTimeline(
+      {source: scroller, axis: 'block'});
+  const inlineScrollTimeline = new ScrollTimeline(
+      {source: scroller, axis: 'inline'});
+  const horizontalScrollTimeline = new ScrollTimeline(
+      {source: scroller, axis: 'horizontal'});
+  const verticalScrollTimeline = new ScrollTimeline(
+      {source: scroller, axis: 'vertical'});
+
+  // Unscrolled, all timelines should read a current time of 0 even though the
+  // X-axis will have started at the right hand side for vertical-rl.
+  assert_percents_equal(blockScrollTimeline.currentTime, 0,
+                        'Unscrolled block timeline');
+  assert_percents_equal(inlineScrollTimeline.currentTime, 0,
+                        'Unscrolled inline timeline');
+  assert_percents_equal(horizontalScrollTimeline.currentTime, 0,
+                        'Unscrolled horizontal timeline');
+  assert_percents_equal(verticalScrollTimeline.currentTime, 0,
+                        'Unscrolled vertical timeline');
+
+  // For vertical-rl, the X-axis starts on the right-hand-side and is the block
+  // axis. The Y-axis is normal but is the inline axis. For the
+  // horizontal/vertical cases, horizontal starts on the right-hand-side and
+  // vertical is normal.
+  scroller.scrollTop = 0.1 * verticalScrollRange;
+  scroller.scrollLeft = -0.8 * horizontalScrollRange;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  assert_percents_equal(blockScrollTimeline.currentTime, 80,
+                        'Scrolled block timeline');
+  assert_percents_equal(inlineScrollTimeline.currentTime, 10,
+                        'Scrolled inline timeline');
+  assert_percents_equal(horizontalScrollTimeline.currentTime, 80,
+                        'Scrolled horizontal timeline');
+  assert_percents_equal(verticalScrollTimeline.currentTime, 10,
+                        'Scrolled vertical timeline');
+}, 'currentTime handles writing-mode: vertical-rl correctly');
+
+promise_test(async t => {
+  const scrollerOverrides = new Map([['writing-mode', 'vertical-lr']]);
+  const scroller = setupScrollTimelineTest(scrollerOverrides);
+  const verticalScrollRange = scroller.scrollHeight - scroller.clientHeight;
+  const horizontalScrollRange = scroller.scrollWidth - scroller.clientWidth;
+
+  const blockScrollTimeline = new ScrollTimeline(
+      {source: scroller, axis: 'block'});
+  const inlineScrollTimeline = new ScrollTimeline(
+      {source: scroller, axis: 'inline'});
+  const horizontalScrollTimeline = new ScrollTimeline(
+      {source: scroller, axis: 'horizontal'});
+  const verticalScrollTimeline = new ScrollTimeline(
+      {source: scroller, axis: 'vertical'});
+
+  // Unscrolled, all timelines should read a current time of 0.
+  assert_percents_equal(blockScrollTimeline.currentTime, 0,
+                        'Unscrolled block timeline');
+  assert_percents_equal(inlineScrollTimeline.currentTime, 0,
+                        'Unscrolled inline timeline');
+  assert_percents_equal(horizontalScrollTimeline.currentTime, 0,
+                        'Unscrolled horizontal timeline');
+  assert_percents_equal(verticalScrollTimeline.currentTime, 0,
+                        'Unscrolled vertical timeline');
+
+  // For vertical-lr, both axes start at their 'normal' positions but the X-axis
+  // is the block direction and the Y-axis is the inline direction. This does
+  // not affect horizontal/vertical.
+  scroller.scrollTop = 0.1 * verticalScrollRange;
+  scroller.scrollLeft = 0.2 * horizontalScrollRange;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  assert_percents_equal(blockScrollTimeline.currentTime, 20,
+                        'Scrolled block timeline');
+  assert_percents_equal(inlineScrollTimeline.currentTime, 10,
+                        'Scrolled inline timeline');
+  assert_percents_equal(horizontalScrollTimeline.currentTime, 20,
+                        'Scrolled horizontal timeline');
+  assert_percents_equal(verticalScrollTimeline.currentTime, 10,
+                        'Scrolled vertical timeline');
+}, 'currentTime handles writing-mode: vertical-lr correctly');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/effect-updateTiming.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/effect-updateTiming.html
new file mode 100644
index 0000000000..78ec8e1de5
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/effect-updateTiming.html
@@ -0,0 +1,602 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>Scroll based animation: AnimationEffect.updateTiming</title>
+<!-- Adapted to progressed based scroll animations from "wpt\web-animations\interfaces\AnimationEffect\updateTiming.html" -->
+<link rel="help" href="https://drafts.csswg.org/web-animations-1/#dom-animationeffect-updatetiming">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<script src="/web-animations/resources/easing-tests.js"></script>
+<script src="/web-animations/resources/timing-tests.js"></script>
+<style>
+  .scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+    will-change: transform;
+  }
+  .contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+// ------------------------------
+//  delay
+// ------------------------------
+
+promise_test(async t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, {duration: 1000, delay: 200})
+  anim.play();
+
+  assert_equals(anim.effect.getTiming().delay, 200, 'initial delay 200');
+  assert_equals(anim.effect.getComputedTiming().delay, 200,
+                'getComputedTiming() initially delay 200');
+
+  anim.effect.updateTiming({ delay: 100 });
+  assert_equals(anim.effect.getTiming().delay, 100, 'set delay 100');
+  assert_equals(anim.effect.getComputedTiming().delay, 100,
+                'getComputedTiming() after set delay 100');
+}, 'Allows setting the delay to a positive number');
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, {duration: 100, delay: -100})
+  anim.play();
+  anim.effect.updateTiming({ delay: -100 });
+  assert_equals(anim.effect.getTiming().delay, -100, 'set delay -100');
+  assert_equals(anim.effect.getComputedTiming().delay, -100,
+                'getComputedTiming() after set delay -100');
+  assert_percents_equal(anim.effect.getComputedTiming().endTime, 0,
+                        'getComputedTiming().endTime after set delay -100');
+}, 'Allows setting the delay to a negative number');
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, {duration: 100})
+  anim.play();
+  anim.effect.updateTiming({ delay: 100 });
+  assert_equals(anim.effect.getComputedTiming().progress, null);
+  assert_equals(anim.effect.getComputedTiming().currentIteration, null);
+}, 'Allows setting the delay of an animation in progress: positive delay that'
+   + ' causes the animation to be no longer in-effect');
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { fill: 'both', duration: 100 })
+  anim.play();
+  anim.effect.updateTiming({ delay: -50 });
+  assert_equals(anim.effect.getComputedTiming().progress, 0.5);
+}, 'Allows setting the delay of an animation in progress: negative delay that'
+   + ' seeks into the active interval');
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { fill: 'both', duration: 100 })
+  anim.play();
+  anim.effect.updateTiming({ delay: -100 });
+  assert_equals(anim.effect.getComputedTiming().progress, 1);
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 0);
+}, 'Allows setting the delay of an animation in progress: large negative delay'
+   + ' that causes the animation to be finished');
+
+for (const invalid of gBadDelayValues) {
+  test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t)
+  anim.play();
+    assert_throws_js(TypeError, () => {
+      anim.effect.updateTiming({ delay: invalid });
+    });
+  }, `Throws when setting invalid delay value: ${invalid}`);
+}
+
+
+// ------------------------------
+//  endDelay
+// ------------------------------
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { duration: 2000 })
+  anim.play();
+  anim.effect.updateTiming({ endDelay: 123.45 });
+  assert_time_equals_literal(anim.effect.getTiming().endDelay, 123.45,
+                             'set endDelay 123.45');
+  assert_time_equals_literal(anim.effect.getComputedTiming().endDelay, 123.45,
+                             'getComputedTiming() after set endDelay 123.45');
+}, 'Allows setting the endDelay to a positive number');
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { duration: 2000 })
+  anim.play();
+  anim.effect.updateTiming({ endDelay: -1000 });
+  assert_equals(anim.effect.getTiming().endDelay, -1000, 'set endDelay -1000');
+  assert_equals(anim.effect.getComputedTiming().endDelay, -1000,
+                'getComputedTiming() after set endDelay -1000');
+}, 'Allows setting the endDelay to a negative number');
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { duration: 2000 })
+  anim.play();
+  assert_throws_js(TypeError, () => {
+    anim.effect.updateTiming({ endDelay: Infinity });
+  });
+}, 'Throws when setting the endDelay to infinity');
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { duration: 2000 })
+  anim.play();
+  assert_throws_js(TypeError, () => {
+    anim.effect.updateTiming({ endDelay: -Infinity });
+  });
+}, 'Throws when setting the endDelay to negative infinity');
+
+
+// ------------------------------
+//  fill
+// ------------------------------
+
+for (const fill of ['none', 'forwards', 'backwards', 'both']) {
+  test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { duration: 100 })
+  anim.play();
+    anim.effect.updateTiming({ fill });
+    assert_equals(anim.effect.getTiming().fill, fill, 'set fill ' + fill);
+    assert_equals(anim.effect.getComputedTiming().fill, fill,
+                  'getComputedTiming() after set fill ' + fill);
+  }, `Allows setting the fill to '${fill}'`);
+}
+
+
+// ------------------------------
+//  iterationStart
+// ------------------------------
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { iterationStart: 0.2,
+                                      iterations: 1,
+                                      fill: 'both',
+                                      duration: 100,
+                                      delay: 1 })
+  anim.play();
+  anim.effect.updateTiming({ iterationStart: 2.5 });
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.5);
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 2);
+}, 'Allows setting the iterationStart of an animation in progress:'
+   + ' backwards-filling');
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { iterationStart: 0.2,
+                                      iterations: 1,
+                                      fill: 'both',
+                                      duration: 100,
+                                      delay: 0 })
+  anim.play();
+  anim.effect.updateTiming({ iterationStart: 2.5 });
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.5);
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 2);
+}, 'Allows setting the iterationStart of an animation in progress:'
+   + ' active phase');
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { iterationStart: 0.3,
+                                      iterations: 1,
+                                      fill: 'both',
+                                      duration: 200,
+                                      delay: 0 })
+  anim.play();
+  assert_percents_equal(anim.currentTime, 0);
+  assert_percents_equal(anim.effect.getComputedTiming().localTime, 0, "localTime");
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.3);
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 0);
+
+  anim.finish();
+  assert_percents_equal(anim.currentTime, 100);
+  assert_percents_equal(anim.effect.getComputedTiming().localTime, 100, "localTime");
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.3);
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 1);
+
+  anim.effect.updateTiming({ iterationStart: 2.5 });
+  assert_percents_equal(anim.currentTime, 100);
+  assert_percents_equal(anim.effect.getComputedTiming().localTime, 100, "localTime");
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.5);
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 3);
+}, 'Allows setting the iterationStart of an animation in progress:'
+   + ' forwards-filling');
+
+for (const invalid of gBadIterationStartValues) {
+  test(t => {
+    const anim = createScrollLinkedAnimationWithTiming(t)
+    anim.play();
+    assert_throws_js(TypeError, () => {
+      anim.effect.updateTiming({ iterationStart: invalid });
+    }, `setting ${invalid}`);
+  }, `Throws when setting invalid iterationStart value: ${invalid}`);
+}
+
+// ------------------------------
+//  iterations
+// ------------------------------
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { duration: 2000 })
+  anim.play();
+  anim.effect.updateTiming({ iterations: 2 });
+  assert_equals(anim.effect.getTiming().iterations, 2, 'set duration 2');
+  assert_equals(anim.effect.getComputedTiming().iterations, 2,
+                'getComputedTiming() after set iterations 2');
+}, 'Allows setting iterations to a double value');
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { duration: 2000 })
+  anim.play();
+  assert_throws_js(TypeError, () => {
+    anim.effect.updateTiming({ iterations: Infinity });
+  }, "test");
+}, `Throws when setting iterations to Infinity`);
+
+
+// progress based animations behave a bit differently than time based animations
+// when changing iterations.
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t,  { duration: 100000, fill: 'both' })
+  anim.play();
+  anim.finish();
+
+  assert_equals(anim.effect.getComputedTiming().progress, 1,
+                'progress when animation is finished');
+  assert_percents_equal(anim.effect.getComputedTiming().duration, 100,
+                        'duration when animation is finished');
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 0,
+                'current iteration when animation is finished');
+
+  anim.effect.updateTiming({ iterations: 2 });
+
+  assert_equals(anim.effect.getComputedTiming().progress, 1,
+                'progress after adding an iteration');
+  assert_percents_equal(anim.effect.getComputedTiming().duration, 50,
+                        'duration after adding an iteration');
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 1,
+                'current iteration after adding an iteration');
+
+  anim.effect.updateTiming({ iterations: 4 });
+
+  assert_equals(anim.effect.getComputedTiming().progress, 1,
+                'progress after setting iterations to 4');
+  assert_percents_equal(anim.effect.getComputedTiming().duration, 25,
+                        'duration after setting iterations to 4');
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 3,
+                'current iteration after setting iterations to 4');
+
+  anim.effect.updateTiming({ iterations: 0 });
+
+  assert_equals(anim.effect.getComputedTiming().progress, 0,
+                'progress after setting iterations to zero');
+  assert_percents_equal(anim.effect.getComputedTiming().duration, 0,
+                        'duration after setting iterations to zero');
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 0,
+                'current iteration after setting iterations to zero');
+}, 'Allows setting the iterations of an animation in progress');
+
+// Added test for checking duration "auto"
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t,  { fill: 'both' })
+  anim.play();
+  anim.finish();
+
+  assert_equals(anim.effect.getComputedTiming().progress, 1,
+                'progress when animation is finished');
+  assert_percents_equal(anim.effect.getComputedTiming().duration, 100,
+                        'duration when animation is finished');
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 0,
+                'current iteration when animation is finished');
+
+  anim.effect.updateTiming({ iterations: 2 });
+
+  assert_equals(anim.effect.getComputedTiming().progress, 1,
+                'progress after adding an iteration');
+  assert_percents_equal(anim.effect.getComputedTiming().duration, 50,
+                        'duration after adding an iteration');
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 1,
+                'current iteration after adding an iteration');
+
+  anim.effect.updateTiming({ iterations: 4 });
+
+  assert_equals(anim.effect.getComputedTiming().progress, 1,
+                'progress after setting iterations to 4');
+  assert_percents_equal(anim.effect.getComputedTiming().duration, 25,
+                        'duration after setting iterations to 4');
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 3,
+                'current iteration after setting iterations to 4');
+
+  anim.effect.updateTiming({ iterations: 0 });
+
+  assert_equals(anim.effect.getComputedTiming().progress, 0,
+                'progress after setting iterations to zero');
+  assert_percents_equal(anim.effect.getComputedTiming().duration, 0,
+                        'duration after setting iterations to zero');
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 0,
+                'current iteration after setting iterations to zero');
+}, 'Allows setting the iterations of an animation in progress with duration "auto"');
+
+
+// ------------------------------
+//  duration
+// ------------------------------
+// adapted for progress based animations
+const gGoodDurationValuesForProgressBased = [
+  // until duration returns a CSSNumberish which can handle percentages, 100%
+  // will be represented as 100
+  { specified: 123.45, computed: 100 },
+  { specified: 'auto', computed: 100 },
+];
+
+for (const duration of gGoodDurationValuesForProgressBased) {
+  test(t => {
+    const anim = createScrollLinkedAnimationWithTiming(t, 2000)
+    anim.play();
+    anim.effect.updateTiming({ duration: duration.specified });
+    if (typeof duration.specified === 'number') {
+      assert_time_equals_literal(anim.effect.getTiming().duration,
+                                 duration.specified,
+                                 'Updates specified duration');
+    } else {
+      assert_equals(anim.effect.getTiming().duration, duration.specified,
+                    'Updates specified duration');
+    }
+    assert_percents_equal(anim.effect.getComputedTiming().duration,
+                          duration.computed,
+                          'Updates computed duration');
+  }, `Allows setting the duration to ${duration.specified}`);
+}
+
+// adapted for progress based animations
+const gBadDurationValuesForProgressBased = [
+  -1, NaN, Infinity, -Infinity, 'abc', '100'
+];
+
+for (const invalid of gBadDurationValuesForProgressBased) {
+  test(t => {
+    assert_throws_js(TypeError, () => {
+      const anim = createScrollLinkedAnimationWithTiming(t,  { duration: invalid })
+      anim.play();
+    });
+  }, 'Throws when setting invalid duration: '
+     + (typeof invalid === 'string' ? `"${invalid}"` : invalid));
+}
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t,  { duration: 100000, fill: 'both' })
+  anim.play();
+  anim.finish();
+  assert_equals(anim.effect.getComputedTiming().progress, 1,
+                'progress when animation is finished');
+  anim.effect.updateTiming({ duration: anim.effect.getTiming().duration * 2 });
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 1,
+                             'progress after doubling the duration');
+  anim.effect.updateTiming({ duration: 0 });
+  assert_equals(anim.effect.getComputedTiming().progress, 1,
+                'progress after setting duration to zero');
+  anim.effect.updateTiming({ duration: 'auto' });
+  assert_equals(anim.effect.getComputedTiming().progress, 1,
+                'progress after setting duration to \'auto\'');
+}, 'Allows setting the duration of an animation in progress');
+
+promise_test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t,  { duration: 100000, fill: 'both' })
+  anim.play();
+  return anim.ready.then(() => {
+    const originalStartTime   = anim.startTime;
+    const originalCurrentTime = anim.currentTime;
+    assert_percents_equal(
+      anim.effect.getComputedTiming().duration,
+      100,
+      'Initial duration should be as set on KeyframeEffect'
+    );
+
+    anim.effect.updateTiming({ duration: 200000 });
+    assert_percents_equal(
+      anim.effect.getComputedTiming().duration,
+      100,
+      'Effect duration should remain at 100% for progress based animations'
+    );
+    assert_percents_equal(anim.startTime, originalStartTime,
+                          'startTime should be unaffected by changing effect ' +
+                          'duration');
+
+    assert_percents_equal(anim.currentTime, originalCurrentTime,
+                          'currentTime should be unaffected by changing ' +
+                          'effect duration');
+  });
+}, 'Allows setting the duration of an animation in progress such that the' +
+   ' the start and current time do not change');
+
+
+// ------------------------------
+//  direction
+// ------------------------------
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { duration: 2000 })
+  anim.play();
+
+  const directions = ['normal', 'reverse', 'alternate', 'alternate-reverse'];
+  for (const direction of directions) {
+    anim.effect.updateTiming({ direction: direction });
+    assert_equals(anim.effect.getTiming().direction, direction,
+                  `set direction to ${direction}`);
+  }
+}, 'Allows setting the direction to each of the possible keywords');
+
+promise_test(async t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { duration: 10000, direction: 'normal' });
+
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  anim.play();
+  await anim.ready;
+  scroller.scrollTop = maxScroll * 0.07
+  await waitForNextFrame();
+
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.07,
+                             'progress before updating direction');
+
+  anim.effect.updateTiming({ direction: 'reverse' });
+
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.93,
+                             'progress after updating direction');
+}, 'Allows setting the direction of an animation in progress from \'normal\' to'
+   + ' \'reverse\'');
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { duration: 10000, direction: 'normal' });
+  anim.play();
+  assert_equals(anim.effect.getComputedTiming().progress, 0,
+                'progress before updating direction');
+
+  anim.effect.updateTiming({ direction: 'reverse' });
+
+  assert_equals(anim.effect.getComputedTiming().progress, 1,
+                'progress after updating direction');
+}, 'Allows setting the direction of an animation in progress from \'normal\' to'
+   + ' \'reverse\' while at start of active interval');
+
+test(t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { fill: 'backwards',
+                                      duration: 10000,
+                                      delay: 10000,
+                                      direction: 'normal' });
+  anim.play();
+  assert_equals(anim.effect.getComputedTiming().progress, 0,
+                'progress before updating direction');
+
+  anim.effect.updateTiming({ direction: 'reverse' });
+
+  assert_equals(anim.effect.getComputedTiming().progress, 1,
+                'progress after updating direction');
+}, 'Allows setting the direction of an animation in progress from \'normal\' to'
+   + ' \'reverse\' while filling backwards');
+
+promise_test(async t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { iterations: 2,
+                                      duration: 10000,
+                                      direction: 'normal' });
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  anim.play();
+  await anim.ready;
+  scroller.scrollTop = maxScroll * 0.17 // 34% through first iteration
+  await waitForNextFrame();
+
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.34,
+                             'progress before updating direction');
+
+  anim.effect.updateTiming({ direction: 'alternate' });
+
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.34,
+                             'progress after updating direction');
+}, 'Allows setting the direction of an animation in progress from \'normal\' to'
+   + ' \'alternate\'');
+
+promise_test(async t => {
+  const anim = createScrollLinkedAnimationWithTiming(t, { iterations: 2,
+                                      duration: 10000,
+                                      direction: 'alternate' });
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  anim.play();
+  await anim.ready;
+  scroller.scrollTop = maxScroll * 0.17
+  await waitForNextFrame();
+  // anim.currentTime = 17000; // 34% through first iteration
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.34,
+                             'progress before updating direction');
+
+  anim.effect.updateTiming({ direction: 'alternate-reverse' });
+
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.66,
+                             'progress after updating direction');
+}, 'Allows setting the direction of an animation in progress from \'alternate\''
+   + ' to \'alternate-reverse\'');
+
+
+// ------------------------------
+//  easing
+// ------------------------------
+
+async function assert_progress(animation, scrollPercent, easingFunction) {
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = maxScroll * scrollPercent
+  await waitForNextFrame();
+  assert_approx_equals(animation.effect.getComputedTiming().progress,
+                       easingFunction(scrollPercent),
+                       0.01,
+                       'The progress of the animation should be approximately'
+                       + ` ${easingFunction(scrollPercent)} at ${scrollPercent}%`);
+}
+
+for (const options of gEasingTests) {
+  promise_test(async t => {
+    const anim = createScrollLinkedAnimationWithTiming(t, { duration: 100000,
+                                  fill: 'forwards' });
+    anim.play();
+    anim.effect.updateTiming({ easing: options.easing });
+    assert_equals(anim.effect.getTiming().easing,
+                  options.serialization || options.easing);
+
+    const easing = options.easingFunction;
+    await assert_progress(anim, 0, easing);
+    await assert_progress(anim, 0.25, easing);
+    await assert_progress(anim, 0.5, easing);
+    await assert_progress(anim, 0.75, easing);
+    await assert_progress(anim, 1, easing);
+  }, `Allows setting the easing to a ${options.desc}`);
+}
+
+for (const easing of gRoundtripEasings) {
+  test(t => {
+    const anim = createScrollLinkedAnimationWithTiming(t);
+    anim.play();
+    anim.effect.updateTiming({ easing: easing });
+    assert_equals(anim.effect.getTiming().easing, easing);
+  }, `Updates the specified value when setting the easing to '${easing}'`);
+}
+
+// Because of the delay being so large, this progress based animation is always
+// in the finished state with progress 1. Included here because it is in the
+// original test file for time based animations.
+promise_test(async t => {
+  const delay = 1000000;
+
+  const anim = createScrollLinkedAnimationWithTiming(t,
+    { duration: 1000000, fill: 'both', delay: delay, easing: 'steps(2, start)' });
+
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  anim.play();
+  await anim.ready;
+
+  anim.effect.updateTiming({ easing: 'steps(2, end)' });
+  assert_equals(anim.effect.getComputedTiming().progress, 0,
+                'easing replace to steps(2, end) at before phase');
+
+  scroller.scrollTop = maxScroll * 0.875
+  await waitForNextFrame();
+
+  assert_equals(anim.effect.getComputedTiming().progress, 0.5,
+                'change currentTime to active phase');
+
+  anim.effect.updateTiming({ easing: 'steps(2, start)' });
+  assert_equals(anim.effect.getComputedTiming().progress, 1,
+                'easing replace to steps(2, start) at active phase');
+
+  scroller.scrollTop = maxScroll * 1.25
+  await waitForNextFrame();
+
+  anim.effect.updateTiming({ easing: 'steps(2, end)' });
+  assert_equals(anim.effect.getComputedTiming().progress, 1,
+                'easing replace to steps(2, end) again at after phase');
+}, 'Allows setting the easing of an animation in progress');
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/finish-animation.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/finish-animation.html
new file mode 100644
index 0000000000..fd768651b4
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/finish-animation.html
@@ -0,0 +1,449 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Finishing an animation</title>
+<link rel="help"
+  href="https://drafts.csswg.org/web-animations/#finishing-an-animation-section">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+.scroller {
+  overflow: auto;
+  height: 200px;
+  width: 100px;
+  will-change: transform;
+}
+
+.contents {
+  height: 1000px;
+  width: 100%;
+}
+</style>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.playbackRate = 0;
+
+  assert_throws_dom('InvalidStateError', () => {
+    animation.finish();
+  });
+}, 'Finishing an animation with a zero playback rate throws');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.finish();
+
+  assert_percents_equal(animation.currentTime, 100,
+    'After finishing, the currentTime should be set to the end of the'
+    + ' active duration');
+}, 'Finishing an animation seeks to the end time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  // 1% past effect end
+  animation.currentTime = CSSNumericValue.parse(animation.effect.getComputedTiming().endTime.value + 1 + "%");
+  animation.finish();
+
+  assert_percents_equal(animation.currentTime, 100,
+    'After finishing, the currentTime should be set back to the end of the'
+    + ' active duration');
+}, 'Finishing an animation with a current time past the effect end jumps'
+  + ' back to the end');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  scroller.scrollTop = maxScroll;
+  await animation.finished;
+
+  animation.playbackRate = -1;
+  animation.finish();
+
+  assert_percents_equal(animation.currentTime, 0,
+                        'After finishing a reversed animation the ' +
+                        'currentTime should be set to zero');
+}, 'Finishing a reversed animation jumps to zero time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.currentTime = CSSNumericValue.parse("100%");
+  await animation.finished;
+
+  animation.playbackRate = -1;
+  animation.currentTime = CSSNumericValue.parse("-1000%");
+  animation.finish();
+
+  assert_percents_equal(animation.currentTime, 0,
+                        'After finishing a reversed animation the ' +
+                        'currentTime should be set back to zero');
+}, 'Finishing a reversed animation with a current time less than zero'
+  + ' makes it jump back to zero');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.playbackRate = 0.5;
+  animation.finish();
+
+  assert_false(animation.pending);
+  assert_equals(animation.playState, 'finished',
+                'The play state of a play-pending animation should become ' +
+                '"finished"');
+  assert_percents_equal(animation.startTime,
+                        animation.timeline.currentTime.value - 100 / 0.5,
+                        'The start time of a play-pending animation should ' +
+                        'be set');
+}, 'Finishing an animation while play-pending resolves the pending'
+  + ' task immediately');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  // Make the scroll timeline inactive.
+  scroller.style.overflow = 'visible';
+  scroller.scrollTop;
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.finish();
+
+  await animation.finished;
+
+  assert_true(animation.pending);
+  assert_equals(animation.playState, 'finished',
+                'The play state of a play-pending animation should become ' +
+                '"finished"');
+  assert_percents_equal(animation.currentTime, 100,
+                        'The current time of a play-pending animation should ' +
+                        'be set to the end of the active duration');
+  assert_equals(animation.startTime, null,
+                'The start time of a finished play-pending animation should ' +
+                'be be unresolved');
+}, 'Finishing an animation attached to inactive timeline while play-pending '
+  + 'doesn\'t resolves the pending task');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  let resolvedFinished = false;
+  animation.finished.then(() => {
+    resolvedFinished = true;
+  });
+
+  await animation.ready;
+
+  animation.finish();
+  await Promise.resolve();
+
+  assert_true(resolvedFinished, 'finished promise should be resolved');
+}, 'Finishing an animation resolves the finished promise synchronously');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  const promise = animation.ready;
+  let readyResolved = false;
+
+  animation.finish();
+  animation.ready.then(() => { readyResolved = true; });
+
+  const promiseResult = await animation.finished;
+  await animation.ready;
+
+  assert_equals(promiseResult, animation);
+  assert_equals(animation.ready, promise);
+  assert_true(readyResolved);
+}, 'A pending ready promise is resolved and not replaced when the animation'
+  + ' is finished');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.effect.target.remove();
+
+  const eventWatcher = new EventWatcher(t, animation, 'finish');
+
+  await animation.ready;
+  animation.finish();
+
+  await eventWatcher.wait_for('finish');
+  assert_equals(animation.effect.target.parentNode, null,
+    'finish event should be fired for the animation on an orphaned element');
+}, 'Finishing an animation fires finish event on orphaned element');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  const originalFinishPromise = animation.finished;
+
+  animation.cancel();
+  assert_equals(animation.startTime, null);
+  assert_equals(animation.currentTime, null);
+
+  const resolvedFinishPromise = animation.finished;
+  assert_not_equals(originalFinishPromise, resolvedFinishPromise,
+              'Canceling an animation should create a new finished promise');
+
+  animation.finish();
+  assert_equals(animation.playState, 'finished',
+                'The play state of a canceled animation should become ' +
+                '"finished"');
+  assert_percents_equal(animation.startTime,
+                        animation.timeline.currentTime.value - 100,
+                        'The start time of a finished animation should be set');
+  assert_percents_equal(animation.currentTime, 100,
+                        'Hold time should be set to end boundary of the ' +
+                        'animation');
+
+}, 'Finishing a canceled animation sets the current and start times');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.25 * maxScroll;
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  const eventWatcher = new EventWatcher(t, animation, 'finish');
+  animation.finish();
+  await animation.finished;
+  const finishEvent = await eventWatcher.wait_for('finish');
+  assert_equals(animation.playState, 'finished',
+    'Animation is finished.');
+  assert_percents_equal(animation.currentTime, 100,
+    'The current time is the end of the active duration in finished state.');
+  assert_percents_equal(animation.startTime, -75,
+    'The start time is calculated to match the current time.');
+  assert_percents_equal(finishEvent.currentTime, 100,
+    'event.currentTime is the animation current time.');
+  assert_percents_equal(finishEvent.timelineTime, 25,
+    'event.timelineTime is timeline.currentTime');
+}, 'Finishing idle animation produces correct state and fires finish event.');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  // Make the scroll timeline inactive.
+  scroller.style.overflow = 'visible';
+  await waitForNextFrame();
+  assert_equals(animation.timeline.currentTime, null,
+                'Sanity check the timeline is inactive.');
+  animation.finish();
+  assert_equals(animation.playState, 'paused', 'Animation is paused.');
+}, 'Finishing idle animation attached to inactive timeline pauses the ' +
+    'animation.');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.25 * maxScroll;
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  const eventWatcher = new EventWatcher(t, animation, 'finish');
+  animation.finish();
+  await animation.finished;
+  const finishEvent = await eventWatcher.wait_for('finish');
+  assert_equals(animation.playState, 'finished',
+    'Animation is finished.');
+  assert_percents_equal(animation.currentTime, 100,
+    'The current time is the end of active duration in finished state.');
+  assert_percents_equal(animation.startTime, -75,
+    'The start time is calculated to match animation current time.');
+  assert_percents_equal(finishEvent.currentTime, 100,
+    'event.currentTime is the animation current time.');
+  assert_percents_equal(finishEvent.timelineTime, 25,
+    'event.timelineTime is timeline.currentTime');
+}, 'Finishing running animation produces correct state and fires finish event.');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  // Make the scroll timeline inactive.
+  scroller.style.overflow = 'visible';
+  scroller.scrollTop;
+  await waitForNextFrame();
+  assert_equals(animation.timeline.currentTime, null,
+                'Sanity check the timeline is inactive.');
+  assert_equals(animation.playState, 'running',
+                'Sanity check the animation is running.');
+
+  animation.finish();
+  assert_equals(animation.playState, 'paused', 'Animation is paused.');
+}, 'Finishing running animation attached to inactive timeline pauses the ' +
+    'animation.');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.pause();
+  await animation.ready;
+
+  animation.finish();
+
+  assert_equals(animation.playState, 'finished',
+                'The play state of a paused animation should become ' +
+                '"finished"');
+  assert_percents_equal(animation.startTime,
+                    animation.timeline.currentTime.value - 100,
+                    'The start time of a paused animation should be set');
+}, 'Finishing a paused animation resolves the start time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  // Update playbackRate so we can test that the calculated startTime
+  // respects it
+  animation.playbackRate = 2;
+  animation.pause();
+  // While animation is still pause-pending call finish()
+  animation.finish();
+
+  assert_false(animation.pending);
+  assert_equals(animation.playState, 'finished',
+                'The play state of a pause-pending animation should become ' +
+                '"finished"');
+  assert_percents_equal(animation.startTime,
+                        animation.timeline.currentTime.value - 100 / 2,
+                        'The start time of a pause-pending animation should ' +
+                        'be set');
+}, 'Finishing a pause-pending animation resolves the pending task'
+  + ' immediately and update the start time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.playbackRate = -2;
+  animation.pause();
+  animation.finish();
+
+  assert_false(animation.pending);
+  assert_equals(animation.playState, 'finished',
+                'The play state of a pause-pending animation should become ' +
+                '"finished"');
+  assert_percents_equal(animation.startTime,
+                        animation.timeline.currentTime,
+                        'The start time of a pause-pending animation should ' +
+                        'be set');
+}, 'Finishing a pause-pending animation with negative playback rate'
+  + ' resolves the pending task immediately');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  animation.pause();
+  animation.play();
+  // We are now in the unusual situation of being play-pending whilst having
+  // a resolved start time. Check that finish() still triggers a transition
+  // to the finished state immediately.
+  animation.finish();
+
+  assert_equals(animation.playState, 'finished',
+                'After aborting a pause then finishing an animation its play ' +
+                'state should become "finished" immediately');
+}, 'Finishing an animation during an aborted pause makes it finished'
+  + ' immediately');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  animation.updatePlaybackRate(2);
+  assert_true(animation.pending);
+
+  animation.finish();
+  assert_false(animation.pending);
+  assert_equals(animation.playbackRate, 2);
+  assert_percents_equal(animation.currentTime, 100);
+}, 'A pending playback rate should be applied immediately when an animation'
+  + ' is finished');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  animation.updatePlaybackRate(0);
+
+  assert_throws_dom('InvalidStateError', () => {
+    animation.finish();
+  });
+}, 'An exception should be thrown if the effective playback rate is zero');
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/idlharness.window.js b/testing/web-platform/tests/scroll-animations/scroll-timelines/idlharness.window.js
new file mode 100644
index 0000000000..90157580ce
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/idlharness.window.js
@@ -0,0 +1,16 @@
+// META: script=/resources/WebIDLParser.js
+// META: script=/resources/idlharness.js
+
+'use strict';
+
+idl_test(
+  ['scroll-animations'],
+  // The css-pseudo dependency shouldn't be necessary, but is:
+  // https://github.com/web-platform-tests/wpt/issues/12574
+  ['web-animations', 'css-pseudo', 'dom'],
+  idl_array => {
+    idl_array.add_objects({
+      ScrollTimeline: ['new ScrollTimeline()'],
+    });
+  }
+);
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/layout-changes-on-percentage-based-timeline.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/layout-changes-on-percentage-based-timeline.html
new file mode 100644
index 0000000000..c5a46a501e
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/layout-changes-on-percentage-based-timeline.html
@@ -0,0 +1,84 @@
+<html class="reftest-wait">
+<title>Layout changes on percentage-based scroll timeline</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations/">
+<meta name="assert" content="Scroll timeline should properly handle
+layout changes on percentage-based scroll offset">
+<link rel="match" href="animation-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+    color: red;
+  }
+
+  #scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+    will-change: transform;
+  }
+
+  #contents {
+    height: 500px;
+    width: 100%;
+  }
+
+  #spacer {
+    height: 80px;
+  }
+
+  .invisible {
+    display: none;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered">Scrolling Test</div>
+<div id="scroller">
+  <div id="contents"></div>
+  <div id="spacer" class="invisible"></div>
+</div>
+
+<script>
+  const box = document.getElementById('box');
+  const effect = new KeyframeEffect(box,
+    [
+      { transform: 'translateY(0)', opacity: 1 },
+      { transform: 'translateY(200px)', opacity: 0 }
+    ]
+  );
+
+  const scroller = document.getElementById('scroller');
+  const timeline = new ScrollTimeline({
+    source: scroller
+  });
+  const animation = new Animation(effect, timeline);
+  animation.play();
+  animation.ready.then(_ => {
+    // Moves the scroller to the end point (240px).
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = maxScroll * 0.6;
+
+    // Makes sure that the animation runs on compositor with current scroll offset
+    waitForAnimationFrames(2).then(_ => {
+      // Adds 80px to scroll height which pushes scroll progress back to 50%.
+      const spacer = document.getElementById('spacer');
+      spacer.classList.remove('invisible');
+      // Makes sure that the change is propagated to the compositor.
+      waitForAnimationFrames(2).then(_ => {
+        takeScreenshot();
+      });
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/null-scroll-source-crash.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/null-scroll-source-crash.html
new file mode 100644
index 0000000000..53ad0d9285
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/null-scroll-source-crash.html
@@ -0,0 +1,24 @@
+<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1088319">
+<meta name="assert" content="Playing animation with null scroll source should not crash.">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  html {
+    overflow: scroll;
+  }
+
+  body {
+    overflow: scroll;
+  }
+</style>
+<div id="box"></div>
+<script>
+  test(() => {
+    const effect = new KeyframeEffect(box, []);
+    const timeline = new ScrollTimeline();
+    const animation = new Animation(effect, timeline);
+    assert_equals(document.scrollingElement, null,
+      "This test relies on scrolling element being nil");
+    animation.play();
+  }, "Playing animation with null scroll source should not crash");
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/pause-animation.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/pause-animation.html
new file mode 100644
index 0000000000..9b263f843d
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/pause-animation.html
@@ -0,0 +1,210 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Pausing an animation</title>
+<link rel="help"
+  href="https://drafts.csswg.org/web-animations/#pausing-an-animation-section">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+.scroller {
+  overflow: auto;
+  height: 100px;
+  width: 100px;
+  will-change: transform;
+}
+
+.contents {
+  height: 1000px;
+  width: 100%;
+}
+</style>
+<body>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  const startTimeBeforePausing = animation.startTime;
+
+  animation.pause();
+  assert_percents_equal(animation.startTime, startTimeBeforePausing,
+                        'The start time does not change when pausing-pending');
+
+  await animation.ready;
+
+  assert_equals(animation.startTime, null,
+                'The start time is unresolved when paused');
+}, 'Pausing clears the start time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  animation.pause();
+  assert_not_equals(animation.startTime, null,
+                    'The start time is resolved when pause-pending');
+
+  animation.play();
+  assert_not_equals(animation.startTime, null,
+                    'The start time is preserved when a pause is aborted');
+}, 'Aborting a pause preserves the start time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  const promise = animation.ready;
+  animation.pause();
+
+  const promiseResult = await promise;
+
+  assert_equals(promiseResult, animation);
+  assert_equals(animation.ready, promise);
+  assert_false(animation.pending, 'No longer pause-pending');
+}, 'A pending ready promise should be resolved and not replaced when the'
+   + ' animation is paused');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  // Let animation start roughly half-way through
+  animation.currentTime = CSSNumericValue.parse("50%");
+  await animation.ready;
+
+  // Go pause-pending and also set a pending playback rate
+  animation.pause();
+  animation.updatePlaybackRate(0.5);
+
+  await animation.ready;
+  // If the current time was updated using the new playback rate it will jump
+  // back to 25s but if we correctly used the old playback rate the current time
+  // will be >= 50s.
+  assert_greater_than_equal(animation.currentTime.value, 50);
+}, 'A pause-pending animation maintains the current time when applying a'
+   + ' pending playback rate');
+
+promise_test(async t => {
+  // This test does not cover a specific step in the algorithm but serves as a
+  // high-level sanity check that pausing does, in fact, freeze the current
+  // time.
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  animation.pause();
+  await animation.ready;
+
+  const currentTimeAfterPausing = animation.currentTime;
+
+  scroller.scrollTop = 0.2 * maxScroll;
+  await waitForNextFrame();
+  assert_percents_equal(animation.timeline.currentTime, 20,
+                        'Sanity check timeline time changed');
+
+  assert_percents_equal(animation.currentTime, currentTimeAfterPausing,
+                        'Animation.currentTime is unchanged after pausing');
+}, 'The animation\'s current time remains fixed after pausing');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+
+  const originalReadyPromise = animation.ready;
+  animation.cancel();
+  assert_equals(animation.startTime, null);
+  assert_equals(animation.currentTime, null);
+
+  const readyPromise = animation.ready;
+  assert_not_equals(originalReadyPromise, readyPromise,
+                    'Canceling an animation should create a new ready promise');
+
+  animation.pause();
+  assert_equals(animation.playState, 'paused',
+                'Pausing a canceled animation should update the play state');
+  assert_true(animation.pending, 'animation should be pause-pending');
+  await animation.ready;
+  assert_false(animation.pending,
+               'animation should no longer be pause-pending');
+  assert_equals(animation.startTime, null, 'start time should be unresolved');
+  assert_percents_equal(animation.currentTime, 0,
+                        'current time should be set to zero');
+}, 'Pausing a canceled animation sets the current time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  // Make the scroll timeline inactive.
+  scroller.style.overflow = 'visible';
+  scroller.scrollTop;
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  assert_equals(animation.timeline.currentTime, null,
+                'Sanity check the timeline is inactive.');
+  // Pause the animation when the timeline is inactive.
+  animation.pause();
+  assert_equals(animation.currentTime, null,
+                'The current time is null when the timeline is inactive.');
+  assert_percents_equal(animation.startTime, 0,
+                        'The start time is zero in Pending state.');
+  await waitForNextFrame();
+  assert_true(animation.pending,
+      'Animation has pause pending task while the timeline is inactive.');
+  assert_equals(animation.playState, 'paused',
+                `State is 'paused' in Pending state.`);
+}, `Pause pending task doesn't run when the timeline is inactive.`);
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.2 * maxScroll;
+  // Make the scroll timeline inactive.
+  scroller.style.overflow = 'visible';
+  scroller.scrollTop;
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  assert_equals(animation.timeline.currentTime, null,
+                'Sanity check the timeline is inactive.');
+  // Play the animation when the timeline is inactive.
+  animation.pause();
+
+  // Make the scroll timeline active.
+  scroller.style.overflow = 'auto';
+  await animation.ready;
+  // Ready promise is resolved as a result of the timeline becoming active.
+  assert_percents_equal(animation.currentTime, 20,
+      'Animation current time is resolved when the animation is ready.');
+  assert_equals(animation.startTime, null,
+      'Animation start time is unresolved when the animation is ready.');
+}, 'Animation start and current times are correct if scroll timeline is ' +
+   'activated after animation.pause call.');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/play-animation.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/play-animation.html
new file mode 100644
index 0000000000..d408547f55
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/play-animation.html
@@ -0,0 +1,233 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Playing an animation</title>
+<link rel="help"
+      href="https://drafts.csswg.org/web-animations/#playing-an-animation-section">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+.scroller {
+  overflow: auto;
+  height: 100px;
+  width: 100px;
+  will-change: transform;
+}
+
+.contents {
+  height: 1000px;
+  width: 100%;
+}
+</style>
+<body>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.currentTime = CSSNumericValue.parse("10%");
+  assert_percents_equal(animation.currentTime, 10);
+  animation.play();
+  assert_percents_equal(animation.currentTime, 10);
+}, 'Playing a running animation leaves the current time unchanged');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.finish();
+  assert_percents_equal(animation.currentTime, 100);
+  animation.play();
+  assert_percents_equal(animation.currentTime, 0);
+}, 'Playing a finished animation seeks back to the start');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.playbackRate = -1;
+  animation.currentTime = CSSNumericValue.parse("0%");
+  assert_percents_equal(animation.currentTime, 0);
+  animation.play();
+  assert_percents_equal(animation.currentTime, 100);
+}, 'Playing a finished and reversed animation seeks to end');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.finish();
+
+  // Initiate a pause then abort it
+  animation.pause();
+  animation.play();
+
+  // Wait to return to running state
+  await animation.ready;
+
+  assert_percents_equal(animation.currentTime, 0,
+      'After aborting a pause when finished, the current time should jump ' +
+      'back to the start of the animation');
+}, 'Playing a pause-pending but previously finished animation seeks back to'
+   + ' to the start');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.finish();
+  await animation.ready;
+
+  animation.play();
+  assert_percents_equal(animation.startTime, 0, 'start time is zero');
+}, 'Playing a finished animation clears the start time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.cancel();
+  const promise = animation.ready;
+  animation.play();
+  assert_not_equals(animation.ready, promise);
+}, 'The ready promise should be replaced if the animation is not already'
+   + ' pending');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  const promise = animation.ready;
+  const promiseResult = await promise;
+  assert_equals(promiseResult, animation);
+  assert_equals(animation.ready, promise);
+}, 'A pending ready promise should be resolved and not replaced when the'
+   + ' animation enters the running state');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.currentTime = CSSNumericValue.parse("50%");
+  await animation.ready;
+
+  animation.pause();
+  await animation.ready;
+
+  const holdTime = animation.currentTime;
+
+  animation.play();
+  await animation.ready;
+
+  assert_percents_equal(
+      animation.startTime,
+      animation.timeline.currentTime.value - holdTime.value);
+}, 'Resuming an animation from paused calculates start time from hold time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  // Go to pause-pending state
+  animation.pause();
+  assert_true(animation.pending, 'Animation is pending');
+  const pauseReadyPromise = animation.ready;
+
+  // Now play again immediately (abort the pause)
+  animation.play();
+  assert_true(animation.pending, 'Animation is still pending');
+  assert_equals(animation.ready, pauseReadyPromise,
+      'The pause Promise is re-used when playing while waiting to pause');
+
+  // Sanity check: Animation proceeds to running state
+  await animation.ready;
+  assert_true(!animation.pending && animation.playState === 'running',
+      'Animation is running after aborting a pause');
+}, 'If a pause operation is interrupted, the ready promise is reused');
+
+promise_test(async t => {
+  // Seek animation beyond target end
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.currentTime = CSSNumericValue.parse("-100%");
+  await animation.ready;
+
+  // Set pending playback rate to the opposite direction
+  animation.updatePlaybackRate(-1);
+  assert_true(animation.pending);
+  assert_equals(animation.playbackRate, 1);
+
+  // When we play, we should seek to the target end, NOT to zero (which
+  // is where we would seek to if we used the playbackRate of 1.
+  animation.play();
+  assert_percents_equal(animation.currentTime, 100);
+}, 'A pending playback rate is used when determining auto-rewind behavior');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.cancel();
+  assert_equals(animation.startTime, null,
+                'Start time should be unresolved');
+
+  animation.play();
+  assert_true(animation.pending, 'Animation should be play-pending');
+
+  await animation.ready;
+
+  assert_false(animation.pending, 'animation should no longer be pending');
+  assert_percents_equal(animation.startTime, 0,
+      'The start time of the playing animation should be zero');
+}, 'Playing a canceled animation sets the start time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.playbackRate = -1;
+  animation.cancel();
+  assert_equals(animation.startTime, null, 'Start time should be unresolved');
+
+  animation.play();
+  assert_true(animation.pending, 'Animation should be play-pending');
+
+  await animation.ready;
+
+  assert_false(animation.pending, 'Animation should no longer be pending');
+  assert_percents_equal(animation.startTime, 100,
+      'The start time of the playing animation should be set');
+}, 'Playing a canceled animation backwards sets the start time');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/progress-based-effect-delay-ref.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/progress-based-effect-delay-ref.html
new file mode 100644
index 0000000000..59366a88dd
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/progress-based-effect-delay-ref.html
@@ -0,0 +1,45 @@
+<!DOCTYPE html>
+<title>Reference for Web Animation with scroll timeline and effect delay tests</title>
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    transform: translate(0, 100px);
+    opacity: 0.5;
+    will-change: transform; /* force compositing */
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  #scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+    will-change: transform; /* force compositing */
+  }
+
+  #contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+<div id="scroller">
+  <div id="contents"></div>
+</div>
+
+<script>
+  window.addEventListener('load', function() {
+    // Move the scroller to halfway.
+    const scroller = document.getElementById("scroller");
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.75 * maxScroll;
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/progress-based-effect-delay.tentative.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/progress-based-effect-delay.tentative.html
new file mode 100644
index 0000000000..525d8448ff
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/progress-based-effect-delay.tentative.html
@@ -0,0 +1,69 @@
+<html class="reftest-wait">
+<title>Animation effect delays should be accounted for when using a progress based timeline</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations/">
+<meta name="assert" content="Effect delay should be accounted for by progress based animations">
+<link rel="match" href="progress-based-effect-delay-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  #scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+    will-change: transform; /* force compositing */
+  }
+
+  #contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+<div id="scroller">
+  <div id="contents"><p>Scrolling Contents</p></div>
+</div>
+<script>
+  const box = document.getElementById('box');
+  const effect = new KeyframeEffect(box,
+    [
+    { transform: 'translateY(0)', opacity: 1},
+    { transform: 'translateY(200px)', opacity: 0}
+    ], {
+      delay: 1000,
+      duration: 1000
+    }
+  );
+
+  const scroller = document.getElementById('scroller');
+  const timeline = new ScrollTimeline(
+      { source: scroller, orientation: 'block' });
+  const animation = new Animation(effect, timeline);
+
+  animation.play();
+
+  animation.ready.then(() => {
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.75 * maxScroll;
+
+    waitForAnimationFrames(2).then(_ => {
+        takeScreenshot();
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/reverse-animation.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/reverse-animation.html
new file mode 100644
index 0000000000..cf6173b60a
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/reverse-animation.html
@@ -0,0 +1,240 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Reversing an animation</title>
+<link rel="help"
+      href="https://drafts.csswg.org/web-animations/#reversing-an-animation-section">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+.scroller {
+  overflow: auto;
+  height: 100px;
+  width: 100px;
+  will-change: transform;
+}
+
+.contents {
+  height: 1000px;
+  width: 100%;
+}
+</style>
+<body>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  animation.currentTime = CSSNumericValue.parse("50%");
+  const previousPlaybackRate = animation.playbackRate;
+  animation.reverse();
+  assert_equals(animation.playbackRate, previousPlaybackRate,
+                'Playback rate should not have changed');
+  await animation.ready;
+
+  assert_equals(animation.playbackRate, -previousPlaybackRate,
+                'Playback rate should be inverted');
+}, 'Reversing an animation inverts the playback rate');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.currentTime = CSSNumericValue.parse("50%");
+  animation.reverse();
+
+  assert_percents_equal(animation.currentTime, 50,
+                        'The current time should not change it is in the ' +
+                        'middle of the animation duration');
+}, 'Reversing an animation maintains the same current time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  assert_true(animation.pending,
+              'The animation is pending before we call reverse');
+
+  animation.reverse();
+
+  assert_true(animation.pending,
+              'The animation is still pending after calling reverse');
+}, 'Reversing an animation does not cause it to leave the pending state');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  let readyResolved = false;
+  animation.ready.then(() => { readyResolved = true; });
+
+  animation.reverse();
+
+  await Promise.resolve();
+  assert_false(readyResolved,
+               'ready promise should not have been resolved yet');
+}, 'Reversing an animation does not cause it to resolve the ready promise');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.currentTime = CSSNumericValue.parse("200%");
+  animation.reverse();
+
+  assert_percents_equal(animation.currentTime, 100,
+                        'reverse() should start playing from the animation ' +
+                        'effect end if the playbackRate > 0 and the ' +
+                        'currentTime > effect end');
+}, 'Reversing an animation when playbackRate > 0 and currentTime > effect ' +
+   'end should make it play from the end');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+
+  animation.currentTime = CSSNumericValue.parse("-200%");
+  animation.reverse();
+
+  assert_percents_equal(animation.currentTime, 100,
+                        'reverse() should start playing from the animation ' +
+                        'effect end if the playbackRate > 0 and the ' +
+                        'currentTime < 0');
+}, 'Reversing an animation when playbackRate > 0 and currentTime < 0 should ' +
+   'make it play from the end');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.playbackRate = -1;
+  animation.currentTime = CSSNumericValue.parse("-200%");
+  animation.reverse();
+
+  assert_percents_equal(animation.currentTime, 0,
+                        'reverse() should start playing from the start of ' +
+                        'animation time if the playbackRate < 0 and the ' +
+                        'currentTime < 0');
+}, 'Reversing an animation when playbackRate < 0 and currentTime < 0 should ' +
+   'make it play from the start');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.playbackRate = -1;
+  animation.currentTime = CSSNumericValue.parse("200%");
+  animation.reverse();
+
+  assert_percents_equal(animation.currentTime, 0,
+                        'reverse() should start playing from the start of ' +
+                        'animation time if the playbackRate < 0 and the ' +
+                        'currentTime > effect end');
+}, 'Reversing an animation when playbackRate < 0 and currentTime > effect ' +
+   'end should make it play from the start');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.playbackRate = 0;
+  animation.currentTime = CSSNumericValue.parse("50%");
+  animation.reverse();
+
+  await animation.ready;
+  assert_equals(animation.playbackRate, 0,
+      'reverse() should preserve playbackRate if the playbackRate == 0');
+  assert_percents_equal(animation.currentTime, 50,
+      'reverse() should not affect the currentTime if the playbackRate == 0');
+}, 'Reversing when when playbackRate == 0 should preserve the current time ' +
+   'and playback rate');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  assert_equals(animation.currentTime, null);
+
+  animation.reverse();
+  await animation.ready;
+
+  assert_percents_equal(animation.startTime, 100,
+                        'animation.startTime should be at its effect end');
+  assert_percents_equal(animation.currentTime, 100,
+                        'animation.currentTime should be at its effect end');
+}, 'Reversing an idle animation from starts playing the animation');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  // Make the scroll timeline inactive.
+  scroller.style.overflow = 'visible';
+  scroller.scrollTop;
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  assert_throws_dom('InvalidStateError', () => { animation.reverse(); });
+}, 'Reversing an animation without an active timeline throws an ' +
+   'InvalidStateError');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.currentTime = CSSNumericValue.parse("50%");
+  animation.pause();
+
+  await animation.ready;
+
+  animation.reverse();
+  await animation.ready;
+
+  assert_equals(animation.playState, 'running',
+                'Animation.playState should be "running" after reverse()');
+}, 'Reversing an animation plays a pausing animation');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  animation.updatePlaybackRate(2);
+  animation.reverse();
+
+  await animation.ready;
+  assert_equals(animation.playbackRate, -2);
+}, 'Reversing should use the negative pending playback rate');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation-effect-fill-modes.tentative.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation-effect-fill-modes.tentative.html
new file mode 100644
index 0000000000..b9cc154676
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation-effect-fill-modes.tentative.html
@@ -0,0 +1,137 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Verify applied effect output for all fill modes in all timeline states: before start, at start, in range, at end, after end while using various effect delay values</title>
+<meta name="timeout" content="long">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+  .scroller {
+    overflow: hidden;
+    height: 200px;
+    width: 200px;
+  }
+  .contents {
+    /* Make scroll range 1000 to simplify the math and avoid rounding errors */
+    height: 1200px;
+    width: 100%;
+  }
+</style>
+<div id="log"></div>
+<script>
+  'use strict';
+
+  test(t => {
+    const effect = new KeyframeEffect(createDiv(t), { opacity: [0.3, 0.7] });
+    const animation = new Animation(effect, createScrollTimeline(t));
+
+    assert_equals(effect.getTiming().fill, "auto");
+    assert_equals(effect.getComputedTiming().fill, "none");
+  }, "Scroll based animation effect fill mode should return 'auto' for" +
+     " getTiming() and should return 'none' for getComputedTiming().")
+
+  /* All interesting transitions:
+      before start delay
+      at start delay
+      within active phase
+      at effect end
+      after effect end
+
+      test_case data structure:
+      fill_mode: {
+        scroll_percentage: ["state description", expected applied effect value]
+      }
+  */
+  const test_cases = {
+    "none": {
+      0.10: ["before start delay", 1],
+      0.25: ["at start delay", 0.3],
+      0.50: ["at midpoint", 0.5],
+      0.75: ["at effect end", 1],
+      0.90: ["after effect end", 1]
+    },
+    "backwards": {
+      0.10: ["before start delay", 0.3],
+      0.25: ["at start delay", 0.3],
+      0.50: ["at midpoint", 0.5],
+      0.75: ["at effect end", 1],
+      0.90: ["after effect end", 1]
+    },
+    "forwards": {
+      0.10: ["before timeline start", 1],
+      0.25: ["at timeline start", 0.3],
+      0.50: ["in timeline range", 0.5],
+      0.75: ["at timeline end", 0.7],
+      0.90: ["after timeline end", 0.7]
+    },
+    "both": {
+      0.10: ["before timeline start", 0.3],
+      0.25: ["at timeline start", 0.3],
+      0.50: ["in timeline range", 0.5],
+      0.75: ["at timeline end", 0.7],
+      0.90: ["after timeline end", 0.7]
+    },
+    "auto": {
+      0.10: ["before timeline start", 1],
+      0.25: ["at timeline start", 0.3],
+      0.50: ["in timeline range", 0.5],
+      0.75: ["at timeline end", 1],
+      0.90: ["after timeline end", 1]
+    }
+  }
+
+  for (const fill_mode in test_cases) {
+    const scroll_percents = test_cases[fill_mode]
+
+    for (const scroll_percentage in scroll_percents) {
+      const expectation = scroll_percents[scroll_percentage];
+
+      const [test_name, expected_value] = expectation;
+
+      const description =
+          `Applied effect value ${test_name} with fill: ${fill_mode}`
+      promise_test(
+          create_scroll_timeline_fill_test(
+              fill_mode, scroll_percentage, expected_value),
+          description);
+    }
+  }
+
+  function create_scroll_timeline_fill_test(
+      fill_mode, scroll_percentage, expected){
+    return async t => {
+      const target = createDiv(t);
+
+      const timeline = createScrollTimeline(t);
+      const effect =
+          new KeyframeEffect(target,
+                             { opacity: [0.3, 0.7] },
+                             {
+                               fill: fill_mode,
+                               /* Animation times normalized to fill scroll
+                                  range */
+                               duration: 2000,
+                               delay: 1000,
+                               endDelay: 1000
+                             });
+      const animation = new Animation(effect, timeline);
+      const scroller = timeline.source;
+      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+      animation.play();
+
+      await animation.ready;
+
+      scroller.scrollTop = scroll_percentage * maxScroll;
+
+      // Wait for new animation frame which allows the timeline to compute
+      // new current time.
+      await waitForNextFrame();
+
+      assert_equals(parseFloat(window.getComputedStyle(target).opacity),
+                    expected,
+                    "animation effect applied property value");
+    }
+  }
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation-effect-phases.tentative.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation-effect-phases.tentative.html
new file mode 100644
index 0000000000..5ded56bbf7
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation-effect-phases.tentative.html
@@ -0,0 +1,480 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Verify timeline time, animation time, effect time, and effect progress for all timeline states: before start, at start, in range, at end, after end while using various effect delay values</title>
+<meta name="timeout" content="long">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+  .scroller {
+    overflow: hidden;
+    height: 200px;
+    width: 200px;
+  }
+  .contents {
+    /* Make scroll range 1000 to simplify the math and avoid rounding errors */
+    height: 1200px;
+    width: 100%;
+  }
+</style>
+<div id="log"></div>
+<script>
+  'use strict';
+ // Note: effects are scaled to fill the timeline.
+
+  // Each entry is [[test input], [test expectations]]
+  // test input = ["description", delay, end_delay, scroll percent]
+  // test expectations = [timeline time, animation current time,
+  //                      effect local time, effect progress, effect phase,
+  //                      opacity]
+
+  /* All interesting transitions:
+      at timeline start
+      before effect delay
+      at effect start
+      in active range
+      at effect end
+      after effect end
+      at timeline end
+  */
+  const test_cases = [
+    // Case 1: No delays.
+    // Boundary at end of active phase is inclusive.
+    [
+      ["at start", 0, 0, 0],
+      [0, 0, 0, 0, "active", 0.3]
+    ],
+    [
+      ["in active range", 0, 0, 0.50],
+      [50, 50, 50, 0.5, "active", 0.5]
+    ],
+    [
+      ["at effect end time", 0, 0, 1.0],
+      [100, 100, 100, 1.0, "active", 0.7]
+    ],
+
+    // Case 2: Positive start delay and no end delay.
+    // Boundary at end of active phase is inclusive.
+    [
+      ["at timeline start", 500, 0, 0],
+      [0, 0, 0, null, "before", 1]
+    ],
+    [
+      ["before start delay", 500, 0, 0.25],
+      [25, 25, 25, null, "before", 1]
+    ],
+    [
+      ["at start delay", 500, 0, 0.5],
+      [50, 50, 50, 0, "active", 0.3]
+    ],
+    [
+      ["in active range", 500, 0, 0.75],
+      [75, 75, 75, 0.5, "active", 0.5]
+    ],
+    [
+      ["at effect end time", 500, 0, 1.0],
+      [100, 100, 100, 1.0, "active", 0.7]
+    ],
+
+    // case 3: No start delay, Positive end delay.
+    // Boundary at end of active phase is exclusive.
+    [
+      ["at timeline start", 0, 500, 0],
+      [0, 0, 0, 0, "active", 0.3]
+    ],
+    [
+      ["in active range", 0, 500, 0.25],
+      [25, 25, 25, 0.5, "active", 0.5]
+    ],
+    [
+      ["at effect end time", 0, 500, 0.5],
+      [50, 50, 50, null, "after", 1.0]
+    ],
+    [
+      ["after effect end time", 0, 500, 0.75],
+      [75, 75, 75, null, "after", 1.0]
+    ],
+    [
+      ["at timeline boundary", 0, 500, 1.0],
+      [100, 100, 100, null, "after", 1.0]
+    ],
+
+    // case 4: Positive start and end delays.
+    // Boundary at end of active phase is exclusive.
+    [
+      ["at timeline start", 250, 250, 0],
+      [0, 0, 0, null, "before", 1]
+    ],
+    [
+      ["before start delay", 250, 250, 0.1],
+      [10, 10, 10, null, "before", 1]
+    ],
+    [
+      ["at start delay", 250, 250, 0.25],
+      [25, 25, 25, 0, "active", 0.3]
+    ],
+    [
+      ["in active range", 250, 250, 0.5],
+      [50, 50, 50, 0.5, "active", 0.5]
+    ],
+    [
+      ["at effect end time", 250, 250, 0.75],
+      [75, 75, 75, null, "after", 1.0]
+    ],
+    [
+      ["after effect end time", 250, 250, 0.9],
+      [90, 90, 90, null, "after", 1.0]
+    ],
+    [
+      ["at timeline boundary", 250, 250, 1.0],
+      [100, 100, 100, null, "after", 1.0]
+    ],
+
+    // Case 5: Negative start and end delays.
+    // Effect boundaries are not reachable.
+    [
+      ["at timeline start", -125, -125, 0],
+      [0, 0, 0, 0.25, "active", 0.4]
+    ],
+    [
+      ["in active range", -125, -125, 0.5],
+      [50, 50, 50, 0.5, "active", 0.5]
+    ],
+    [
+      ["at timeline end", -125, -125, 1.0],
+      [100, 100, 100, 0.75, "active", 0.6]
+    ]
+  ];
+
+  for (const test_case of test_cases) {
+    const [inputs, expected] = test_case;
+    const [test_name, delay, end_delay, scroll_percentage] = inputs;
+
+    const description = `Current times and effect phase ${test_name} when` +
+      ` delay = ${delay} and endDelay = ${end_delay} |`;
+
+    promise_test(
+        create_scroll_timeline_delay_test(
+            delay, end_delay, scroll_percentage, expected),
+        description);
+  }
+
+  function create_scroll_timeline_delay_test(
+      delay, end_delay, scroll_percentage, expected){
+    return async t => {
+      const target = createDiv(t);
+      const timeline = createScrollTimeline(t);
+      const effect = new KeyframeEffect(
+        target,
+        {
+          opacity: [0.3, 0.7]
+        },
+        {
+          duration: 500,
+          delay: delay,
+          endDelay: end_delay
+        }
+      );
+      const animation = new Animation(effect, timeline);
+      t.add_cleanup(() => {
+        animation.cancel();
+      });
+      const scroller = timeline.source;
+      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+      animation.play();
+
+      await animation.ready;
+
+      scroller.scrollTop = scroll_percentage * maxScroll;
+
+      // Wait for new animation frame which allows the timeline to compute
+      // new current time.
+      await waitForNextFrame();
+
+      const [expected_timeline_current_time,
+          expected_animation_current_time,
+          expected_effect_local_time,
+          expected_effect_progress,
+          expected_effect_phase,
+          expected_opacity] = expected;
+
+      assert_percents_equal(
+          animation.timeline.currentTime,
+          expected_timeline_current_time,
+          "timeline current time");
+      assert_percents_equal(
+          animation.currentTime,
+          expected_animation_current_time,
+          "animation current time");
+      assert_percents_equal(
+          animation.effect.getComputedTiming().localTime,
+          expected_effect_local_time,
+          "animation effect local time");
+      assert_approx_equals_or_null(
+          animation.effect.getComputedTiming().progress,
+          expected_effect_progress,
+          0.001,
+          "animation effect progress");
+      assert_phase_at_time(
+          animation, expected_effect_phase, animation.currentTime);
+      assert_approx_equals(
+          parseFloat(getComputedStyle(target).opacity), expected_opacity,
+          0.001,
+          'target opacity');
+    }
+  }
+
+  function createKeyframeEffectOpacity(test){
+    return new KeyframeEffect(
+      createDiv(test),
+      {
+        opacity: [0.3, 0.7]
+      },
+      {
+        duration: 1000
+      }
+    );
+  }
+
+  function verifyEffectBeforePhase(animation) {
+    // If currentTime is null, we are either idle, or running with an
+    // inactive timeline. Either way, the animation is not in effect and cannot
+    // be in the before phase.
+    assert_true(animation.currentTime != null,
+                'Animation is not in effect');
+
+    const fillMode = animation.effect.getTiming().fill;
+    animation.effect.updateTiming({ fill: 'none' });
+
+    // progress == null AND opacity == 1 implies we are in the effect before
+    // or after phase.
+    assert_equals(animation.effect.getComputedTiming().progress, null);
+    assert_equals(
+        window.getComputedStyle(animation.effect.target)
+            .getPropertyValue("opacity"),
+        "1");
+
+    // If the progress is no longer null after adding fill: backwards, then we
+    // are in the before phase.
+    animation.effect.updateTiming({ fill: 'backwards' });
+    assert_true(animation.effect.getComputedTiming().progress != null);
+    assert_equals(
+        window.getComputedStyle(animation.effect.target)
+            .getPropertyValue("opacity"),
+        "0.3");
+
+    // Reset fill mode to avoid side-effects.
+    animation.effect.updateTiming({ fill: fillMode });
+  }
+
+  function createScrollLinkedOpacityAnimationWithDelays(t) {
+    const animation = new Animation(
+      createKeyframeEffectOpacity(t),
+      createScrollTimeline(t)
+    );
+    t.add_cleanup(() => {
+      animation.cancel();
+    });
+    animation.effect.updateTiming({
+       duration: 1000,
+       delay: 500,
+       endDelay: 500
+    });
+    return animation;
+  }
+
+  promise_test(async t => {
+    const animation = createScrollLinkedOpacityAnimationWithDelays(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+    animation.play();
+    await animation.ready;
+    verifyEffectBeforePhase(animation);
+
+    animation.pause();
+    await waitForNextFrame();
+    verifyEffectBeforePhase(animation);
+
+    animation.play();
+    await waitForNextFrame();
+
+    verifyEffectBeforePhase(animation);
+  }, 'Verify that (play -> pause -> play) doesn\'t change phase/progress.');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedOpacityAnimationWithDelays(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+    animation.play();
+    await animation.ready;
+    verifyEffectBeforePhase(animation);
+
+    animation.pause();
+    await waitForNextFrame();
+    verifyEffectBeforePhase(animation);
+
+    // Scrolling should not cause the animation effect to change.
+    scroller.scrollTop = 0.5 * maxScroll;
+    await waitForNextFrame();
+
+    // Check timeline phase
+    assert_percents_equal(animation.timeline.currentTime, 50);
+    assert_percents_equal(animation.currentTime, 0);
+    assert_percents_equal(animation.effect.getComputedTiming().localTime, 0,
+        "effect local time");
+
+    // Make sure the effect is still in the before phase even though the
+    // timeline is not.
+    verifyEffectBeforePhase(animation);
+  }, 'Pause in before phase, scroll timeline into active phase, animation ' +
+     'should remain in the before phase');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedOpacityAnimationWithDelays(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+    animation.play();
+    await animation.ready;
+    verifyEffectBeforePhase(animation);
+
+    animation.pause();
+    await waitForNextFrame();
+    verifyEffectBeforePhase(animation);
+
+    // Setting the current time should force the animation into effect.
+    const expected_time = 50;
+    animation.currentTime = CSS.percent(expected_time);
+    await waitForNextFrame();
+    assert_percents_equal(animation.timeline.currentTime, 0);
+    assert_percents_equal(animation.currentTime, expected_time,
+                          'Current time matches set value');
+    assert_percents_equal(
+        animation.effect.getComputedTiming().localTime,
+        expected_time, "Effect local time after setting animation.currentTime");
+    assert_equals(animation.effect.getComputedTiming().progress, 0.5,
+                  "Progress after setting animation.currentTime");
+    assert_equals(
+        window.getComputedStyle(animation.effect.target)
+            .getPropertyValue("opacity"),
+        "0.5", "Opacity after setting animation.currentTime");
+
+    // Scrolling should not cause the animation effect to change since
+    // paused.
+    scroller.scrollTop = 0.75 * maxScroll; // scroll so that timeline is 75%
+    await waitForNextFrame();
+    assert_percents_equal(animation.timeline.currentTime, 75);
+
+    // animation and effect timings are unchanged.
+    assert_percents_equal(animation.currentTime, expected_time,
+                          "Current time after scrolling while paused");
+    assert_percents_equal(
+        animation.effect.getComputedTiming().localTime,
+        expected_time,
+        "Effect local time after scrolling while paused");
+    assert_equals(animation.effect.getComputedTiming().progress, 0.5,
+                  "Progress after scrolling while paused");
+    assert_equals(
+        window.getComputedStyle(animation.effect.target)
+            .getPropertyValue("opacity"),
+        "0.5", "Opacity after scrolling while paused");
+  }, 'Pause in before phase, set animation current time to be in active ' +
+     'range, animation should become active. Scrolling should have no effect.');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedOpacityAnimationWithDelays(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+    animation.play();
+    await animation.ready;
+
+    // Causes the timeline to be inactive
+    scroller.style.overflow = "visible";
+    await waitForNextFrame();
+    await waitForNextFrame();
+
+    // Verify that he timeline is inactive
+    assert_equals(animation.timeline.currentTime, null,
+                  "Timeline is inactive");
+    assert_equals(
+        animation.currentTime, null,
+        "Current time for running animation with an inactive timeline");
+    assert_equals(animation.effect.getComputedTiming().localTime, null,
+        "effect local time with inactive timeline");
+
+    // Setting the current time while timeline is inactive should pause the
+    // animation at the specified time.
+    animation.currentTime = CSS.percent(50);
+    await waitForNextFrame();
+    await waitForNextFrame();
+
+    // Verify that animation currentTime is properly set despite the inactive
+    // timeline.
+    assert_equals(animation.timeline.currentTime, null);
+    assert_percents_equal(animation.currentTime, 50);
+    assert_percents_equal(animation.effect.getComputedTiming().localTime, 50,
+        "effect local time after setting animation current time");
+
+    // Check effect phase
+    // progress == 0.5 AND opacity == 0.5 shows we are in the effect active
+    // phase.
+    assert_equals(animation.effect.getComputedTiming().progress, 0.5,
+                  "effect progress");
+    assert_equals(
+        window.getComputedStyle(animation.effect.target)
+            .getPropertyValue("opacity"),
+        "0.5",
+        "effect opacity after setting animation current time");
+  }, 'Make scroller inactive, then set current time to an in range time');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedOpacityAnimationWithDelays(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+    // Update timeline.currentTime.
+    await waitForNextFrame();
+
+    animation.pause();
+    await animation.ready;
+    // verify effect is applied.
+    const expected_progress = 0.5;
+    assert_equals(
+        animation.effect.getComputedTiming().progress,
+        expected_progress,
+        "Verify effect progress after pausing.");
+
+    // cause the timeline to become inactive
+    scroller.style.overflow = 'visible';
+    await waitForAnimationFrames(2);
+    assert_equals(animation.timeline.currentTime, null,
+        'Sanity check the timeline is inactive.');
+    assert_equals(
+        animation.effect.getComputedTiming().progress,
+        expected_progress,
+        "Verify effect progress after the timeline goes inactive.");
+  }, 'Animation effect is still applied after pausing and making timeline ' +
+     'inactive.');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedOpacityAnimationWithDelays(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+    animation.play();
+    await animation.ready;
+
+    // cause the timeline to become inactive
+    scroller.style.overflow = 'visible';
+
+    scroller.scrollTop;
+
+    animation.pause();
+  }, 'Make timeline inactive, force style update then pause the animation. ' +
+     'No crashing indicates test success.');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation-inactive-timeline.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation-inactive-timeline.html
new file mode 100644
index 0000000000..236452eead
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation-inactive-timeline.html
@@ -0,0 +1,170 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Test basic functionality of scroll linked animation.</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+  .scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+    will-change: transform;
+  }
+
+  .contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+<div id="log"></div>
+<script>
+'use strict';
+
+promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+
+    // Ensure we have a valid animation frame time before continuing the test.
+    // This is so that we can properly determine frame advancement after the
+    // style change.
+    await waitForNextFrame();
+
+    // Make the scroll timeline inactive.
+    scroller.style.overflow = 'visible';
+    // Wait for new animation frame which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    assert_equals(animation.timeline.currentTime, null,
+                  'Sanity check the timeline is inactive.');
+    // Play the animation when the timeline is inactive.
+    animation.play();
+    assert_equals(animation.currentTime, null,
+                  'The current time is null when the timeline is inactive.');
+    assert_percents_equal(animation.startTime, 0,
+                          'The start time is zero in Pending state.');
+    await waitForNextFrame();
+    assert_true(animation.pending,
+                'Animation has play pending task while timeline is inactive.');
+    assert_equals(animation.playState, 'running',
+                  `State is 'running' in Pending state.`);
+}, `Play pending task doesn't run when the timeline is inactive.`);
+
+promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+
+    await waitForNextFrame();
+
+    // Make the scroll timeline inactive.
+    scroller.style.overflow = 'visible';
+    // Wait for new animation frame which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    assert_equals(animation.timeline.currentTime, null,
+                  'Sanity check the timeline is inactive.');
+    // Play the animation when the timeline is inactive.
+    animation.play();
+
+    // Make the scroll timeline active.
+    scroller.style.overflow = 'auto';
+    await animation.ready;
+    // Ready promise is resolved as a result of the timeline becoming active.
+    assert_percents_equal(animation.currentTime, 0,
+        'Animation current time is resolved when the animation is ready.');
+    assert_percents_equal(animation.startTime, 0,
+        'Animation start time is resolved when the animation is ready.');
+}, 'Animation start and current times are correct if scroll timeline is ' +
+   'activated after animation.play call.');
+
+promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const target = animation.effect.target;
+
+    await waitForNextFrame();
+
+    // Make the scroll timeline inactive.
+    scroller.style.overflow = 'visible';
+    scroller.scrollTop;
+    // Wait for new animation frame which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    assert_equals(animation.timeline.currentTime, null,
+                  'Sanity check the timeline is inactive.');
+    // Set start time when the timeline is inactive.
+    animation.startTime = CSSNumericValue.parse("0%");
+    assert_equals(animation.currentTime, null,
+                  'Sanity check current time is unresolved when the timeline ' +
+                  'is inactive.');
+
+    // Make the scroll timeline active.
+    scroller.style.overflow = 'auto';
+    // Wait for new animation frame which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+
+    assert_percents_equal(animation.currentTime, 0,
+        'Animation current time is resolved when the timeline is active.');
+    assert_percents_equal(animation.startTime, 0,
+                          'Animation start time is resolved.');
+    assert_percents_equal(animation.effect.getComputedTiming().localTime, 0,
+        'Effect local time is resolved when the timeline is active.');
+    assert_equals(Number(getComputedStyle(target).opacity), 0,
+        'Animation has an effect when the timeline is active.');
+}, 'Animation start and current times are correct if scroll timeline is ' +
+   'activated after setting start time.');
+
+promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    const target = animation.effect.target;
+
+    await waitForNextFrame();
+
+    // Advance the scroller.
+    scroller.scrollTop = 0.2 * maxScroll;
+
+    // Wait for new animation frame which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    // Play the animation when the timeline is active.
+    animation.play();
+    await animation.ready;
+
+    // Make the scroll timeline inactive.
+    scroller.style.overflow = 'visible';
+    scroller.scrollTop;
+    await waitForNextFrame();
+    assert_equals(animation.timeline.currentTime, null,
+                  'Sanity check the timeline is inactive.');
+    assert_equals(animation.playState, 'running',
+                  `State is 'running' when the timeline is inactive.`);
+    assert_equals(animation.currentTime, null,
+                  'Current time is unresolved when the timeline is inactive.');
+    assert_percents_equal(animation.startTime, 0,
+                          'Start time is zero when the timeline is inactive.');
+    assert_equals(animation.effect.getComputedTiming().localTime, null,
+                  'Effect local time is null when the timeline is inactive.');
+    assert_equals(Number(getComputedStyle(target).opacity), 1,
+        'Animation does not have an effect when the timeline is inactive.');
+
+    // Make the scroll timeline active.
+    scroller.style.overflow = 'auto';
+    await waitForNextFrame();
+
+    assert_equals(animation.playState, 'running',
+                  `State is 'running' when the timeline is active.`);
+    assert_percents_equal(animation.currentTime, 20,
+        'Current time is resolved when the timeline is active.');
+    assert_percents_equal(animation.startTime, 0,
+                          'Start time is zero when the timeline is active.');
+    assert_percents_equal(animation.effect.getComputedTiming().localTime, 20,
+        'Effect local time is resolved when the timeline is active.');
+    assert_equals(Number(getComputedStyle(target).opacity), 0.2,
+        'Animation has an effect when the timeline is active.');
+}, 'Animation current time is correct when the timeline becomes newly ' +
+   'inactive and then active again.');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation.html
new file mode 100644
index 0000000000..bc7ab4a476
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-animation.html
@@ -0,0 +1,216 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Test basic functionality of scroll linked animation.</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+  .scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+    will-change: transform;
+  }
+  .contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+<div id="log"></div>
+<script>
+  'use strict';
+  promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+
+    // Verify initial start and current times in Idle state.
+    assert_equals(animation.currentTime, null,
+      "The current time is null in Idle state.");
+    assert_equals(animation.startTime, null,
+      "The start time is null in Idle state.");
+    animation.play();
+    assert_true(animation.pending, "Animation is in pending state.");
+    // Verify initial start and current times in Pending state.
+    assert_percents_equal(animation.currentTime, 0,
+      "The current time is zero in Pending state.");
+    assert_percents_equal(animation.startTime, 0,
+      "The start time is zero in Pending state.");
+
+    await animation.ready;
+    // Verify initial start and current times in Playing state.
+    assert_percents_equal(animation.currentTime, 0,
+      "The current time is zero in Playing state.");
+    assert_percents_equal(animation.startTime, 0,
+      "The start time is zero in Playing state.");
+
+    // Now do some scrolling and make sure that the Animation current time is
+    // correct.
+    scroller.scrollTop = 0.4 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    assert_percents_equal(animation.currentTime, animation.timeline.currentTime,
+      "The current time corresponds to the scroll position of the scroller.");
+    assert_times_equal(
+      animation.effect.getComputedTiming().progress,
+      // Division by 100 is temporary, it will be removed when progress returns
+      // a CSSNumericValue instead of a double 0-1
+      animation.timeline.currentTime.value / 100,
+      'Effect progress corresponds to the scroll position of the scroller.');
+}, 'Animation start and current times are correct for each animation state.');
+
+promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+    // Advance the scroller.
+    scroller.scrollTop = 0.2 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+
+    // Verify initial start and current times in Idle state.
+    assert_equals(animation.currentTime, null,
+      "The current time is null in Idle state.");
+    assert_equals(animation.startTime, null,
+      "The start time is null in Idle state.");
+    animation.play();
+    // Verify initial start and current times in Pending state.
+    assert_percents_equal(animation.currentTime, animation.timeline.currentTime,
+      "The current time is a hold time in Pending state.");
+    assert_percents_equal(animation.startTime, 0,
+      "The start time is zero in Pending state.");
+
+    await animation.ready;
+    // Verify initial start and current times in Playing state.
+    assert_percents_equal(animation.currentTime, animation.timeline.currentTime,
+      "The current corresponds to the scroll position of the scroller.");
+      assert_percents_equal(animation.startTime, 0,
+      "The start time is zero in Playing state.");
+}, 'Animation start and current times are correct for each animation state' +
+    ' when the animation starts playing with advanced scroller.');
+
+promise_test(async t => {
+    const timeline = createScrollTimeline(t);
+    const animation1 = createScrollLinkedAnimation(t, timeline);
+    const animation2 = createScrollLinkedAnimation(t, timeline);
+    const scroller = timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+    // Advance the scroller.
+    scroller.scrollTop = 0.4 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+
+    // Verify initial start and current times in Idle state.
+    assert_equals(animation1.currentTime, null,
+      "The current time is null in Idle state.");
+    assert_equals(animation1.startTime, null,
+      "The start time is null in Idle state.");
+    assert_equals(animation2.currentTime, null,
+      "The current time is null in Idle state.");
+    assert_equals(animation2.startTime, null,
+      "The start time is null in Idle state.");
+    animation1.play();
+    animation2.play();
+    // Verify initial start and current times in Pending state.
+    assert_percents_equal(animation1.currentTime, timeline.currentTime,
+      "The current time corresponds to the scroll position of the scroller" +
+        " in Pending state.");
+    assert_percents_equal(animation1.startTime, 0,
+      "The start time is zero in Pending state.");
+    assert_percents_equal(animation2.currentTime, timeline.currentTime,
+      "The current time corresponds to the scroll position of the scroller" +
+        " in Pending state.");
+    assert_percents_equal(animation2.startTime, 0,
+      "The start time is zero in Pending state.");
+
+    await animation1.ready;
+    await animation2.ready;
+    // Verify initial start and current times in Playing state.
+    assert_percents_equal(animation1.currentTime, timeline.currentTime,
+      "The current corresponds to the scroll position of the scroller.");
+    assert_percents_equal(animation1.startTime, 0,
+      "The start time is zero in Playing state.");
+    assert_percents_equal(animation2.currentTime, timeline.currentTime,
+      "The current corresponds to the scroll position of the scroller.");
+    assert_percents_equal(animation2.startTime, 0,
+      "The start time is zero in Playing state.");
+}, 'Animation start and current times are correct when multiple animations' +
+    ' are attached to the same timeline.');
+
+promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+    animation.play();
+    await animation.ready;
+    // Advance the scroller to max position.
+    scroller.scrollTop = maxScroll;
+
+    await animation.finished;
+
+    assert_equals(animation.playState, 'finished',
+      'Animation state is in finished state.');
+      assert_percents_equal(animation.currentTime, 100,
+        'Animation current time is at 100% on reverse scrolling.');
+
+    // Scroll back.
+    scroller.scrollTop = 0.2 * maxScroll;
+
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    // Verify animation state and current time on reverse scrolling.
+    assert_equals(animation.playState, 'running',
+      'Animation state is playing on reverse scrolling.');
+    assert_percents_equal(animation.currentTime, 20,
+      'Animation current time is updated on reverse scrolling.');
+}, 'Finished animation plays on reverse scrolling.');
+
+promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+    animation.play();
+    await animation.ready;
+
+    // Advance the scroller to max position.
+    scroller.scrollTop =  maxScroll;
+    await animation.finished;
+
+    var sent_finish_event = false;
+    animation.onfinish = function() {
+      sent_finish_event = true;
+    };
+
+    // Scroll back.
+    scroller.scrollTop = 0.2 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    assert_false(sent_finish_event,
+                 "No animation finished event is sent on reverse scroll.");
+
+    scroller.scrollTop = maxScroll;
+    await animation.finished;
+
+    // Wait for next frame to allow the animation to send finish events. The
+    // finished promise fires before events are sent.
+    await waitForNextFrame();
+
+    assert_true(sent_finish_event,
+                 "Animation finished event is sent on reaching max scroll.");
+}, 'Sending animation finished events by finished animation on reverse ' +
+   'scrolling.');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-timeline-in-removed-iframe-crash.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-timeline-in-removed-iframe-crash.html
new file mode 100644
index 0000000000..07edbd83f1
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-timeline-in-removed-iframe-crash.html
@@ -0,0 +1,20 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title>Starting an animation with a scroll timeline in a removed iframe
+         should not crash</title>
+</head>
+<body>
+  <div id="target"></div>
+  <iframe id="frame"></iframe>
+</body>
+<script type="text/javascript">
+  const target = document.getElementById('target');
+  const frame = document.getElementById('frame');
+  const timeline = new frame.contentWindow.ScrollTimeline();
+  frame.remove();
+  target.animate(null, {timeline: timeline});
+</script>
+</html>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-timeline-invalidation.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-timeline-invalidation.html
new file mode 100644
index 0000000000..f35d878c69
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-timeline-invalidation.html
@@ -0,0 +1,128 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>ScrollTimeline invalidation</title>
+<link rel="help" href="https://wicg.github.io/scroll-animations/#current-time-algorithm">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+  .scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+    will-change: transform;
+  }
+  .contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+<div id="log"></div>
+
+<script>
+'use strict';
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const effect_duration = 350;
+  animation.effect.updateTiming({ duration: effect_duration });
+  const scroller = animation.timeline.source;
+  let maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.2 * maxScroll;
+  const initial_progress = (scroller.scrollTop / maxScroll) * 100;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  animation.play();
+  await animation.ready;
+
+  // Animation current time is at 20% because scroller was scrolled to 20%
+  // assert_equals(animation.currentTime.value, 20);
+  assert_equals(animation.currentTime.value, 20);
+  assert_equals(scroller.scrollTop, 180);
+  assert_equals(maxScroll, 900);
+
+  // Shrink scroller content size (from 1000 to 500).
+  // scroller.scrollTop maintains the same offset, which means shrinking the
+  // content has the effect of skipping the animation forward.
+  scroller.firstChild.style.height = "500px";
+  maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  assert_equals(scroller.scrollTop, 180);
+  assert_equals(maxScroll, 400);
+  await waitForNextFrame();
+
+  const expected_progress = (scroller.scrollTop / maxScroll) * 100;
+  assert_true(expected_progress > initial_progress)
+  assert_equals(animation.currentTime.value, expected_progress); // 45%
+  assert_equals(animation.timeline.currentTime.value, expected_progress); // 45%
+  assert_equals(animation.effect.getComputedTiming().localTime.value, expected_progress); // 45%
+}, 'Animation current time and effect local time are updated after scroller ' +
+   'content size changes.');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  animation.effect.updateTiming({ duration: 350 });
+  const scroller = animation.timeline.source;
+  let maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  const scrollOffset = 0.2 * maxScroll
+  scroller.scrollTop = scrollOffset;
+  const initial_progress = (scroller.scrollTop / maxScroll) * 100;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  animation.play();
+  await animation.ready;
+
+  // Animation current time is at 20% because scroller was scrolled to 20%
+  // assert_equals(animation.currentTime.value, 20);
+  assert_equals(animation.currentTime.value, 20);
+  assert_equals(scroller.scrollTop, scrollOffset);
+  assert_equals(maxScroll, 900);
+
+  // Change scroller size.
+  scroller.style.height = "500px";
+  maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  assert_equals(scroller.scrollTop, scrollOffset);
+  assert_equals(maxScroll, 500);
+  await waitForNextFrame();
+
+  const expected_progress = (scroller.scrollTop / maxScroll) * 100;
+  assert_true(expected_progress > initial_progress);
+  assert_equals(animation.currentTime.value, expected_progress); // 45%
+  assert_equals(animation.timeline.currentTime.value, expected_progress); // 45%
+  assert_equals(animation.effect.getComputedTiming().localTime.value, expected_progress); // 45%
+}, 'Animation current time and effect local time are updated after scroller ' +
+   'size changes.');
+
+promise_test(async t => {
+  const timeline = createScrollTimeline(t);
+  const scroller = timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  // Instantiate scroll animation that resizes its scroll timeline scroller.
+  const animation = new Animation(
+    new KeyframeEffect(
+      timeline.source.firstChild,
+      [{ height: '1000px' }, { height: '2000px' }],
+      { duration: 1000, }
+    ), timeline);
+  animation.play();
+  await animation.ready;
+  await waitForNextFrame();
+  scroller.scrollTop = 0.2 * maxScroll;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  assert_times_equal(timeline.currentTime.value, 20,
+    'Timeline current time is updated after animation frame.');
+  await waitForNextFrame();
+  assert_times_equal(timeline.currentTime.value, 16.3636,
+    'Timeline current time is updated after two animation frames and ' +
+    'reflects single layout run.');
+}, 'If scroll animation resizes its scroll timeline scroller, ' +
+   'layout runs only once to reflect the initial update.');
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-timeline-snapshotting.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-timeline-snapshotting.html
new file mode 100644
index 0000000000..1e43699d5b
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/scroll-timeline-snapshotting.html
@@ -0,0 +1,44 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>ScrollTimeline snapshotting</title>
+<link rel="help" href="https://wicg.github.io/scroll-animations/#avoiding-cycles">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/resources/testdriver.js"></script>
+<script src="/resources/testdriver-vendor.js"></script>
+<script src="/resources/testdriver-actions.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="./testcommon.js"></script>
+
+<style>
+  body {
+    height: 800px;
+    width: 800px;
+  }
+</style>
+<div id="log"></div>
+
+<script>
+'use strict';
+
+promise_test(async t => {
+  const scroller = document.scrollingElement;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  const timeline = new ScrollTimeline({source: scroller});
+  scroller.scrollTo(0, 0);
+  assert_equals(scroller.scrollTop, 0, "verify test pre-condition");
+
+  scroller.scrollBy({top: 100, behavior: 'smooth'});
+  // Wait for the scroll to change.
+  const startScroll = scroller.scrollTop;
+  do {
+    await waitForNextFrame();
+  } while(scroller.scrollTop == startScroll);
+  assert_percents_equal(
+      timeline.currentTime,
+      (scroller.scrollTop / maxScroll) * 100,
+      'Scroll timeline current time corresponds to the scroll position.');
+}, 'ScrollTimeline current time is updated after programmatic animated ' +
+   'scroll.');
+
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/set-current-time-before-play.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/set-current-time-before-play.html
new file mode 100644
index 0000000000..cd3faeeee4
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/set-current-time-before-play.html
@@ -0,0 +1,69 @@
+<html class="reftest-wait">
+<title>Setting current time before play should not timeout</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations/">
+<meta name="assert" content="Regression test to make sure the ready promise is correctly resolved">
+<link rel="match" href="animation-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  #scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+    will-change: transform; /* force compositing */
+  }
+
+  #contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+<div id="scroller">
+  <div id="contents"></div>
+</div>
+
+<script>
+  const box = document.getElementById('box');
+  const effect = new KeyframeEffect(box,
+    [
+    { transform: 'translateY(0)', opacity: 1},
+    { transform: 'translateY(200px)', opacity: 0}
+    ], {
+      duration: 1000,
+    }
+  );
+
+  const scroller = document.getElementById('scroller');
+  const timeline = new ScrollTimeline(
+      { source: scroller, orientation: 'block' });
+  const animation = new Animation(effect, timeline);
+  animation.currentTime = CSS.percent(0);
+  animation.play();
+
+  animation.ready.then(() => {
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+
+    waitForAnimationFrames(2).then(_ => {
+      takeScreenshot();
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/setting-current-time.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/setting-current-time.html
new file mode 100644
index 0000000000..f6c826db69
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/setting-current-time.html
@@ -0,0 +1,286 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Setting the current time of an animation</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations-1/#setting-the-current-time-of-an-animation">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+.scroller {
+  overflow: auto;
+  height: 200px;
+  width: 100px;
+  will-change: transform;
+}
+.contents {
+  height: 1000px;
+  width: 100%;
+}
+</style>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.25 * maxScroll;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+
+  await animation.ready;
+
+  assert_throws_js(TypeError, () => {
+    animation.currentTime = null;
+  });
+}, 'Setting animation current time to null throws TypeError.');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  assert_throws_dom('NotSupportedError', () => {
+    animation.currentTime = CSSNumericValue.parse("300");
+  });
+  assert_throws_dom('NotSupportedError', () => {
+    animation.currentTime = CSSNumericValue.parse("300ms");
+  });
+  assert_throws_dom('NotSupportedError', () => {
+    animation.currentTime = CSSNumericValue.parse("0.3s");
+  });
+}, 'Setting the current time to an absolute time value throws exception');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.25 * maxScroll;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  animation.currentTime = CSSNumericValue.parse("33.3%");
+
+  assert_percents_equal(animation.currentTime, 33.3,
+    "Animation current time should be equal to the set value."
+  );
+}, 'Set animation current time to a valid value without playing.');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.25 * maxScroll;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+
+  await animation.ready;
+  animation.currentTime = CSSNumericValue.parse("33.3%");
+
+  assert_percents_equal(animation.currentTime, 33.3,
+    "Animation current time should be equal to the set value."
+  );
+}, 'Set animation current time to a valid value while playing.');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.25 * maxScroll;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+
+  await animation.ready;
+  animation.currentTime = CSSNumericValue.parse("200%");
+
+  assert_equals(animation.playState, "finished");
+  assert_percents_equal(animation.currentTime, 200,
+    "Animation current time should be equal to the set value."
+  );
+}, 'Set animation current time to a value beyond effect end.');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.25 * maxScroll;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+
+  await animation.ready;
+  animation.currentTime = CSSNumericValue.parse("-10%");
+
+  assert_equals(animation.playState, "running");
+  assert_percents_equal(animation.currentTime, -10,
+    "Animation current time should be equal to the set value."
+  );
+}, 'Set animation current time to a negative value.');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.25 * maxScroll;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+
+  animation.currentTime = CSSNumericValue.parse("30%");
+
+  assert_equals(animation.playState, "running");
+  assert_true(animation.pending);
+  assert_percents_equal(animation.currentTime, 30);
+}, "Setting current time while play pending overrides the current time");
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.25 * maxScroll;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+
+  await animation.ready;
+  animation.currentTime = CSSNumericValue.parse("33.3%");
+
+  assert_percents_equal(animation.currentTime, 33.3,
+    "Animation current time should be equal to the set value."
+  );
+
+  // Cancel the animation and play it again, check that current time has reset
+  // to scroll offset based current time.
+  animation.cancel();
+  animation.play();
+  await animation.ready;
+
+  assert_percents_equal(animation.currentTime, animation.timeline.currentTime,
+    "Animation current time should return to a value matching its" +
+    " timeline current time after animation is cancelled and played again."
+  );
+}, 'Setting animation.currentTime then restarting the animation should' +
+  ' reset the current time.');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.25 * maxScroll;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+
+  await animation.ready;
+  const originalCurrentTime = animation.currentTime.value;
+
+  // Set the current time to something other than where the scroll offset.
+  animation.currentTime = CSSNumericValue.parse("50%");
+
+  // Setting current time is internally setting the start time to
+  // scrollTimeline.currentTime - newAnimationCurrentTime.
+  // Which results in current time of (timeline.currentTime - start_time).
+  // This behavior puts the animation in a strange "out of sync" state between
+  // the scroller and the animation effect, this is currently expected
+  // behavior.
+
+  const expectedStartTime = originalCurrentTime - animation.currentTime.value;
+  assert_percents_equal(animation.startTime, expectedStartTime,
+    "Animation current time should be updated when setting the current time" +
+    " to a time within the range of the animation.");
+
+  scroller.scrollTop = 0.7 * maxScroll;
+  // Wait for new animation frame  which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  assert_percents_equal(animation.startTime, expectedStartTime,
+    "Animation start time should remain unchanged when the scroller changes" +
+    " position."
+  );
+  assert_percents_equal(animation.currentTime,
+    animation.timeline.currentTime.value - animation.startTime.value,
+    "Animation current time should return to a value equal to" +
+    " (timeline.currentTime - animation.startTime) after timeline scroll" +
+    " source has been scrolled."
+  );
+}, 'Set Animation current time then scroll.');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  // Make the timeline inactive.
+  scroller.style.overflow = 'visible';
+  scroller.scrollTop;
+  await waitForNextFrame();
+
+  assert_equals(animation.currentTime, null,
+    'Current time is unresolved when the timeline is inactive.');
+
+  animation.currentTime = CSSNumericValue.parse("30%");
+  assert_percents_equal(animation.currentTime, 30,
+    'Animation current time should be equal to the set value.');
+  assert_equals(animation.playState, 'paused',
+    'Animation play state is \'paused\' when current time is set and ' +
+    'timeline is inactive.');
+}, 'Animation current time and play state are correct when current time is ' +
+   'set while the timeline is inactive.');
+
+promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+
+    // Wait for new animation frame which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    animation.play();
+    await animation.ready;
+
+    // Make the timeline inactive.
+    scroller.style.overflow = 'visible';
+    scroller.scrollTop;
+    await waitForNextFrame();
+
+    assert_equals(animation.timeline.currentTime, null,
+      'Current time is unresolved when the timeline is inactive.');
+
+    animation.currentTime = CSSNumericValue.parse("30%");
+    assert_percents_equal(animation.currentTime, 30,
+      'Animation current time should be equal to the set value.');
+    assert_equals(animation.playState, 'paused',
+      'Animation play state is \'paused\' when current time is set and ' +
+      'timeline is inactive.');
+
+    // Make the timeline active.
+    scroller.style.overflow = 'auto';
+    scroller.scrollTop;
+    await waitForNextFrame();
+
+    assert_percents_equal(animation.timeline.currentTime, 0,
+      'Current time is resolved when the timeline is active.');
+    assert_percents_equal(animation.currentTime, 30,
+      'Animation current time holds the set value.');
+    assert_equals(animation.playState, 'paused',
+      'Animation holds \'paused\' state.');
+}, 'Animation current time set while the timeline is inactive holds when the ' +
+   'timeline becomes active again.');
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/setting-playback-rate.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/setting-playback-rate.html
new file mode 100644
index 0000000000..4d6fecae05
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/setting-playback-rate.html
@@ -0,0 +1,323 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Setting the playback rate of an animation that is using a ScrollTimeline</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#setting-the-playback-rate-of-an-animation">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+.scroller {
+  overflow: auto;
+  height: 100px;
+  width: 100px;
+  will-change: transform;
+}
+.contents {
+  height: 1000px;
+  width: 100%;
+}
+</style>
+<body>
+<script>
+  'use strict';
+
+  promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    // this forces a layout which results in an active timeline
+    scroller.scrollTop = 0;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+
+    animation.playbackRate = 0.5;
+    animation.play();
+
+    assert_percents_equal(animation.currentTime, 0,
+        'Zero current time is not affected by playbackRate change.');
+  }, 'Zero current time is not affected by playbackRate set while the ' +
+       'animation is in idle state.');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    // this forces a layout which results in an active timeline
+    scroller.scrollTop = 0;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+
+    animation.play();
+    animation.playbackRate = 0.5;
+
+    assert_percents_equal(animation.currentTime, 0,
+        'Zero current time is not affected by playbackRate change.');
+  }, 'Zero current time is not affected by playbackRate set while the ' +
+      'animation is in play-pending state.');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.2 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+
+    animation.playbackRate = 0.5;
+    animation.play();
+    await animation.ready;
+    assert_percents_equal(animation.currentTime, 10,
+        'Initial current time is scaled by playbackRate change.');
+  }, 'Initial current time is scaled by playbackRate set while ' +
+      'scroll-linked animation is in running state.');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    const playbackRate = 2;
+
+    scroller.scrollTop = 0.2 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+
+    animation.play();
+    await animation.ready;
+    // Set playback rate while the animation is playing.
+    animation.playbackRate = playbackRate;
+    assert_percents_equal(animation.currentTime, 40,
+        'The current time is scaled by the playback rate.');
+  }, 'The current time is scaled by playbackRate set while the ' +
+      'scroll-linked animation is in play state.');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+    // Set playback rate while the animation is in 'idle' state.
+    animation.playbackRate = 2;
+    animation.play();
+    await animation.ready;
+    scroller.scrollTop = 0.2 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+
+    assert_percents_equal(animation.currentTime, 40,
+                          'The current time should increase two times faster ' +
+                          'than timeline time.');
+  }, 'The playback rate set before scroll-linked animation started playing ' +
+      'affects the rate of progress of the current time');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    animation.play();
+
+    await animation.ready;
+
+    animation.playbackRate = 2;
+    scroller.scrollTop = 0.25 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+
+    assert_percents_equal(
+        animation.currentTime,
+        animation.timeline.currentTime.value * animation.playbackRate,
+        'The current time should increase two times faster than timeline time');
+  }, 'The playback rate affects the rate of progress of the current time' +
+     ' when scrolling');
+
+    promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.25 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    animation.play();
+
+    animation.playbackRate = 2;
+
+    assert_equals(animation.playState, "running");
+    assert_true(animation.pending);
+    assert_percents_equal(animation.currentTime, 50);
+  }, 'Setting the playback rate while play-pending scales the current time' +
+    ' from scrollTimeline.');
+
+  test(t => {
+    const animation = createScrollLinkedAnimation(t);
+    animation.play();
+    animation.currentTime = CSSNumericValue.parse("25%");
+    animation.playbackRate = 2;
+
+    assert_equals(animation.playState, "running");
+    assert_true(animation.pending);
+    assert_percents_equal(animation.currentTime, 50);
+  }, 'Setting the playback rate while play-pending scales the set current' +
+    ' time.');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.25 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    animation.play();
+    assert_percents_equal(animation.currentTime, 25);
+
+    await animation.ready;
+    animation.playbackRate = 2;
+
+    assert_percents_equal(animation.currentTime, 50);
+  }, 'Setting the playback rate while playing scales the current time' +
+    ' from scrollTimeline.');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+
+    /* Wait for animation frame is here for now to avoid a renderer crash
+    caused by crbug.com/1042924. Once that is fixed, these can be removed */
+    await waitForAnimationFrames(2);
+
+    animation.play();
+
+    animation.currentTime = CSSNumericValue.parse("25%");
+    await animation.ready;
+    animation.playbackRate = 2;
+
+    assert_percents_equal(animation.currentTime, 50);
+  }, 'Setting the playback rate while playing scales the set current time.');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    animation.playbackRate = -1;
+    scroller.scrollTop = 0.3 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    animation.play();
+
+    await animation.ready;
+    const expectedCurrentTime = 100 - animation.timeline.currentTime.value;
+    assert_percents_equal(animation.currentTime, expectedCurrentTime);
+  }, 'Negative initial playback rate should correctly modify initial current' +
+    ' time.');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    animation.play();
+
+    await animation.ready;
+    const startingTimelineTime = animation.timeline.currentTime;
+    const startingCurrentTime = animation.currentTime;
+    assert_percents_equal(startingCurrentTime, 50);
+    assert_percents_equal(startingTimelineTime, 50);
+
+    animation.playbackRate = -1;
+
+    scroller.scrollTop = 0.8 * maxScroll;
+    await waitForNextFrame();
+    // -300 = 500 - 800
+
+    // let timelineDiff =
+    //     startingTimelineTime.value - animation.timeline.currentTime.value;
+    // // 200 = 500 + (-300)
+    // let expected = startingCurrentTime.value + timelineDiff;
+    assert_percents_equal(animation.timeline.currentTime, 80);
+    assert_percents_equal(animation.currentTime, 20);
+
+    scroller.scrollTop = 0.2 * maxScroll;
+    await waitForNextFrame();
+    // // 300 = 500 - 200
+    // timelineDiff =
+    //     startingTimelineTime.value - animation.timeline.currentTime.value;
+    // // 800 = 500 + 300
+    // expected = startingCurrentTime.value + timelineDiff;
+    assert_percents_equal(animation.timeline.currentTime, 20);
+    assert_percents_equal(animation.currentTime, 80);
+  }, 'Reversing the playback rate while playing correctly impacts current' +
+    ' time during future scrolls');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    animation.playbackRate = 0;
+    scroller.scrollTop = 0.3 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    animation.play();
+
+    await animation.ready;
+    assert_percents_equal(animation.currentTime, 0);
+  }, 'Zero initial playback rate should correctly modify initial current' +
+    ' time.');
+
+  promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.2 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    animation.play();
+
+    await animation.ready;
+    assert_percents_equal(animation.currentTime, 20);
+    animation.playbackRate = 0;
+    scroller.scrollTop = 0.5 * maxScroll;
+    await waitForNextFrame();
+
+    // Ensure that current time does not change.
+    assert_percents_equal(animation.timeline.currentTime, 50);
+    assert_percents_equal(animation.currentTime, 0);
+  }, 'Setting a zero playback rate while running preserves the start time');
+
+
+promise_test(async t => {
+    const animation = createScrollLinkedAnimation(t);
+    const scroller = animation.timeline.source;
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.2 * maxScroll;
+    // Wait for new animation frame  which allows the timeline to compute new
+    // current time.
+    await waitForNextFrame();
+    animation.play();
+
+    await animation.ready;
+    assert_percents_equal(animation.timeline.currentTime, 20);
+    assert_percents_equal(animation.currentTime, 20);
+    animation.startTime = animation.currentTime;
+    // timeline current time [0%, 100%] --> animation current time [-20%, 80%].
+    assert_percents_equal(animation.currentTime, 0);
+
+    animation.playbackRate = -1;
+    // timeline current time [0%, 100%] --> animation current time [80%, -20%].
+    // timeline @ 20% --> animation current time @ (20% - 80%) * (-1) = 60%.
+    assert_percents_equal(animation.timeline.currentTime, 20);
+    assert_percents_equal(animation.currentTime, 60);
+  }, 'Reversing an animation with non-boundary aligned start time ' +
+     'symmetrically adjusts the start time');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/setting-start-time.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/setting-start-time.html
new file mode 100644
index 0000000000..d506df65bf
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/setting-start-time.html
@@ -0,0 +1,439 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Setting the start time of scroll animation</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#setting-the-start-time-of-an-animation">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+.scroller {
+  overflow: auto;
+  height: 200px;
+  width: 100px;
+  will-change: transform;
+}
+
+.contents {
+  height: 1000px;
+  width: 100%;
+}
+</style>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  assert_throws_dom('NotSupportedError', () => {
+    animation.startTime = CSSNumericValue.parse("300");
+  });
+  assert_throws_dom('NotSupportedError', () => {
+    animation.startTime = CSSNumericValue.parse("300ms");
+  });
+  assert_throws_dom('NotSupportedError', () => {
+    animation.startTime = CSSNumericValue.parse("0.3s");
+  });
+}, 'Setting the start time to an absolute time value throws exception');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.2 * maxScroll;
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  // So long as a hold time is set, querying the current time will return
+  // the hold time.
+
+  // Since the start time is unresolved at this point, setting the current time
+  // will set the hold time
+  animation.currentTime = CSSNumericValue.parse("30%");
+  assert_equals(animation.startTime, null, 'The start time stays unresolved');
+  assert_percents_equal(animation.currentTime, 30,
+                        'The current time is calculated from the hold time');
+
+  // If we set the start time, however, we should clear the hold time.
+  animation.startTime = CSSNumericValue.parse("0%");
+  assert_percents_equal(animation.startTime, 0,
+                        'The start time is set to the requested value');
+  assert_percents_equal(animation.currentTime, 20,
+                        'The current time is calculated from the start time, ' +
+                        'not the hold time');
+  // Sanity check
+  assert_equals(animation.playState, 'running',
+                'Animation reports it is running after setting a resolved ' +
+                'start time');
+}, 'Setting the start time clears the hold time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  // Make the scroll timeline inactive.
+  scroller.style.overflow = 'visible';
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  assert_equals(animation.timeline.currentTime, null,
+                'Sanity check the timeline is inactive');
+
+  // So long as a hold time is set, querying the current time will return
+  // the hold time.
+
+  // Since the start time is unresolved at this point, setting the current time
+  // will set the hold time
+  animation.currentTime = CSSNumericValue.parse("30%");
+  assert_equals(animation.startTime, null, 'The start time stays unresolved');
+  assert_percents_equal(animation.currentTime, 30,
+                        'The current time is calculated from the hold time');
+
+  // If we set the start time, however, we should clear the hold time.
+  animation.startTime = CSSNumericValue.parse("0%");
+  assert_percents_equal(animation.startTime, 0,
+                        'The start time is set to the requested value');
+  assert_equals(animation.currentTime, null,
+                'The current time is calculated from the start time, not' +
+                ' the hold time');
+  // Sanity check
+  assert_equals(animation.playState, 'running',
+                'Animation reports it is running after setting a resolved ' +
+                'start time');
+}, 'Setting the start time clears the hold time when the timeline is inactive');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  scroller.scrollTop = 0.2 * maxScroll;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  // Set up a running animation (i.e. both start time and current time
+  // are resolved).
+  animation.startTime = CSSNumericValue.parse("5%");
+  assert_equals(animation.playState, 'running');
+  assert_percents_equal(animation.startTime, 5,
+                        'The start time is set to the requested value');
+  assert_percents_equal(animation.currentTime, 15,
+                        'Current time is resolved for a running animation');
+
+  // Clear start time
+  animation.startTime = null;
+  assert_equals(animation.startTime, null,
+                'The start time is set to the requested value');
+  assert_percents_equal(animation.currentTime, 15,
+                        'Hold time is set after start time is made unresolved');
+  assert_equals(animation.playState, 'paused',
+                'Animation reports it is paused after setting an unresolved'
+                + ' start time');
+}, 'Setting an unresolved start time sets the hold time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  // Make the scroll timeline inactive.
+  scroller.style.overflow = 'visible';
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  assert_equals(animation.timeline.currentTime, null,
+                'Sanity check the timeline is inactive');
+
+  // Set up a running animation (i.e. both start time and current time
+  // are resolved).
+  animation.startTime = CSSNumericValue.parse("5%");
+  assert_equals(animation.playState, 'running');
+  assert_percents_equal(animation.startTime, 5,
+                        'The start time is set to the requested value');
+  assert_equals(animation.currentTime, null,
+                'Current time is unresolved for a running animation when the ' +
+                'timeline is inactive');
+
+  // Clear start time
+  animation.startTime = null;
+  assert_equals(animation.startTime, null,
+                'The start time is set to the requested value');
+  assert_equals(animation.currentTime, null,
+                'Hold time is set to unresolved after start time is made ' +
+                'unresolved');
+  assert_equals(animation.playState, 'idle',
+                'Animation reports it is idle after setting an unresolved'
+                + ' start time');
+}, 'Setting an unresolved start time sets the hold time to unresolved when ' +
+   'the timeline is inactive');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  let readyPromiseCallbackCalled = false;
+  animation.ready.then(() => { readyPromiseCallbackCalled = true; } );
+
+  // Put the animation in the play-pending state
+  animation.play();
+
+  // Sanity check
+  assert_true(animation.pending && animation.playState === 'running',
+              'Animation is in play-pending state');
+
+  // Setting the start time should resolve the 'ready' promise, i.e.
+  // it should schedule a microtask to run the promise callbacks.
+  animation.startTime = CSSNumericValue.parse("10%");
+  assert_percents_equal(animation.startTime, 10,
+                        'The start time is set to the requested value');
+  assert_false(readyPromiseCallbackCalled,
+               'Ready promise callback is not called synchronously');
+
+  // If we schedule another microtask then it should run immediately after
+  // the ready promise resolution microtask.
+  await Promise.resolve();
+  assert_true(readyPromiseCallbackCalled,
+              'Ready promise callback called after setting startTime');
+}, 'Setting the start time resolves a pending ready promise');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  // Make the scroll timeline inactive.
+  scroller.style.overflow = 'visible';
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  assert_equals(animation.timeline.currentTime, null,
+                'Sanity check the timeline is inactive');
+
+  let readyPromiseCallbackCalled = false;
+  animation.ready.then(() => { readyPromiseCallbackCalled = true; } );
+
+  // Put the animation in the play-pending state
+  animation.play();
+
+  // Sanity check
+  assert_true(animation.pending && animation.playState === 'running',
+              'Animation is in play-pending state');
+
+  // Setting the start time should resolve the 'ready' promise, i.e.
+  // it should schedule a microtask to run the promise callbacks.
+  animation.startTime = CSSNumericValue.parse("10%");
+  assert_percents_equal(animation.startTime, 10,
+                        'The start time is set to the requested value');
+  assert_false(readyPromiseCallbackCalled,
+               'Ready promise callback is not called synchronously');
+
+  // If we schedule another microtask then it should run immediately after
+  // the ready promise resolution microtask.
+  await Promise.resolve();
+  assert_true(readyPromiseCallbackCalled,
+              'Ready promise callback called after setting startTime');
+}, 'Setting the start time resolves a pending ready promise when the timeline' +
+   'is inactive');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  // Put the animation in the play-pending state
+  animation.play();
+
+  // Sanity check
+  assert_true(animation.pending, 'Animation is pending');
+  assert_equals(animation.playState, 'running', 'Animation is play-pending');
+  assert_percents_equal(animation.startTime, 0, 'Start time is zero');
+
+  // Setting start time should cancel the pending task.
+  animation.startTime = null;
+  assert_false(animation.pending, 'Animation is no longer pending');
+  assert_equals(animation.playState, 'paused', 'Animation is paused');
+}, 'Setting an unresolved start time on a play-pending animation makes it'
+   + ' paused');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  // Make the scroll timeline inactive.
+  scroller.style.overflow = 'visible';
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  assert_equals(animation.timeline.currentTime, null,
+                'Sanity check the timeline is inactive');
+
+  // Put the animation in the play-pending state
+  animation.play();
+
+  // Sanity check
+  assert_true(animation.pending, 'Animation is pending');
+  assert_equals(animation.playState, 'running', 'Animation is play-pending');
+  assert_percents_equal(animation.startTime, 0, 'Start time is zero');
+
+  // Setting start time should cancel the pending task.
+  animation.startTime = null;
+  assert_false(animation.pending, 'Animation is no longer pending');
+  assert_equals(animation.playState, 'idle', 'Animation is idle');
+}, 'Setting an unresolved start time on a play-pending animation makes it'
+   + ' idle when the timeline is inactive');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+
+  // Set start time such that the current time is past the end time
+  animation.startTime = CSSNumericValue.parse("-110%");
+  assert_percents_equal(animation.startTime, -110,
+                        'The start time is set to the requested value');
+  assert_equals(animation.playState, 'finished',
+                'Seeked to finished state using the startTime');
+
+  // If the 'did seek' flag is true, the current time should be greater than
+  // the effect end.
+  assert_greater_than(animation.currentTime.value,
+                      animation.effect.getComputedTiming().endTime.value,
+                      'Setting the start time updated the finished state with'
+                      + ' the \'did seek\' flag set to true');
+
+  // Furthermore, that time should persist if we have correctly updated
+  // the hold time
+  const finishedCurrentTime = animation.currentTime;
+  await waitForNextFrame();
+  assert_percents_equal(animation.currentTime, finishedCurrentTime,
+                        'Current time does not change after seeking past the ' +
+                        'effect end time by setting the current time');
+}, 'Setting the start time updates the finished state');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+
+  await animation.ready;
+  assert_equals(animation.playState, 'running');
+
+  // Setting the start time updates the finished state. The hold time is not
+  // constrained by the effect end time.
+  animation.startTime = CSSNumericValue.parse("-110%");
+  assert_equals(animation.playState, 'finished');
+
+  assert_percents_equal(animation.currentTime, 110);
+}, 'Setting the start time on a running animation updates the play state');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  // Setting the start time updates the finished state. The hold time is not
+  // constrained by the normal range of the animation time.
+  animation.currentTime = CSSNumericValue.parse("100%");
+  assert_equals(animation.playState, 'finished', 'Animation is finished');
+  animation.playbackRate = -1;
+  assert_equals(animation.playState, 'running', 'Animation is running');
+  animation.startTime = CSSNumericValue.parse("-200%");
+  assert_equals(animation.playState, 'finished', 'Animation is finished');
+  assert_percents_equal(animation.currentTime, -200);
+}, 'Setting the start time on a reverse running animation updates the play '
+   + 'state');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  let readyPromiseCallbackCalled = false;
+  animation.ready.then(() => { readyPromiseCallbackCalled = true; } );
+  animation.pause();
+
+  // Sanity check
+  assert_true(animation.pending && animation.playState === 'paused',
+              'Animation is in pause-pending state');
+
+  // Setting the start time should resolve the 'ready' promise although
+  // the resolution callbacks when be run in a separate microtask.
+  animation.startTime = null;
+  assert_false(readyPromiseCallbackCalled,
+               'Ready promise callback is not called synchronously');
+
+  await Promise.resolve();
+  assert_true(readyPromiseCallbackCalled,
+              'Ready promise callback called after setting startTime');
+}, 'Setting the start time resolves a pending pause task');
+
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.play();
+
+  // We should be play-pending now
+  assert_true(anim.pending);
+  assert_equals(anim.playState, 'running');
+
+  // Apply a pending playback rate
+  anim.updatePlaybackRate(2);
+  assert_equals(anim.playbackRate, 1);
+  assert_true(anim.pending);
+
+  // Setting the start time should apply the pending playback rate
+  anim.startTime = CSSNumericValue.parse(
+    anim.timeline.currentTime.value - 2500 + "%");
+  assert_equals(anim.playbackRate, 2);
+  assert_false(anim.pending);
+
+  // Sanity check that the start time is preserved and current time is
+  // calculated using the new playback rate
+  assert_percents_equal(anim.startTime,
+                        anim.timeline.currentTime.value - 2500);
+  assert_percents_equal(anim.currentTime, 5000);
+}, 'Setting the start time of a play-pending animation applies a pending '
+   + 'playback rate');
+
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.play();
+  await anim.ready;
+
+  // We should be running now
+  assert_false(anim.pending);
+  assert_equals(anim.playState, 'running');
+
+  // Apply a pending playback rate
+  anim.updatePlaybackRate(2);
+  assert_equals(anim.playbackRate, 1);
+  assert_true(anim.pending);
+
+  // Setting the start time should apply the pending playback rate
+  anim.startTime =  CSSNumericValue.parse(
+    anim.timeline.currentTime.value - 25 + "%");
+  assert_equals(anim.playbackRate, 2);
+  assert_false(anim.pending);
+
+  // Sanity check that the start time is preserved and current time is
+  // calculated using the new playback rate
+  assert_percents_equal(anim.startTime,
+                        anim.timeline.currentTime.value - 25);
+  assert_percents_equal(anim.currentTime, 50);
+}, 'Setting the start time of a playing animation applies a pending playback '
+   + 'rate');
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/setting-timeline.tentative.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/setting-timeline.tentative.html
new file mode 100644
index 0000000000..69b40cb6c4
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/setting-timeline.tentative.html
@@ -0,0 +1,392 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Setting the timeline of scroll animation</title>
+<link rel="help"
+      href="https://drafts.csswg.org/web-animations-1/#setting-the-timeline">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+  .scroller {
+    overflow: auto;
+    height: 200px;
+    width: 100px;
+    will-change: transform;
+  }
+  .contents {
+    /* The height is set to align scrolling in pixels with logical time in ms */
+    height: 1200px;
+    width: 100%;
+  }
+  @keyframes anim {
+    from { opacity: 0; }
+    to { opacity: 1; }
+  }
+  .anim {
+    animation: anim 1s paused linear;
+  }
+</style>
+<body>
+<script>
+'use strict';
+
+function createAnimation(t) {
+  const elem = createDiv(t);
+  const animation = elem.animate({ opacity: [1, 0] }, 1000);
+  return animation;
+}
+
+function createPausedCssAnimation(t) {
+  const elem = createDiv(t);
+  elem.classList.add('anim');
+  return elem.getAnimations()[0];
+}
+
+function updateScrollPosition(timeline, offset) {
+  const scroller = timeline.source;
+  assert_true(!!scroller, 'source is resolved');
+  scroller.scrollTop = offset;
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  return waitForNextFrame();
+}
+
+function assert_timeline_current_time(animation, timeline_current_time) {
+  if (animation.currentTime instanceof CSSUnitValue){
+    assert_percents_equal(animation.timeline.currentTime, timeline_current_time,
+                          `Timeline's currentTime aligns with the scroll ` +
+                          `position even when paused`);
+  }
+  else {
+    assert_times_equal(animation.timeline.currentTime, timeline_current_time,
+                       `Timeline's currentTime aligns with the scroll ` +
+                       `position even when paused`);
+  }
+}
+
+function assert_scroll_synced_times(animation, timeline_current_time,
+                                    animation_current_time) {
+  assert_timeline_current_time(animation, timeline_current_time);
+  if (animation.currentTime instanceof CSSUnitValue){
+    assert_percents_equal(animation.currentTime, animation_current_time,
+        `Animation's currentTime aligns with the scroll position`);
+  }
+  else {
+    assert_times_equal(animation.currentTime, animation_current_time,
+        `Animation's currentTime aligns with the scroll position`);
+  }
+}
+
+function assert_paused_times(animation, timeline_current_time,
+                             animation_current_time) {
+  assert_timeline_current_time(animation, timeline_current_time);
+  if (animation.currentTime instanceof CSSUnitValue){
+    assert_percents_equal(animation.currentTime, animation_current_time,
+                          `Animation's currentTime is fixed while paused`);
+  }
+  else {
+    assert_times_equal(animation.currentTime, animation_current_time,
+                       `Animation's currentTime is fixed while paused`);
+  }
+}
+
+promise_test(async t => {
+  const scrollTimeline = createScrollTimeline(t);
+  await updateScrollPosition(scrollTimeline, 100);
+
+  const animation = createAnimation(t);
+  animation.timeline = scrollTimeline;
+  assert_true(animation.pending);
+  await animation.ready;
+
+  assert_equals(animation.playState, 'running');
+  assert_scroll_synced_times(animation, 10, 10);
+}, 'Setting a scroll timeline on a play-pending animation synchronizes ' +
+   'currentTime of the animation with the scroll position.');
+
+promise_test(async t => {
+  const scrollTimeline = createScrollTimeline(t);
+  await updateScrollPosition(scrollTimeline, 100);
+
+  const animation = createAnimation(t);
+  animation.pause();
+  animation.timeline = scrollTimeline;
+  assert_true(animation.pending);
+  await animation.ready;
+
+  assert_equals(animation.playState, 'paused');
+  assert_paused_times(animation, 10, 0);
+
+  await updateScrollPosition(animation.timeline, 200);
+
+  assert_equals(animation.playState, 'paused');
+  assert_paused_times(animation, 20, 0);
+
+  animation.play();
+  await animation.ready;
+
+  assert_scroll_synced_times(animation, 20, 20);
+}, 'Setting a scroll timeline on a pause-pending animation fixes the ' +
+   'currentTime of the animation based on the scroll position once resumed');
+
+promise_test(async t => {
+  const scrollTimeline = createScrollTimeline(t);
+  await updateScrollPosition(scrollTimeline, 100);
+
+  const animation = createAnimation(t);
+  animation.reverse();
+  animation.timeline = scrollTimeline;
+  await animation.ready;
+
+  assert_equals(animation.playState, 'running');
+  assert_scroll_synced_times(animation, 10, 90);
+},  'Setting a scroll timeline on a reversed play-pending animation ' +
+    'synchronizes the currentTime of the animation with the scroll ' +
+    'position.');
+
+promise_test(async t => {
+  const scrollTimeline = createScrollTimeline(t);
+  await updateScrollPosition(scrollTimeline, 100);
+
+  const animation = createAnimation(t);
+  await animation.ready;
+
+  animation.timeline = scrollTimeline;
+  assert_false(animation.pending);
+  assert_equals(animation.playState, 'running');
+  assert_scroll_synced_times(animation, 10, 10);
+},  'Setting a scroll timeline on a running animation synchronizes the ' +
+    'currentTime of the animation with the scroll position.');
+
+promise_test(async t => {
+  const scrollTimeline = createScrollTimeline(t);
+  await updateScrollPosition(scrollTimeline, 100);
+
+  const animation = createAnimation(t);
+  animation.pause();
+  await animation.ready;
+
+  animation.timeline = scrollTimeline;
+  assert_false(animation.pending);
+  assert_equals(animation.playState, 'paused');
+  assert_paused_times(animation, 10, 0);
+
+  animation.play();
+  await animation.ready;
+
+  assert_scroll_synced_times(animation, 10, 10);
+}, 'Setting a scroll timeline on a paused animation fixes the currentTime of ' +
+   'the animation based on the scroll position when resumed');
+
+promise_test(async t => {
+  const scrollTimeline = createScrollTimeline(t);
+  await updateScrollPosition(scrollTimeline, 100);
+
+  const animation = createAnimation(t);
+  animation.reverse();
+  animation.pause();
+  await animation.ready;
+
+  animation.timeline = scrollTimeline;
+  assert_false(animation.pending);
+  assert_equals(animation.playState, 'paused');
+  assert_paused_times(animation, 10, 100);
+
+  animation.play();
+  await animation.ready;
+
+  assert_scroll_synced_times(animation, 10, 90);
+}, 'Setting a scroll timeline on a reversed paused animation ' +
+   'fixes the currentTime of the animation based on the scroll ' +
+   'position when resumed');
+
+promise_test(async t => {
+  const animation = createAnimation(t);
+  const scrollTimeline = createScrollTimeline(t);
+  animation.timeline = scrollTimeline;
+  await animation.ready;
+  await updateScrollPosition(scrollTimeline, 100);
+
+  animation.timeline = document.timeline;
+  assert_times_equal(animation.currentTime, 100);
+}, 'Transitioning from a scroll timeline to a document timeline on a running ' +
+   'animation preserves currentTime');
+
+promise_test(async t => {
+  const animation = createAnimation(t);
+  const scrollTimeline = createScrollTimeline(t);
+  animation.timeline = scrollTimeline;
+  await animation.ready;
+  await updateScrollPosition(scrollTimeline, 100);
+
+  animation.pause();
+  animation.timeline = document.timeline;
+
+  await animation.ready;
+  assert_times_equal(animation.currentTime, 100);
+}, 'Transitioning from a scroll timeline to a document timeline on a ' +
+   'pause-pending animation preserves currentTime');
+
+promise_test(async t => {
+  const animation = createAnimation(t);
+  const scrollTimeline = createScrollTimeline(t);
+  animation.timeline = scrollTimeline;
+  await animation.ready;
+
+  animation.reverse();
+  animation.pause();
+  await updateScrollPosition(scrollTimeline, 100);
+  assert_scroll_synced_times(animation, 10, 90);
+
+  await animation.ready;
+
+  animation.timeline = document.timeline;
+  assert_false(animation.pending);
+  assert_equals(animation.playState, 'paused');
+  assert_times_equal(animation.currentTime, 900);
+}, 'Transition from a scroll timeline to a document timeline on a reversed ' +
+   'paused animation maintains correct currentTime');
+
+promise_test(async t => {
+  const animation = createAnimation(t);
+  const scrollTimeline = createScrollTimeline(t);
+  animation.timeline = scrollTimeline;
+  await animation.ready;
+  await updateScrollPosition(scrollTimeline, 100);
+
+  assert_equals(animation.playState, 'running');
+  animation.timeline = null;
+
+  assert_equals(animation.playState, 'running');
+}, 'Transitioning from a scroll timeline to a null timeline on a running ' +
+    'animation preserves the play state');
+
+promise_test(async t => {
+  const keyframeEfect = new KeyframeEffect(createDiv(t),
+                                           { opacity: [0, 1] },
+                                           1000);
+  const animation = new Animation(keyframeEfect, null);
+  animation.startTime = 0;
+  assert_equals(animation.playState, 'running');
+
+  const scrollTimeline = createScrollTimeline(t);
+  await updateScrollPosition(scrollTimeline, 100);
+
+  animation.timeline = scrollTimeline;
+  assert_equals(animation.playState, 'running');
+  assert_percents_equal(animation.currentTime, 10);
+}, 'Switching from a null timeline to a scroll timeline on an animation with ' +
+   'a resolved start time preserved the play state');
+
+promise_test(async t => {
+  const firstScrollTimeline = createScrollTimeline(t);
+  await updateScrollPosition(firstScrollTimeline, 100);
+
+  const secondScrollTimeline = createScrollTimeline(t);
+  await updateScrollPosition(secondScrollTimeline, 200);
+
+  const animation = createAnimation(t);
+  animation.timeline = firstScrollTimeline;
+  await animation.ready;
+  assert_percents_equal(animation.currentTime, 10);
+
+  animation.timeline = secondScrollTimeline;
+  assert_percents_equal(animation.currentTime, 20);
+}, 'Switching from one scroll timeline to another updates currentTime');
+
+promise_test(async t => {
+  const scrollTimeline = createScrollTimeline(t);
+  await updateScrollPosition(scrollTimeline, 100);
+
+  const animation = createPausedCssAnimation(t);
+  animation.timeline = scrollTimeline;
+  await animation.ready;
+  assert_equals(animation.playState, 'paused');
+  assert_percents_equal(animation.currentTime, 0);
+
+  const target = animation.effect.target;
+  target.style.animationPlayState = 'running';
+  await animation.ready;
+
+  assert_percents_equal(animation.currentTime, 10);
+}, 'Switching from a document timeline to a scroll timeline updates ' +
+   'currentTime when unpaused via CSS.');
+
+promise_test(async t => {
+  const scrollTimeline = createScrollTimeline(t);
+  await updateScrollPosition(scrollTimeline, 100);
+
+  const animation = createAnimation(t);
+  animation.pause();
+  animation.timeline = scrollTimeline;
+  await animation.ready;
+  assert_percents_equal(animation.currentTime, 0);
+
+  animation.currentTime = CSSNumericValue.parse("50%");
+
+  animation.play();
+  await animation.ready;
+  assert_percents_equal(animation.currentTime, 50);
+}, 'Switching from a document timeline to a scroll timeline and updating ' +
+   'currentTime preserves the new value when unpaused.');
+
+promise_test(async t => {
+  const scrollTimeline = createScrollTimeline(t);
+  await updateScrollPosition(scrollTimeline, 100);
+
+  const animation = createAnimation(t);
+  animation.pause();
+  animation.timeline = scrollTimeline;
+  await animation.ready;
+  assert_percents_equal(animation.currentTime, 0);
+  assert_equals(animation.playState, 'paused');
+
+  // Set a start time that will ensure that current time is in the active
+  // region in order not to trigger a seek when play is called.
+  animation.startTime = CSSNumericValue.parse("-10%");
+  assert_equals(animation.playState, 'running');
+
+  animation.play();
+  await animation.ready;
+  assert_percents_equal(animation.startTime, -10);
+}, 'Switching from a document timeline to a scroll timeline and updating ' +
+   'startTime preserves the new value when play is called.');
+
+promise_test(async t => {
+  const elem = createDiv(t);
+  const animation = elem.animate(null, Infinity);
+  await animation.ready;
+
+  animation.timeline = new ScrollTimeline();
+  let timing = animation.effect.getComputedTiming();
+  assert_percents_equal(timing.endTime, 100);
+  assert_percents_equal(timing.activeDuration, 100);
+  assert_percents_equal(timing.duration, 100);
+
+  animation.effect.updateTiming({ iterations: 2 });
+  timing = animation.effect.getComputedTiming();
+  assert_percents_equal(timing.endTime, 100);
+  assert_percents_equal(timing.activeDuration, 100);
+  assert_percents_equal(timing.duration, 50);
+
+  // Blink implementation does not permit setting an infinite number of
+  // iterations on a scroll-linked animation. Workaround by temporarily
+  // switching back to a document timeline.
+  animation.timeline = document.timeline;
+  animation.effect.updateTiming({ iterations: Infinity });
+  animation.timeline = new ScrollTimeline();
+  timing = animation.effect.getComputedTiming();
+  // Having an infinite number of iterations with a finite timeline results in
+  // each iteration having zero duration.
+  assert_percents_equal(timing.duration, 0);
+  // If either the iteration duration or iteration count is zero, the active
+  // duration is always zero.
+  assert_percents_equal(timing.activeDuration, 0);
+  assert_percents_equal(timing.endTime, 0);
+
+}, 'Switching from a document timeline to a scroll timeline on an infinite ' +
+   'duration animation.')
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/source-quirks-mode.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/source-quirks-mode.html
new file mode 100644
index 0000000000..17e95a0519
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/source-quirks-mode.html
@@ -0,0 +1,36 @@
+<!-- Quirks mode -->
+<html>
+<head>
+  <title>ScrollTimeline default source in quirks mode</title>
+  <link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#dom-scrolltimeline-scrolltimeline">
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+  <style>
+    /* This is just to make it possible for #body1 to be
+       "potentially scrollable".
+
+       https://drafts.csswg.org/cssom-view/#potentially-scrollable */
+    html {
+      overflow: hidden;
+    }
+  </style>
+</head>
+<body id=body1></body>
+<script>
+test(() => {
+  try {
+    assert_equals(document.scrollingElement.id, 'body1');
+    assert_equals(new ScrollTimeline({}).source, body1);
+
+    // Make #body1 "potentially scrollable". This causes the scrollingElement
+    // of the document to become null.
+    //
+    // https://drafts.csswg.org/cssom-view/#dom-document-scrollingelement
+    body1.style = 'overflow:scroll';
+    assert_equals(new ScrollTimeline({}).source, null);
+  } finally {
+    body1.style = '';
+  }
+}, 'Style of <body> is reflected in source attribute in quirks mode');
+</script>
+</html>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/testcommon.js b/testing/web-platform/tests/scroll-animations/scroll-timelines/testcommon.js
new file mode 100644
index 0000000000..97e81f494c
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/testcommon.js
@@ -0,0 +1,124 @@
+'use strict';
+
+// Builds a generic structure that looks like:
+//
+// <div class="scroller">  // 100x100 viewport
+//   <div class="contents"></div>  // 500x500
+// </div>
+//
+// The |scrollerOverrides| and |contentOverrides| parameters are maps which
+// are applied to the scroller and contents style after basic setup.
+//
+// Appends the outer 'scroller' element to the document body, and returns it.
+function setupScrollTimelineTest(
+    scrollerOverrides = new Map(), contentOverrides = new Map()) {
+  let scroller = document.createElement('div');
+  scroller.style.width = '100px';
+  scroller.style.height = '100px';
+  // Hide the scrollbars, but maintain the ability to scroll. This setting
+  // ensures that variability in scrollbar sizing does not contribute to test
+  // failures or flakes.
+  scroller.style.overflow = 'hidden';
+  for (const [key, value] of scrollerOverrides) {
+    scroller.style[key] = value;
+  }
+
+  let contents = document.createElement('div');
+  contents.style.width = '500px';
+  contents.style.height = '500px';
+  for (const [key, value] of contentOverrides) {
+    contents.style[key] = value;
+  }
+
+  scroller.appendChild(contents);
+  document.body.appendChild(scroller);
+  return scroller;
+}
+
+// Helper method to calculate the current time, implementing only step 5 of
+// https://wicg.github.io/scroll-animations/#current-time-algorithm
+function calculateCurrentTime(
+    currentScrollOffset, startScrollOffset, endScrollOffset) {
+  return ((currentScrollOffset - startScrollOffset) /
+          (endScrollOffset - startScrollOffset)) *
+         100;
+}
+
+function createScroller(test) {
+  var scroller = createDiv(test);
+  scroller.innerHTML = "<div class='contents'></div>";
+  scroller.classList.add('scroller');
+  // Trigger layout run.
+  scroller.scrollTop;
+  return scroller;
+}
+
+function createScrollerWithStartAndEnd(test, orientationClass = 'vertical') {
+  var scroller = createDiv(test);
+  scroller.innerHTML =
+    `<div class='contents'>
+        <div id='start'></div>
+        <div id='end'></div>
+      </div>`;
+  scroller.classList.add('scroller');
+  scroller.classList.add(orientationClass);
+
+  return scroller;
+}
+
+function createScrollTimeline(test, options) {
+  options = options || {
+    source: createScroller(test)
+  }
+  return new ScrollTimeline(options);
+}
+
+function createScrollLinkedAnimation(test, timeline) {
+  return createScrollLinkedAnimationWithTiming(test, /* duration in ms*/ 1000, timeline);
+}
+
+function createScrollLinkedAnimationWithTiming(test, timing, timeline) {
+  if (timeline === undefined)
+    timeline = createScrollTimeline(test);
+  const KEYFRAMES = { opacity: [0, 1] };
+  return new Animation(
+    new KeyframeEffect(createDiv(test), KEYFRAMES, timing), timeline);
+}
+
+function assert_approx_equals_or_null(actual, expected, tolerance, name) {
+  if (actual === null || expected === null){
+    assert_equals(actual, expected, name);
+  }
+  else {
+    assert_approx_equals(actual, expected, tolerance, name);
+  }
+}
+
+function assert_percents_approx_equal(actual, expected, maxScroll,
+                                      description) {
+  // Base the tolerance on being out by up to half a pixel.
+  const tolerance = 0.5 / maxScroll * 100;
+  assert_equals(actual.unit, "percent", `'actual' unit type must be ` +
+      `'percent' for "${description}"`);
+  assert_true(actual instanceof CSSUnitValue, `'actual' must be of type ` +
+      `CSSNumberish for "${description}"`);
+  if (expected instanceof CSSUnitValue){
+    // Verify that when the expected in a CSSUnitValue, it is the correct unit
+    // type
+    assert_equals(expected.unit, "percent", `'expected' unit type must be ` +
+        `'percent' for "${description}"`);
+    assert_approx_equals(actual.value, expected.value, tolerance,
+        `values do not match for "${description}"`);
+  } else if (typeof expected, "number"){
+    assert_approx_equals(actual.value, expected, tolerance,
+        `values do not match for "${description}"`);
+  }
+}
+
+function assert_percents_equal(actual, expected, description) {
+  // Rough estimate of the default size of the scrollable area based on
+  // sizes in setupScrollTimelineTest.
+  const defaultScrollRange = 400;
+  return assert_percents_approx_equal(actual, expected, defaultScrollRange,
+                                      description);
+}
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/two-animations-attach-to-same-scroll-timeline-cancel-one.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/two-animations-attach-to-same-scroll-timeline-cancel-one.html
new file mode 100644
index 0000000000..ed8e8337a6
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/two-animations-attach-to-same-scroll-timeline-cancel-one.html
@@ -0,0 +1,84 @@
+<html class="reftest-wait">
+<title>Scroll timeline shared by two animation, one gets cancelled</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations/">
+<meta name="assert" content="Cancelling animations should not affect other
+                   animation that is attached to the same timeline.">
+<link rel="match" href="animation-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  #scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+    will-change: transform; /* force compositing */
+  }
+
+  #contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+<div id="scroller">
+  <div id="contents"><p>Scrolling Contents</p></div>
+</div>
+
+<script>
+  const box = document.getElementById('box');
+  const effect = new KeyframeEffect(box,
+    [
+    { transform: 'translateY(0)', opacity: 1},
+    { transform: 'translateY(200px)', opacity: 0}
+    ], {
+      duration: 1000,
+    }
+  );
+  const temporary_effect = new KeyframeEffect(box,
+    [
+    { transform: 'translateX(0)'},
+    { transform: 'translateX(200px)'}
+    ], {
+      duration: 1000,
+    }
+  );
+
+  const scroller = document.getElementById('scroller');
+  const timeline = new ScrollTimeline(
+      { source: scroller, orientation: 'block' });
+  const animation = new Animation(effect, timeline);
+  const temporary_animation = new Animation(temporary_effect, timeline);
+  animation.play();
+  temporary_animation.play();
+
+  Promise.all([animation.ready, temporary_animation.ready]).then(() => {
+    temporary_animation.cancel();
+    temporary_animation.ready.then(() => {
+      waitForAnimationFrames(2).then(_ => {
+        // Move the scroller to the halfway point.
+        const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+        scroller.scrollTop = 0.5 * maxScroll;
+
+        waitForAnimationFrames(2).then(_ => {
+          takeScreenshot();
+        });
+      });
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/two-animations-attach-to-same-scroll-timeline.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/two-animations-attach-to-same-scroll-timeline.html
new file mode 100644
index 0000000000..de50599fba
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/two-animations-attach-to-same-scroll-timeline.html
@@ -0,0 +1,79 @@
+<html class="reftest-wait">
+<title>Scroll timeline shared by two animation</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations/">
+<meta name="assert" content="Should be able to use the same scroll timeline to
+drive two animations">
+<link rel="match" href="animation-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  #scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+    will-change: transform; /* force compositing */
+  }
+
+  #contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+
+<div id="box"></div>
+<div id="covered"></div>
+<div id="scroller">
+  <div id="contents"><p>Scrolling Contents</p></div>
+</div>
+
+<script>
+  const box = document.getElementById('box');
+  const transform_effect = new KeyframeEffect(box,
+    [
+    { transform: 'translateY(0)'},
+    { transform: 'translateY(200px)'}
+    ], {
+      duration: 1000,
+    }
+  );
+  const opacity_effect = new KeyframeEffect(box,
+    [
+    { opacity: 1},
+    { opacity: 0}
+    ], {
+      duration: 1000,
+    }
+  );
+
+  const scroller = document.getElementById('scroller');
+  const timeline = new ScrollTimeline(
+      { source: scroller, orientation: 'block' });
+  const transform_animation = new Animation(transform_effect, timeline);
+  transform_animation.play();
+  const opacity_animation = new Animation(opacity_effect, timeline);
+  opacity_animation.play();
+
+  Promise.all([transform_animation.ready, opacity_animation.ready]).then(() => {
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+
+    waitForAnimationFrames(2).then(_ => {
+      takeScreenshot();
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/update-playback-rate.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/update-playback-rate.html
new file mode 100644
index 0000000000..1db31602f3
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/update-playback-rate.html
@@ -0,0 +1,181 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Seamlessly updating the playback rate of an animation</title>
+<link rel="help"
+  href="https://drafts.csswg.org/web-animations-1/#seamlessly-updating-the-playback-rate-of-an-animation">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+.scroller {
+  overflow: auto;
+  height: 100px;
+  width: 100px;
+  will-change: transform;
+}
+
+.contents {
+  height: 1000px;
+  width: 100%;
+}
+</style>
+<body>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  animation.currentTime = CSSNumericValue.parse("50%");
+
+  animation.updatePlaybackRate(0.5);
+  await animation.ready;
+  assert_percents_equal(animation.currentTime, 50,
+                        'Reducing the playback rate should not change the ' +
+                        'current time of a playing animation');
+
+  animation.updatePlaybackRate(2);
+  await animation.ready;
+  assert_percents_equal(animation.currentTime, 50,
+                        'Increasing the playback rate should not change the ' +
+                        'current time of a playing animation');
+}, 'Updating the playback rate maintains the current time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  await animation.ready;
+
+  assert_false(animation.pending);
+  animation.updatePlaybackRate(2);
+  assert_true(animation.pending);
+}, 'Updating the playback rate while running makes the animation pending');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.currentTime = CSSNumericValue.parse("50%");
+  assert_true(animation.pending);
+
+  animation.updatePlaybackRate(0.5);
+
+  // Check that the hold time is updated as expected
+  assert_percents_equal(animation.currentTime, 50);
+
+  await animation.ready;
+
+  // As above, check that the currentTime is not calculated by simply
+  // substituting in the updated playbackRate without updating the startTime.
+  assert_percents_equal(animation.currentTime, 50,
+                        'Reducing the playback rate should not change the ' +
+                        'current time of a play-pending animation');
+}, 'Updating the playback rate on a play-pending animation maintains the ' +
+   'current time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.currentTime = CSSNumericValue.parse("50%");
+  await animation.ready;
+
+  animation.pause();
+  animation.updatePlaybackRate(0.5);
+
+  assert_percents_equal(animation.currentTime, 50);
+}, 'Updating the playback rate on a pause-pending animation maintains the ' +
+   'current time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+
+  animation.updatePlaybackRate(2);
+  animation.updatePlaybackRate(3);
+  animation.updatePlaybackRate(4);
+
+  assert_equals(animation.playbackRate, 1);
+  await animation.ready;
+
+  assert_equals(animation.playbackRate, 4);
+}, 'If a pending playback rate is set multiple times, the latest wins');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.cancel();
+
+  animation.updatePlaybackRate(2);
+  assert_equals(animation.playbackRate, 2);
+  assert_false(animation.pending);
+}, 'In the idle state, the playback rate is applied immediately');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.pause();
+  await animation.ready;
+
+  animation.updatePlaybackRate(2);
+  assert_equals(animation.playbackRate, 2);
+  assert_false(animation.pending);
+}, 'In the paused state, the playback rate is applied immediately');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.finish();
+  assert_percents_equal(animation.currentTime, 100);
+  assert_false(animation.pending);
+
+  animation.updatePlaybackRate(2);
+  assert_equals(animation.playbackRate, 2);
+  assert_percents_equal(animation.currentTime, 100);
+  assert_false(animation.pending);
+}, 'Updating the playback rate on a finished animation maintains the current ' +
+   'time');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  animation.play();
+  animation.finish();
+  assert_percents_equal(animation.currentTime, 100);
+  assert_false(animation.pending);
+
+  animation.updatePlaybackRate(0);
+  assert_equals(animation.playbackRate, 0);
+  assert_percents_equal(animation.currentTime, 100);
+  assert_false(animation.pending);
+}, 'Updating the playback rate to zero on a finished animation maintains the ' +
+   'current time');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/scroll-timelines/updating-the-finished-state.html b/testing/web-platform/tests/scroll-animations/scroll-timelines/updating-the-finished-state.html
new file mode 100644
index 0000000000..86b52d5aa0
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/scroll-timelines/updating-the-finished-state.html
@@ -0,0 +1,565 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Updating the finished state</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#updating-the-finished-state">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="testcommon.js"></script>
+<style>
+.scroller {
+  overflow: auto;
+  height: 100px;
+  width: 100px;
+  will-change: transform;
+}
+
+.contents {
+  height: 1000px;
+  width: 100%;
+}
+</style>
+<body>
+<script>
+'use strict';
+
+// --------------------------------------------------------------------
+//
+// TESTS FOR UPDATING THE HOLD TIME
+//
+// --------------------------------------------------------------------
+
+// CASE 1: playback rate > 0 and current time >= target effect end
+// (Also the start time is resolved and there is pending task)
+// Did seek = true
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.play();
+
+  await anim.ready;
+
+  anim.currentTime = CSS.percent(200);
+  scroller.scrollTop = 0.7 * maxScroll;
+  await waitForNextFrame();
+
+  assert_percents_equal(anim.currentTime, 200,
+      'Hold time is set so current time should NOT change');
+}, 'Updating the finished state when seeking past end');
+
+// Did seek = false
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.play();
+  await anim.ready;
+
+  scroller.scrollTop = maxScroll;
+  await waitForNextFrame();
+
+  assert_percents_equal(anim.currentTime, 100,
+      'Hold time is set to target end clamping current time');
+}, 'Updating the finished state when playing exactly to end');
+
+// Did seek = true
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  await anim.ready;
+
+  anim.currentTime = CSS.percent(100);
+  scroller.scrollTop = 0.7 * maxScroll;
+  await waitForNextFrame();
+
+  assert_percents_equal(anim.currentTime, 100,
+      'Hold time is set so current time should NOT change');
+}, 'Updating the finished state when seeking exactly to end');
+
+
+// CASE 2: playback rate < 0 and current time <= 0
+// (Also the start time is resolved and there is pending task)
+
+// Did seek = false
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.playbackRate = -1;
+  anim.play(); // Make sure animation is not initially finished
+
+  await anim.ready;
+
+  // Seek to 1ms before 0 and then wait 1ms
+  anim.currentTime = CSS.percent(1);
+  scroller.scrollTop = 0.2 * maxScroll;
+  await waitForNextFrame();
+
+  assert_percents_equal(anim.currentTime, 0,
+      'Hold time is set to zero clamping current time');
+}, 'Updating the finished state when playing in reverse past zero');
+
+// Did seek = true
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.playbackRate = -1;
+  anim.play();
+
+  await anim.ready;
+
+  anim.currentTime = CSS.percent(-100);
+  scroller.scrollTop = 0.2 * maxScroll;
+  await waitForNextFrame();
+
+  assert_percents_equal(anim.currentTime, -100,
+      'Hold time is set so current time should NOT change');
+}, 'Updating the finished state when seeking a reversed animation past zero');
+
+// Did seek = false
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.playbackRate = -1;
+  anim.play();
+  await anim.ready;
+
+  scroller.scrollTop = maxScroll;
+  await waitForNextFrame();
+
+  assert_percents_equal(anim.currentTime, 0,
+      'Hold time is set to target end clamping current time');
+}, 'Updating the finished state when playing  a reversed animation exactly ' +
+   'to zero');
+
+// Did seek = true
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.playbackRate = -1;
+  anim.play();
+  await anim.ready;
+
+  anim.currentTime = CSS.percent(0);
+
+  scroller.scrollTop = 0.2 * maxScroll;
+  await waitForNextFrame();
+
+  assert_percents_equal(anim.currentTime, 0,
+      'Hold time is set so current time should NOT change');
+}, 'Updating the finished state when seeking a reversed animation exactly ' +
+   'to zero');
+
+// CASE 3: playback rate > 0 and current time < target end OR
+//         playback rate < 0 and current time > 0
+// (Also the start time is resolved and there is pending task)
+
+// Did seek = true; playback rate > 0
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.play();
+  anim.finish();
+  await anim.ready;
+  assert_percents_equal(anim.startTime, -100);
+
+  anim.currentTime = CSS.percent(50);
+  // When did seek = true, updating the finished state: (i) updates
+  // the animation's start time and (ii) clears the hold time.
+  // We can test both by checking that the currentTime is initially
+  // updated and then increases.
+  assert_percents_equal(anim.currentTime, 50, 'Start time is updated');
+  assert_percents_equal(anim.startTime, -50);
+
+  scroller.scrollTop = 0.2 * maxScroll;
+  await waitForNextFrame();
+
+  assert_percents_equal(anim.currentTime, 70,
+      'Hold time is not set so current time should increase');
+}, 'Updating the finished state when seeking before end');
+
+// Did seek = false; playback rate < 0
+//
+// Unfortunately it is not possible to test this case. We need to have
+// a hold time set, a resolved start time, and then perform some
+// operation that updates the finished state with did seek set to true.
+//
+// However, the only situation where this could arrive is when we
+// replace the timeline and that procedure is likely to change. For all
+// other cases we either have an unresolved start time (e.g. when
+// paused), we don't have a set hold time (e.g. regular playback), or
+// the current time is zero (and anything that gets us out of that state
+// will set did seek = true).
+
+// Did seek = true; playback rate < 0
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.play();
+  anim.playbackRate = -1;
+  await anim.ready;
+
+  anim.currentTime = CSS.percent(50);
+  assert_percents_equal(anim.startTime, 50, 'Start time is updated');
+  assert_percents_equal(anim.currentTime, 50, 'Current time is updated');
+
+  scroller.scrollTop = 0.2 * maxScroll;
+  await waitForNextFrame();
+
+  assert_percents_equal(anim.currentTime, 30,
+      'Hold time is not set so current time should decrease');
+}, 'Updating the finished state when seeking a reversed animation before end');
+
+
+// CASE 4: playback rate == 0
+
+// current time < 0
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.play();
+  anim.playbackRate = 0;
+  await anim.ready;
+
+  anim.currentTime = CSS.percent(-100);
+
+  scroller.scrollTop = 0.2 * maxScroll;
+  await waitForNextFrame();
+
+  assert_percents_equal(anim.currentTime, -100,
+      'Hold time should not be cleared so current time should NOT change');
+}, 'Updating the finished state when playback rate is zero and the current ' +
+   'time is less than zero');
+
+// current time < target end
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.play();
+
+  anim.playbackRate = 0;
+  await anim.ready;
+
+  anim.currentTime = CSS.percent(50);
+  scroller.scrollTop = 0.2 * maxScroll;
+  await waitForNextFrame();
+
+  assert_percents_equal(anim.currentTime, 50,
+      'Hold time should not be cleared so current time should NOT change');
+}, 'Updating the finished state when playback rate is zero and the current ' +
+   'time is less than end');
+
+// current time > target end
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.play();
+  anim.playbackRate = 0;
+  await anim.ready;
+
+  anim.currentTime = CSS.percent(200);
+  scroller.scrollTop = 0.2 * maxScroll;
+  await waitForNextFrame();
+
+  assert_percents_equal(anim.currentTime, 200,
+      'Hold time should not be cleared so current time should NOT change');
+}, 'Updating the finished state when playback rate is zero and the current' +
+   'time is greater than end');
+
+// CASE 5: current time unresolved
+
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.play();
+  anim.cancel();
+  // Trigger a change that will cause the "update the finished state"
+  // procedure to run.
+  anim.effect.updateTiming({ duration: 2000 });
+  assert_equals(anim.currentTime, null,
+      'The animation hold time / start time should not be updated');
+  // The "update the finished state" procedure is supposed to run after any
+  // change to timing, but just in case an implementation defers that, let's
+  // wait a frame and check that the hold time / start time has still not been
+  // updated.
+  await waitForAnimationFrames(1);
+
+  assert_equals(anim.currentTime, null,
+      'The animation hold time / start time should not be updated');
+}, 'Updating the finished state when current time is unresolved');
+
+// CASE 7: start time unresolved
+
+// Did seek = true
+promise_test(async t => {
+  const anim = createScrollLinkedAnimation(t);
+  // Wait for new animation frame which allows the timeline to compute new
+  // current time.
+  await waitForNextFrame();
+  anim.cancel();
+  anim.currentTime = CSS.percent(150);
+  // Trigger a change that will cause the "update the finished state"
+  // procedure to run.
+  anim.currentTime = CSS.percent(50);
+  assert_percents_equal(anim.currentTime, 50,
+      'The animation hold time should not be updated');
+  assert_equals(anim.startTime, null,
+      'The animation start time should not be updated');
+}, 'Updating the finished state when start time is unresolved and did seek = ' +
+   'true');
+
+// --------------------------------------------------------------------
+//
+// TESTS FOR RUNNING FINISH NOTIFICATION STEPS
+//
+// --------------------------------------------------------------------
+
+function waitForFinishEventAndPromise(animation) {
+  const eventPromise = new Promise(resolve => {
+    animation.onfinish = resolve;
+  });
+  return Promise.all([eventPromise, animation.finished]);
+}
+
+promise_test(t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  animation.play();
+  animation.onfinish =
+      t.unreached_func('Seeking to finish should not fire finish event');
+  animation.finished.then(
+      t.unreached_func(
+          'Seeking to finish should not resolve finished promise'));
+  animation.currentTime = CSS.percent(100);
+  animation.currentTime = CSS.percent(0);
+  animation.pause();
+  scroller.scrollTop = 0.2 * maxScroll;
+  return waitForAnimationFrames(3);
+}, 'Finish notification steps don\'t run when the animation seeks to finish ' +
+   'and then seeks back again');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  animation.play();
+  await animation.ready;
+  scroller.scrollTop = maxScroll;
+
+  return waitForFinishEventAndPromise(animation);
+}, 'Finish notification steps run when the animation completes normally');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+  animation.effect.target = null;
+
+  animation.play();
+  await animation.ready;
+  scroller.scrollTop = maxScroll;
+  return waitForFinishEventAndPromise(animation);
+}, 'Finish notification steps run when an animation without a target effect ' +
+   'completes normally');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  animation.play();
+  await animation.ready;
+
+  animation.currentTime = CSS.percent(101);
+  return waitForFinishEventAndPromise(animation);
+}, 'Finish notification steps run when the animation seeks past finish');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  animation.play();
+  await animation.ready;
+
+  // Register for notifications now since once we seek away from being
+  // finished the 'finished' promise will be replaced.
+  const finishNotificationSteps = waitForFinishEventAndPromise(animation);
+  animation.finish();
+  animation.currentTime = CSS.percent(0);
+  animation.pause();
+  return finishNotificationSteps;
+}, 'Finish notification steps run when the animation completes with ' +
+   '.finish(), even if we then seek away');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  animation.play();
+  scroller.scrollTop = maxScroll;
+  const initialFinishedPromise = animation.finished;
+  await animation.finished;
+
+  animation.currentTime = CSS.percent(0);
+  assert_not_equals(initialFinishedPromise, animation.finished);
+}, 'Animation finished promise is replaced after seeking back to start');
+
+promise_test(async t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  animation.play();
+
+  const initialFinishedPromise = animation.finished;
+  scroller.scrollTop = maxScroll;
+  await animation.finished;
+
+  scroller.scrollTop = 0;
+  await waitForNextFrame();
+
+  animation.play();
+  assert_not_equals(initialFinishedPromise, animation.finished);
+}, 'Animation finished promise is replaced after replaying from start');
+
+async_test(t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  animation.play();
+
+  animation.onfinish = event => {
+    scroller.scrollTop = 0;
+    window.requestAnimationFrame(function() {
+      window.requestAnimationFrame(function() {
+        scroller.scrollTop = maxScroll;
+      });
+    });
+    animation.onfinish = event => {
+      t.done();
+    };
+  };
+  scroller.scrollTop = maxScroll;
+}, 'Animation finish event is fired again after seeking back to start');
+
+async_test(t => {
+  const animation = createScrollLinkedAnimation(t);
+  const scroller = animation.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  animation.play();
+
+  animation.onfinish = event => {
+    scroller.scrollTop = 0;
+    window.requestAnimationFrame(function() {
+      animation.play();
+      scroller.scrollTop = maxScroll;
+      animation.onfinish = event => {
+        t.done();
+      };
+    });
+  };
+  scroller.scrollTop = maxScroll;
+}, 'Animation finish event is fired again after replaying from start');
+
+async_test(t => {
+  const anim = createScrollLinkedAnimation(t);
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  anim.effect.updateTiming({ duration: 800,  endDelay: 200});
+
+  anim.onfinish = t.step_func(event => {
+    assert_unreached('finish event should not be fired');
+  });
+  anim.play();
+  anim.ready.then(() => {
+    scroller.scrollTop = 0.9 * maxScroll;
+    return waitForAnimationFrames(3);
+  }).then(t.step_func(() => {
+    t.done();
+  }));
+}, 'finish event is not fired at the end of the active interval when the ' +
+   'endDelay has not expired');
+
+async_test(t => {
+  const anim = createScrollLinkedAnimation(t);
+  const scroller = anim.timeline.source;
+  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+  anim.effect.updateTiming({ duration: 800,  endDelay: 100});
+  anim.play();
+  anim.ready.then(() => {
+    scroller.scrollTop = 0.95 * maxScroll; // during endDelay
+    anim.onfinish = t.step_func(event => {
+      assert_unreached('onfinish event should not be fired during endDelay');
+    });
+    return waitForAnimationFrames(2);
+  }).then(t.step_func(() => {
+    anim.onfinish = t.step_func(event => {
+      t.done();
+    });
+    scroller.scrollTop = maxScroll;
+    return waitForAnimationFrames(2);
+  }));
+}, 'finish event is fired after the endDelay has expired');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/scroll-animations/view-timelines/block-view-timeline-current-time-vertical-rl.tentative.html b/testing/web-platform/tests/scroll-animations/view-timelines/block-view-timeline-current-time-vertical-rl.tentative.html
new file mode 100644
index 0000000000..5bc4598452
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/view-timelines/block-view-timeline-current-time-vertical-rl.tentative.html
@@ -0,0 +1,97 @@
+<!DOCTYPE html>
+<html id="top">
+<meta charset="utf-8">
+<title>View timeline current-time with vertical-rl writing mode</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#viewtimeline-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="/scroll-animations/view-timelines/testcommon.js"></script>
+<style>
+  #container {
+    writing-mode: vertical-rl;
+    overflow-x: scroll;
+    height:  200px;
+    width: 200px;
+  }
+  .spacer {
+    width:  800px;
+  }
+  #target {
+    background-color:  green;
+    height:  100px;
+    width:  200px;
+  }
+</style>
+<body>
+  <div id="container">
+    <div id="leading-space" class="spacer"></div>
+    <div id="target"></div>
+    <div id="trailing-space" class="spacer"></div>
+  </div>
+</body>
+<script type="text/javascript">
+  promise_test(async t => {
+    container.scrollLeft = 0;
+    await waitForNextFrame();
+
+    const anim = CreateViewTimelineOpacityAnimation(t, target, {axis: 'block'});
+    const timeline = anim.timeline;
+    await anim.ready;
+
+    // Initially before start-offset and animation effect is in the before
+    // phase.
+    assert_percents_equal(timeline.currentTime, -150,
+                          "Timeline's currentTime at container start boundary");
+    assert_percents_equal(anim.currentTime, -150,
+                          "Animation's currentTime at container start boundary");
+    assert_equals(getComputedStyle(target).opacity, "1",
+                  'Effect is inactive in the before phase');
+
+    // Advance to the start offset, which triggers entry to the active phase.
+    container.scrollLeft = -600;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 0,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 0,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '0.3',
+                  'Effect at the start of the active phase');
+
+    // Advance to the midpoint of the animation.
+    container.scrollLeft = -800;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 50,
+                          "Timeline's currentTime at midpoint");
+    assert_percents_equal(anim.currentTime, 50,
+                          "Animation's currentTime at midpoint");
+    assert_equals(getComputedStyle(target).opacity,'0.5',
+                  'Effect at the midpoint of the active range');
+
+    // Advance to the end of the animation.
+    container.scrollLeft = -1000;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 100,
+                          "Timeline's currentTime at end offset");
+    assert_percents_equal(anim.currentTime, 100,
+                          "Animation's currentTime at end offset");
+    assert_equals(getComputedStyle(target).opacity, '0.7',
+                  'Effect is in the active phase at effect end time');
+
+    // Advance to the scroll limit.
+    container.scrollLeft = -1600;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 250,
+                          "Timeline's currentTime at scroll limit");
+    // Hold time set when the animation finishes, which clamps the value of
+    // the animation's currentTime.
+    assert_percents_equal(anim.currentTime, 100,
+                          "Animation's currentTime at scroll limit");
+    // In the after phase, so the effect should not be applied.
+    assert_equals(getComputedStyle(target).opacity, '1',
+                  'After phase at scroll limit');
+  }, 'View timeline with container having vertical-rl layout' );
+
+</script>
+</html>
diff --git a/testing/web-platform/tests/scroll-animations/view-timelines/block-view-timeline-current-time.tentative.html b/testing/web-platform/tests/scroll-animations/view-timelines/block-view-timeline-current-time.tentative.html
new file mode 100644
index 0000000000..a6530f6631
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/view-timelines/block-view-timeline-current-time.tentative.html
@@ -0,0 +1,205 @@
+<!DOCTYPE html>
+<html id="top">
+<meta charset="utf-8">
+<title>View timeline current-time</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#viewtimeline-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="/scroll-animations/view-timelines/testcommon.js"></script>
+<style>
+  #container {
+    border:  10px solid lightgray;
+    overflow-y: scroll;
+    height:  200px;
+    width: 200px;
+  }
+  .spacer {
+    height:  800px;
+  }
+  #target {
+    background-color:  green;
+    height:  200px;
+    width:  100px;
+  }
+</style>
+<body>
+  <div id="container">
+    <div id="leading-space" class="spacer"></div>
+    <div id="target"></div>
+    <div id="trailing-space" class="spacer"></div>
+  </div>
+</body>
+<script type="text/javascript">
+  promise_test(async t => {
+    container.scrollTop = 0;
+    await waitForNextFrame();
+
+    const anim = CreateViewTimelineOpacityAnimation(t, target);
+    const timeline = anim.timeline;
+    await anim.ready;
+
+    // Initially before start-offset and animation effect is in the before
+    // phase.
+    assert_percents_equal(timeline.currentTime, -150,
+                          "Timeline's currentTime at container start boundary");
+    assert_percents_equal(anim.currentTime, -150,
+                          "Animation's currentTime at container start boundary");
+    assert_equals(getComputedStyle(target).opacity, "1",
+                  'Effect is inactive in the before phase');
+
+    // Advance to the start offset, which triggers entry to the active phase.
+    container.scrollTop = 600;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 0,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 0,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '0.3',
+                  'Effect at the start of the active phase');
+
+    // Advance to the midpoint of the animation.
+    container.scrollTop = 800;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 50,
+                          "Timeline's currentTime at midpoint");
+    assert_percents_equal(anim.currentTime, 50,
+                          "Animation's currentTime at midpoint");
+    assert_equals(getComputedStyle(target).opacity,'0.5',
+                  'Effect at the midpoint of the active range');
+
+    // Advance to the end of the animation.
+    container.scrollTop = 1000;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 100,
+                          "Timeline's currentTime at end offset");
+    assert_percents_equal(anim.currentTime, 100,
+                          "Animation's currentTime at end offset");
+    assert_equals(getComputedStyle(target).opacity, '0.7',
+                  'Effect is in the active phase at effect end time');
+
+    // Advance to the scroll limit.
+    container.scrollTop = 1600;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 250,
+                          "Timeline's currentTime at scroll limit");
+    // Hold time set when the animation finishes, which clamps the value of
+    // the animation's currentTime.
+    assert_percents_equal(anim.currentTime, 100,
+                          "Animation's currentTime at scroll limit");
+    // In the after phase, so the effect should not be applied.
+    assert_equals(getComputedStyle(target).opacity, '1',
+                  'After phase at scroll limit');
+  }, 'View timeline with start and end scroll offsets that do not align with ' +
+     'the scroll boundaries' );
+
+  promise_test(async t => {
+    const leading = document.getElementById('leading-space');
+    leading.style = 'display: none';
+    t.add_cleanup(() => {
+      leading.style = null;
+    });
+
+    container.scrollTop = 0;
+    await waitForNextFrame();
+
+    const anim = CreateViewTimelineOpacityAnimation(t, target);
+    const timeline = anim.timeline;
+    await anim.ready;
+
+    assert_percents_equal(timeline.currentTime, 50,
+                          "Timeline's currentTime at container start boundary");
+    assert_percents_equal(anim.currentTime, 50,
+                          "Animation's currentTime at container start boundary");
+    assert_equals(getComputedStyle(target).opacity, "0.5",
+                  'Effect enters active phase at container start boundary');
+
+
+    // Advance to midpoint
+    container.scrollTop = 100;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 75,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 75,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '0.6',
+                  'Effect at the start of the active phase');
+
+    // Advance to end-offset
+    container.scrollTop = 200;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 100,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 100,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '0.7',
+                  'Effect at the start of the active phase');
+
+    // Advance to scroll limit.
+    container.scrollTop = 800;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 250,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 100,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '1',
+                  'Effect at the start of the active phase');
+
+  }, 'View timeline does not clamp starting scroll offset at 0');
+
+  promise_test(async t => {
+    const trailing = document.getElementById('trailing-space');
+    trailing.style = 'display: none';
+    t.add_cleanup(() => {
+      trailing.style = null;
+    });
+
+    container.scrollTop = 0;
+    await waitForNextFrame();
+
+    const anim = CreateViewTimelineOpacityAnimation(t, target);
+    const timeline = anim.timeline;
+    await anim.ready;
+
+    // Initially in before phase.
+    assert_percents_equal(timeline.currentTime, -150,
+                          "Timeline's currentTime at container start boundary");
+    assert_percents_equal(anim.currentTime, -150,
+                          "Animation's currentTime at container start boundary");
+    assert_equals(getComputedStyle(target).opacity, "1",
+                  'Effect enters active phase at container start boundary');
+
+    // Advance to start offset.
+    container.scrollTop = 600;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 0,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 0,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '0.3',
+                  'Effect at the start of the active phase');
+
+    // Advance to midpoint.
+    container.scrollTop = 700;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 25,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 25,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '0.4',
+                  'Effect at the start of the active phase');
+
+    // Advance to end offset.
+    container.scrollTop = 800;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 50,
+                          "Timeline's currentTime at max scroll offset");
+    assert_percents_equal(anim.currentTime, 50,
+                          "Animation's currentTime at max scroll offset");
+    assert_equals(getComputedStyle(target).opacity, "0.5",
+                  'Effect at end of active phase');
+  }, 'View timeline  does not clamp end scroll offset at max scroll');
+
+</script>
+</html>
diff --git a/testing/web-platform/tests/scroll-animations/view-timelines/block-view-timeline-nested-subject.tentative.html b/testing/web-platform/tests/scroll-animations/view-timelines/block-view-timeline-nested-subject.tentative.html
new file mode 100644
index 0000000000..2cc8af882f
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/view-timelines/block-view-timeline-nested-subject.tentative.html
@@ -0,0 +1,113 @@
+<!DOCTYPE html>
+<html id="top">
+<meta charset="utf-8">
+<title>View timeline nested subject</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#viewtimeline-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="/scroll-animations/view-timelines/testcommon.js"></script>
+<style type="text/css">
+  #container {
+    overflow-y: scroll;
+    height:  300px;
+    width: 300px;
+  }
+  .big-spacer {
+    height:  800px;
+  }
+  .small-spacer {
+    height:  100px;
+  }
+  #block {
+    background-color:  #ddd;
+  }
+  #target {
+    background-color:  green;
+    height:  100px;
+    width:  100px;
+  }
+</style>
+<body>
+  <div id="container">
+    <div class="big-spacer"></div>
+    <div id="block">
+      <div class="small-spacer"></div>
+      <div id="target"></div>
+    </div>
+    <div class="big-spacer"></div>
+  </div>
+</body>
+<script type="text/javascript">
+  promise_test(async t => {
+    container.scrollTop = 0;
+    await waitForNextFrame();
+
+    const anim = CreateViewTimelineOpacityAnimation(t, target);
+    const timeline = anim.timeline;
+    await anim.ready;
+
+    // start offset = 800 + 100 - 300 = 600
+    // end offset = 800 + 100 + 100 = 1000
+    // scroll limit = L = 800 + 200 + 800 - 300 = 1500
+    // progress = P =  (current - start) / (end - start)
+    // P(0) = -600 / 400 = -1.5
+    // P(L) = 900 / 400 = 2.5
+
+    // Initially before start-offset and animation effect is in the before
+    // phase.
+    assert_percents_equal(timeline.currentTime, -150,
+                          "Timeline's currentTime at container start boundary");
+    assert_percents_equal(anim.currentTime, -150,
+                          "Animation's currentTime at container start boundary");
+    assert_equals(getComputedStyle(target).opacity, "1",
+                  'Effect is inactive in the before phase');
+
+
+    // Advance to the start offset, which triggers entry to the active phase.
+    container.scrollTop = 600;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 0,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 0,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '0.3',
+                  'Effect at the start of the active phase');
+
+    // Advance to the midpoint of the animation.
+    container.scrollTop = 800;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 50,
+                          "Timeline's currentTime at midpoint");
+    assert_percents_equal(anim.currentTime, 50,
+                          "Animation's currentTime at midpoint");
+    assert_equals(getComputedStyle(target).opacity,'0.5',
+                  'Effect at the midpoint of the active range');
+
+    // Advance to the end of the animation.
+    container.scrollTop = 1000;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 100,
+                          "Timeline's currentTime at end offset");
+    assert_percents_equal(anim.currentTime, 100,
+                          "Animation's currentTime at end offset");
+    assert_equals(getComputedStyle(target).opacity, '0.7',
+                  'Effect is in the active phase at effect end time');
+
+    // Advance to the scroll limit.
+    container.scrollTop = 1600;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 225,
+                          "Timeline's currentTime at scroll limit");
+    // Hold time set when the animation finishes, which clamps the value of
+    // the animation's currentTime.
+    assert_percents_equal(anim.currentTime, 100,
+                          "Animation's currentTime at scroll limit");
+    // In the after phase, so the effect should not be applied.
+    assert_equals(getComputedStyle(target).opacity, '1',
+                  'After phase at scroll limit');
+  }, 'View timeline with subject that is not a direct descendant of the ' +
+     'scroll container');
+</script>
+</html>
diff --git a/testing/web-platform/tests/scroll-animations/view-timelines/inline-view-timeline-current-time.tentative.html b/testing/web-platform/tests/scroll-animations/view-timelines/inline-view-timeline-current-time.tentative.html
new file mode 100644
index 0000000000..412145b04c
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/view-timelines/inline-view-timeline-current-time.tentative.html
@@ -0,0 +1,289 @@
+<!DOCTYPE html>
+<html id="top">
+<meta charset="utf-8">
+<title>View timeline current-time</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#viewtimeline-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="/scroll-animations/view-timelines/testcommon.js"></script>
+<style>
+  #container {
+    border:  10px solid lightgray;
+    overflow-x: scroll;
+    height:  200px;
+    width: 200px;
+  }
+  #content {
+    display:  flex;
+    flex-flow:  row nowrap;
+    justify-content:  flex-start;
+    width:  1800px;
+    margin: 0;
+  }
+  .spacer {
+    width:  800px;
+    display:  inline-block;
+  }
+  #target {
+    background-color:  green;
+    height:  100px;
+    width:  200px;
+    display:  inline-block;
+  }
+</style>
+<body>
+  <div id="container">
+    <div id="content">
+      <div id="leading-space" class="spacer"></div>
+      <div id="target"></div>
+      <div id="trailing-space" class="spacer"></div>
+    </div>
+  </div>
+</body>
+<script type="text/javascript">
+  promise_test(async t => {
+    container.scrollLeft = 0;
+    await waitForNextFrame();
+
+    const anim = CreateViewTimelineOpacityAnimation(t, target,
+                                                    {axis: 'inline'});
+    const timeline = anim.timeline;
+    await anim.ready;
+
+    // Initially before start-offset and animation effect is in the before
+    // phase.
+    assert_percents_equal(timeline.currentTime, -150,
+                          "Timeline's currentTime at container start boundary");
+    assert_percents_equal(anim.currentTime, -150,
+                          "Animation's currentTime at container start boundary");
+    assert_equals(getComputedStyle(target).opacity, "1",
+                  'Effect is inactive in the before phase');
+
+    // Advance to the start offset, which triggers entry to the active phase.
+    container.scrollLeft = 600;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 0,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 0,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '0.3',
+                  'Effect at the start of the active phase');
+
+    // Advance to the midpoint of the animation.
+    container.scrollLeft = 800;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 50,
+                          "Timeline's currentTime at midpoint");
+    assert_percents_equal(anim.currentTime, 50,
+                          "Animation's currentTime at midpoint");
+    assert_equals(getComputedStyle(target).opacity,'0.5',
+                  'Effect at the midpoint of the active range');
+
+    // Advance to the end of the animation.
+    container.scrollLeft = 1000;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 100,
+                          "Timeline's currentTime at end offset");
+    assert_percents_equal(anim.currentTime, 100,
+                          "Animation's currentTime at end offset");
+    assert_equals(getComputedStyle(target).opacity, '0.7',
+                  'Effect is in the active phase at effect end time');
+
+    // Advance to the scroll limit.
+    container.scrollLeft = 1600;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 250,
+                          "Timeline's currentTime at scroll limit");
+    // Hold time set when the animation finishes, which clamps the value of
+    // the animation's currentTime.
+    assert_percents_equal(anim.currentTime, 100,
+                          "Animation's currentTime at scroll limit");
+    // In the after phase, so the effect should not be applied.
+    assert_equals(getComputedStyle(target).opacity, '1',
+                  'After phase at scroll limit');
+  }, 'View timeline with start and end scroll offsets that do not align with ' +
+     'the scroll boundaries' );
+
+  promise_test(async t => {
+    const leading = document.getElementById('leading-space');
+    leading.style = 'display: none';
+    content.style = 'width: 1000px';
+    t.add_cleanup(() => {
+      leading.style = null;
+      content.style = null;
+    });
+
+    container.scrollLeft = 0;
+    await waitForNextFrame();
+
+    const anim = CreateViewTimelineOpacityAnimation(t, target,
+                                                    {axis: 'inline'});
+    const timeline = anim.timeline;
+    await anim.ready;
+
+    assert_percents_equal(timeline.currentTime, 50,
+                          "Timeline's currentTime at container start boundary");
+    assert_percents_equal(anim.currentTime, 50,
+                          "Animation's currentTime at container start boundary");
+    assert_equals(getComputedStyle(target).opacity, "0.5",
+                  'Effect enters active phase at container start boundary');
+
+
+    // Advance to midpoint
+    container.scrollLeft = 100;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 75,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 75,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '0.6',
+                  'Effect at the start of the active phase');
+
+    // Advance to end-offset
+    container.scrollLeft = 200;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 100,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 100,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '0.7',
+                  'Effect at the start of the active phase');
+
+    // Advance to scroll limit.
+    container.scrollLeft = 800;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 250,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 100,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '1',
+                  'Effect at the start of the active phase');
+
+  }, 'View timeline does not clamp starting scroll offset at 0');
+
+  promise_test(async t => {
+    const trailing = document.getElementById('trailing-space');
+    trailing.style = 'display: none';
+    content.style = 'width: 1000px';
+    t.add_cleanup(() => {
+      trailing.style = null;
+      content.style = null;
+    });
+
+    container.scrollLeft = 0;
+    await waitForNextFrame();
+
+    const anim = CreateViewTimelineOpacityAnimation(t, target,
+                                                    {axis: 'inline'});
+    const timeline = anim.timeline;
+    await anim.ready;
+
+    // Initially in before phase.
+    assert_percents_equal(timeline.currentTime, -150,
+                          "Timeline's currentTime at container start boundary");
+    assert_percents_equal(anim.currentTime, -150,
+                          "Animation's currentTime at container start boundary");
+    assert_equals(getComputedStyle(target).opacity, "1",
+                  'Effect enters active phase at container start boundary');
+
+    // Advance to start offset.
+    container.scrollLeft = 600;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 0,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 0,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '0.3',
+                  'Effect at the start of the active phase');
+
+    // Advance to midpoint
+    container.scrollLeft = 700;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 25,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 25,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '0.4',
+                  'Effect at the start of the active phase');
+
+    // Advance to end offset.
+    container.scrollLeft = 800;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 50,
+                          "Timeline's currentTime at max scroll offset");
+    assert_percents_equal(anim.currentTime, 50,
+                          "Animation's currentTime at max scroll offset");
+    assert_equals(getComputedStyle(target).opacity, "0.5",
+                  'Effect at end of active phase');
+  }, 'View timeline does not clamp end scroll offset at max scroll');
+
+
+  promise_test(async t => {
+    container.style = "direction: rtl";
+    container.scrollLeft = 0;
+    t.add_cleanup(() => {
+      content.style = null;
+    });
+    await waitForNextFrame();
+
+    const anim = CreateViewTimelineOpacityAnimation(t, target,
+                                                    {axis: 'inline'});
+    const timeline = anim.timeline;
+    await anim.ready;
+
+    // Initially before start-offset and animation effect is in the before
+    // phase.
+    assert_percents_equal(timeline.currentTime, -150,
+                          "Timeline's currentTime at container start boundary");
+    assert_percents_equal(anim.currentTime, -150,
+                          "Animation's currentTime at container start boundary");
+    assert_equals(getComputedStyle(target).opacity, "1",
+                  'Effect is inactive in the before phase');
+
+    // Advance to the start offset, which triggers entry to the active phase.
+    container.scrollLeft = -600;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 0,
+                          "Timeline's current time at start offset");
+    assert_percents_equal(anim.currentTime, 0,
+                          "Animation's current time at start offset");
+    assert_equals(getComputedStyle(target).opacity, '0.3',
+                  'Effect at the start of the active phase');
+
+    // Advance to the midpoint of the animation.
+    container.scrollLeft = -800;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 50,
+                          "Timeline's currentTime at midpoint");
+    assert_percents_equal(anim.currentTime, 50,
+                          "Animation's currentTime at midpoint");
+    assert_equals(getComputedStyle(target).opacity,'0.5',
+                  'Effect at the midpoint of the active range');
+
+    // Advance to the end of the animation.
+    container.scrollLeft = -1000;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 100,
+                          "Timeline's currentTime at end offset");
+    assert_percents_equal(anim.currentTime, 100,
+                          "Animation's currentTime at end offset");
+    assert_equals(getComputedStyle(target).opacity, '0.7',
+                  'Effect is in the active phase at effect end time');
+
+    // Advance to the scroll limit.
+    container.scrollLeft = -1600;
+    await waitForNextFrame();
+    assert_percents_equal(timeline.currentTime, 250,
+                          "Timeline's currentTime at scroll limit");
+    // Hold time set when the animation finishes, which clamps the value of
+    // the animation's currentTime.
+    assert_percents_equal(anim.currentTime, 100,
+                          "Animation's currentTime at scroll limit");
+    // In the after phase, so the effect should not be applied.
+    assert_equals(getComputedStyle(target).opacity, '1',
+                  'After phase at scroll limit');
+  }, 'View timeline with container having RTL layout' );
+</script>
+</html>
diff --git a/testing/web-platform/tests/scroll-animations/view-timelines/testcommon.js b/testing/web-platform/tests/scroll-animations/view-timelines/testcommon.js
new file mode 100644
index 0000000000..969f282e67
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/view-timelines/testcommon.js
@@ -0,0 +1,137 @@
+'use strict';
+
+function assert_px_equals(observed, expected, description) {
+  assert_equals(observed.unit, 'px',
+                `Unexpected unit type for '${description}'`);
+  assert_approx_equals(observed.value, expected, 0.0001,
+                       `Unexpected value for ${description}`);
+}
+
+function CreateViewTimelineOpacityAnimation(test, target, options) {
+  const viewTimelineOptions = {
+    subject: target,
+    axis: 'block'
+  };
+  if (options) {
+    for (let key in options) {
+      viewTimelineOptions[key] = options[key];
+    }
+  }
+
+  const anim =
+      target.animate(
+          { opacity: [0.3, 0.7] },
+          { timeline: new ViewTimeline(viewTimelineOptions) });
+  test.add_cleanup(() => {
+    anim.cancel();
+  });
+  return anim;
+}
+
+// Verify that range specified in the options aligns with the active range of
+// the animation.
+//
+// Sample call:
+// await runTimelineRangeTest(t, {
+//   timeline: { inset: [ CSS.percent(0), CSS.percent(20)] },
+//   timing: { fill: 'both' }
+//   rangeStart: 600,
+//   rangeEnd: 900
+// });
+async function runTimelineRangeTest(t, options, message) {
+  container.scrollLeft = 0;
+  await waitForNextFrame();
+
+  const anim =
+      options.anim ||
+      CreateViewTimelineOpacityAnimation(t, target, options.timeline);
+  if (options.timing)
+    anim.effect.updateTiming(options.timing);
+
+  const timeline = anim.timeline;
+  await anim.ready;
+
+  // Advance to the start offset, which triggers entry to the active phase.
+  container.scrollLeft = options.rangeStart;
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).opacity, '0.3',
+                `Effect at the start of the active phase: ${message}`);
+
+  // Advance to the midpoint of the animation.
+  container.scrollLeft = (options.rangeStart + options.rangeEnd) / 2;
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).opacity,'0.5',
+                `Effect at the midpoint of the active range: ${message}`);
+
+  // Advance to the end of the animation.
+  container.scrollLeft = options.rangeEnd;
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).opacity, '0.7',
+                `Effect is in the active phase at effect end time: ${message}`);
+
+  // Return the animation so that we can continue testing with the same object.
+  return anim;
+}
+
+// Sets the start and end delays for a view timeline and ensures that the
+// range aligns with expected values.
+//
+// Sample call:
+// await runTimelineDelayTest(t, {
+//   delay: { phase: 'cover', percent: CSS.percent(0) } ,
+//   endDelay: { phase: 'cover', percent: CSS.percent(100) },
+//   rangeStart: 600,
+//   rangeEnd: 900
+// });
+async function runTimelineDelayTest(t, options) {
+  const delayToString = delay => {
+    const parts = [];
+    if (delay.phase)
+      parts.push(delay.phase);
+    if (delay.percent)
+      parts.push(`${delay.percent.value}%`);
+    return parts.join(' ');
+  };
+  const range =
+     `${delayToString(options.delay)} to ` +
+     `${delayToString(options.endDelay)}`;
+
+  options.timeline = {
+    axis: 'inline'
+  };
+  options.timing = {
+    delay: options.delay,
+    endDelay: options.endDelay,
+    // Set fill to accommodate floating point precision errors at the
+    // endpoints.
+    fill: 'both'
+  };
+
+  return runTimelineRangeTest(t, options, range);
+}
+
+// Sets the Inset for a view timeline and ensures that the range aligns with
+// expected values.
+//
+// Sample call:
+// await runTimelineDelayTest(t, {
+//   inset: [ CSS.px(20), CSS.px(40) ]
+//   rangeStart: 600,
+//   rangeEnd: 900
+// });
+async function runTimelineInsetTest(t, options) {
+  options.timeline = {
+    axis: 'inline',
+    inset: options.inset
+  };
+  options.timing = {
+    // Set fill to accommodate floating point precision errors at the
+    // endpoints.
+    fill: 'both'
+  }
+  const length = options.inset.length;
+  const range =
+      (options.inset instanceof Array) ? options.inset.join(' ')
+                                       : options.inset;
+  return runTimelineRangeTest(t, options, range);
+}
diff --git a/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-delay-large-subject.html b/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-delay-large-subject.html
new file mode 100644
index 0000000000..edb37c1379
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-delay-large-subject.html
@@ -0,0 +1,93 @@
+<!DOCTYPE html>
+<html id="top">
+<meta charset="utf-8">
+<title>View timeline delay</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#viewtimeline-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="/scroll-animations/view-timelines/testcommon.js"></script>
+<style>
+  #container {
+    border:  10px solid lightgray;
+    overflow-x: scroll;
+    height:  200px;
+    width: 200px;
+  }
+  #content {
+    display:  flex;
+    flex-flow:  row nowrap;
+    justify-content:  flex-start;
+    width:  2100px;
+    margin: 0;
+  }
+  .spacer {
+    width:  800px;
+    display:  inline-block;
+  }
+  #target {
+    background-color:  green;
+    height:  100px;
+    /* target size > viewport size, which changes interpretation of the
+       contain range */
+    width:  400px;
+    display:  inline-block;
+  }
+</style>
+<body>
+  <div id="container">
+    <div id="content">
+      <div class="spacer"></div>
+      <div id="target"></div>
+      <div class="spacer"></div>
+    </div>
+  </div>
+</body>
+<script type="text/javascript">
+  promise_test(async t => {
+    await runTimelineDelayTest(t, {
+      delay: { phase: 'cover', percent: CSS.percent(0) } ,
+      endDelay: { phase: 'cover', percent: CSS.percent(100) },
+      rangeStart: 600,
+      rangeEnd: 1200
+    });
+    await runTimelineDelayTest(t, {
+      delay: { phase: 'contain', percent: CSS.percent(0) } ,
+      endDelay: { phase: 'contain', percent: CSS.percent(100) },
+      rangeStart: 800,
+      rangeEnd: 1000
+    });
+    await runTimelineDelayTest(t, {
+      delay: { phase: 'enter', percent: CSS.percent(0) },
+      endDelay: { phase: 'enter', percent: CSS.percent(100) },
+      rangeStart: 600,
+      rangeEnd: 800
+    });
+    await runTimelineDelayTest(t, {
+      delay:  { phase: 'exit', percent: CSS.percent(0) },
+      endDelay: { phase: 'exit', percent: CSS.percent(100) },
+      rangeStart: 1000,
+      rangeEnd: 1200
+    });
+    await runTimelineDelayTest(t, {
+      delay: { phase: 'contain', percent: CSS.percent(-50) },
+      endDelay: { phase: 'enter', percent: CSS.percent(200) },
+      rangeStart: 700,
+      rangeEnd: 1000
+    });
+    await runTimelineDelayTest(t, {
+      delay: { phase: 'enter' },
+      endDelay: { phase: 'exit' },
+      rangeStart: 600,
+      rangeEnd: 1200
+    });
+    await runTimelineDelayTest(t, {
+      delay: { percent: CSS.percent(0) },
+      endDelay: { percent: CSS.percent(100) },
+      rangeStart: 600,
+      rangeEnd: 1200
+    });
+
+  }, 'View timeline with range set via delays.' );
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-delay.html b/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-delay.html
new file mode 100644
index 0000000000..1377dc339c
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-delay.html
@@ -0,0 +1,103 @@
+<!DOCTYPE html>
+<html id="top">
+<meta charset="utf-8">
+<title>View timeline delay</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#viewtimeline-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="/scroll-animations/view-timelines/testcommon.js"></script>
+<style>
+  #container {
+    border:  10px solid lightgray;
+    overflow-x: scroll;
+    height:  200px;
+    width: 200px;
+  }
+  #content {
+    display:  flex;
+    flex-flow:  row nowrap;
+    justify-content:  flex-start;
+    width:  1800px;
+    margin: 0;
+  }
+  .spacer {
+    width:  800px;
+    display:  inline-block;
+  }
+  #target {
+    background-color:  green;
+    height:  100px;
+    width:  100px;
+    display:  inline-block;
+  }
+</style>
+<body>
+  <div id="container">
+    <div id="content">
+      <div class="spacer"></div>
+      <div id="target"></div>
+      <div class="spacer"></div>
+    </div>
+  </div>
+</body>
+<script type="text/javascript">
+  promise_test(async t => {
+    // Delays are associated with the animation and not with the timeline.
+    // Thus adjusting the delays has no effect on the timeline offsets.  The
+    // offsets always correspond to the 'cover' range.
+    const verifyTimelineOffsets = anim => {
+      const timeline = anim.timeline;
+      assert_px_equals(timeline.startOffset, 600, 'startOffset');
+      assert_px_equals(timeline.endOffset, 900, 'endOffset');
+    };
+    await runTimelineDelayTest(t, {
+      delay: { phase: 'cover', percent: CSS.percent(0) } ,
+      endDelay: { phase: 'cover', percent: CSS.percent(100) },
+      rangeStart: 600,
+      rangeEnd: 900
+    }).then(anim => {
+      verifyTimelineOffsets(anim);
+    });
+    await runTimelineDelayTest(t, {
+      delay: { phase: 'contain', percent: CSS.percent(0) } ,
+      endDelay: { phase: 'contain', percent: CSS.percent(100) },
+      rangeStart: 700,
+      rangeEnd: 800
+    }).then(anim => {
+      verifyTimelineOffsets(anim);
+    });
+    await runTimelineDelayTest(t, {
+      delay: { phase: 'enter', percent: CSS.percent(0) },
+      endDelay: { phase: 'enter', percent: CSS.percent(100) },
+      rangeStart: 600,
+      rangeEnd: 700
+    });
+    await runTimelineDelayTest(t, {
+      delay:  { phase: 'exit', percent: CSS.percent(0) },
+      endDelay: { phase: 'exit', percent: CSS.percent(100) },
+      rangeStart: 800,
+      rangeEnd: 900
+    });
+    await runTimelineDelayTest(t, {
+      delay: { phase: 'contain', percent: CSS.percent(-50) },
+      endDelay: { phase: 'enter', percent: CSS.percent(200) },
+      rangeStart: 650,
+      rangeEnd: 800
+    });
+    await runTimelineDelayTest(t, {
+      delay: { phase: 'enter' },
+      endDelay: { phase: 'exit' },
+      rangeStart: 600,
+      rangeEnd: 900
+    });
+    await runTimelineDelayTest(t, {
+      delay: { percent: CSS.percent(0) },
+      endDelay: { percent: CSS.percent(100) },
+      rangeStart: 600,
+      rangeEnd: 900
+    });
+
+  }, 'View timeline with range set via delays.' );
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-get-current-time-range-name.html b/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-get-current-time-range-name.html
new file mode 100644
index 0000000000..8f385e7b6e
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-get-current-time-range-name.html
@@ -0,0 +1,145 @@
+<!DOCTYPE html>
+<html id="top">
+<meta charset="utf-8">
+<title>View timeline delay</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#viewtimeline-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="/scroll-animations/view-timelines/testcommon.js"></script>
+<style>
+  #container {
+    border:  10px solid lightgray;
+    overflow-x: scroll;
+    height:  200px;
+    width: 200px;
+  }
+  #content {
+    display:  flex;
+    flex-flow:  row nowrap;
+    justify-content:  flex-start;
+    width:  1800px;
+    margin: 0;
+  }
+  .spacer {
+    width:  800px;
+    display:  inline-block;
+  }
+  #target {
+    background-color:  green;
+    height:  100px;
+    width:  100px;
+    display:  inline-block;
+  }
+</style>
+<body>
+  <div id="container">
+    <div id="content">
+      <div class="spacer"></div>
+      <div id="target"></div>
+      <div class="spacer"></div>
+    </div>
+  </div>
+</body>
+<script type="text/javascript">
+  const MAX_SCROLL = 1600;
+
+  promise_test(async t => {
+    // Points of interest along view timeline:
+    //   600 px cover start, enter start
+    //   700 px contain start, enter end
+    //   800 px contain end, exit start
+    //   900 px cover end, exit end
+    const anim =
+        CreateViewTimelineOpacityAnimation(t, target,
+                                           { axis: 'inline', fill: 'both' });
+    let timeline = anim.timeline;
+
+    container.scrollLeft = 600;
+    await waitForNextFrame();
+
+    assert_percents_approx_equal(timeline.getCurrentTime('cover'), 0,
+                                 MAX_SCROLL, 'Scroll aligned with cover start');
+    assert_percents_approx_equal(timeline.getCurrentTime('enter'), 0,
+                                 MAX_SCROLL, 'Scroll aligned with enter start');
+    assert_percents_approx_equal(timeline.getCurrentTime(), 0,
+                                 MAX_SCROLL,
+                                 'Scroll aligned with timeline start offset');
+
+    container.scrollLeft = 650;
+    await waitForNextFrame();
+
+    assert_percents_approx_equal(timeline.getCurrentTime('enter'), 50,
+                                 MAX_SCROLL, 'Scroll at enter midpoint');
+
+    container.scrollLeft = 700;
+    await waitForNextFrame();
+
+    assert_percents_approx_equal(timeline.getCurrentTime('enter'), 100,
+                                 MAX_SCROLL, 'Scroll at enter end');
+    assert_percents_approx_equal(timeline.getCurrentTime('contain'), 0,
+                                 MAX_SCROLL, 'Scroll at contain start');
+
+    container.scrollLeft = 750;
+    await waitForNextFrame();
+
+    assert_percents_approx_equal(timeline.getCurrentTime('contain'), 50,
+                                 MAX_SCROLL, 'Scroll at contain midpoint');
+    assert_percents_approx_equal(timeline.getCurrentTime(), 50,
+                                 MAX_SCROLL, 'Scroll at timeline midpoint');
+
+    container.scrollLeft = 800;
+    await waitForNextFrame();
+
+    assert_percents_approx_equal(timeline.getCurrentTime('exit'), 0,
+                                 MAX_SCROLL, 'Scroll at exit start');
+    assert_percents_approx_equal(timeline.getCurrentTime('contain'), 100,
+                                 MAX_SCROLL, 'Scroll at contain end');
+
+    container.scrollLeft = 850;
+    await waitForNextFrame();
+
+    assert_percents_approx_equal(timeline.getCurrentTime('exit'), 50,
+                                 MAX_SCROLL, 'Scroll at exit midpoint');
+
+    container.scrollLeft = 900;
+    await waitForNextFrame();
+
+    assert_percents_approx_equal(timeline.getCurrentTime('exit'), 100,
+                                 MAX_SCROLL, 'Scroll at exit end');
+    assert_percents_approx_equal(timeline.getCurrentTime('cover'), 100,
+                                 MAX_SCROLL, 'Scroll at cover end');
+    assert_percents_approx_equal(timeline.getCurrentTime(), 100,
+                                 MAX_SCROLL, 'Scroll at end of timeline');
+
+    assert_equals(timeline.getCurrentTime('gibberish'), null,
+                  'No current time for unknown named range');
+
+    // Add insets to force the start and end offsets to align. This forces
+    // the timeline to become inactive.
+    // start_offset = target_offset - viewport_size + end_side_inset
+    //              = 600 + end_side_inset
+    // end_offset = target_offset + target_size - start_side_inset
+    //            = 900 - start_side_inset
+    // Equating start_offset and end_offset:
+    //    end_side_inset = 300 - start_side_inset;
+    timeline =
+        new ViewTimeline ({
+          subject: target,
+          axis: 'inline',
+          inset: [ CSS.px(150), CSS.px(150) ]
+        });
+    anim.timeline = timeline;
+    await waitForNextFrame();
+
+    assert_equals(timeline.currentTime, null,
+                  'Current time is null when scroll-range is zero');
+    assert_equals(timeline.getCurrentTime(), null,
+                  'getCurrentTime with an inactive timeline.');
+    assert_equals(timeline.getCurrentTime('contain'), null,
+                  'getCurrentTime on a ranged name with an inactive timeline.');
+
+  }, 'View timeline current time for named range');
+
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-inset.html b/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-inset.html
new file mode 100644
index 0000000000..72480ea9f3
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-inset.html
@@ -0,0 +1,226 @@
+<!DOCTYPE html>
+<html id="top">
+<meta charset="utf-8">
+<title>View timeline delay</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#viewtimeline-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="/scroll-animations/view-timelines/testcommon.js"></script>
+<style>
+    #container {
+    border:  10px solid lightgray;
+    overflow-x: scroll;
+    height:  200px;
+    width: 200px;
+  }
+  #content {
+    display:  flex;
+    flex-flow:  row nowrap;
+    justify-content:  flex-start;
+    width:  1800px;
+    margin: 0;
+  }
+  .spacer {
+    width:  800px;
+    display:  inline-block;
+  }
+  #target {
+    background-color:  green;
+    height:  100px;
+    width:  100px;
+    display:  inline-block;
+    font-size:  16px;
+  }
+  #target.big-font {
+    font-size:  20px;
+  }
+  #container.scroll-padded {
+    scroll-padding-inline: 10px 20px;
+  }
+</style>
+</style>
+<body>
+  <div id="container">
+    <div id="content">
+      <div class="spacer"></div>
+      <div id="target"></div>
+      <div class="spacer"></div>
+    </div>
+  </div>
+</body>
+<script type="text/javascript">
+
+  function verifyTimelineOffsets(anim, start, end)  {
+    const timeline = anim.timeline;
+    assert_px_equals(timeline.startOffset, start, 'startOffset');
+    assert_px_equals(timeline.endOffset, end, 'endOffset');
+  };
+
+  promise_test(async t => {
+    // These tests are all based on the cover range, which has bounds
+    // [600, 900] if there are no insets.
+    // rangeStart = target_pos - viewport_size + end_side_inset
+    //            = 600 + end_side_inset
+    // rangeEnd = target_pos + target_size - start_side_inset
+    //          = 900 - start_side_inset
+    await runTimelineInsetTest(t, {
+      inset: [ CSS.px(0), CSS.px(0) ],
+      rangeStart: 600,
+      rangeEnd: 900
+    }).then(anim => verifyTimelineOffsets(anim, 600, 900));
+    await runTimelineInsetTest(t, {
+      inset: [ CSS.px(10), CSS.px(20) ],
+      rangeStart: 620,
+      rangeEnd: 890
+    }).then(anim => verifyTimelineOffsets(anim, 620, 890));
+    await runTimelineInsetTest(t, {
+      inset: [ CSS.px(10) ],
+      rangeStart: 610,
+      rangeEnd: 890
+    }).then(anim => verifyTimelineOffsets(anim, 610, 890));
+  }, 'View timeline with px based inset.');
+
+  promise_test(async t => {
+    // These tests are all based on the cover range, which has bounds
+    // [600, 900].
+    // Percentages are relative to the viewport size, which is 200 for this
+    // test.
+    await runTimelineInsetTest(t, {
+      inset: [ CSS.percent(0), CSS.percent(0) ],
+      rangeStart: 600,
+      rangeEnd: 900
+    }).then(anim => verifyTimelineOffsets(anim, 600, 900));
+    await runTimelineInsetTest(t, {
+      inset: [ CSS.percent(10), CSS.percent(20) ],
+      rangeStart: 640,
+      rangeEnd: 880
+    }).then(anim => verifyTimelineOffsets(anim, 640, 880));
+    await runTimelineInsetTest(t, {
+      inset: [ CSS.percent(10) ],
+      rangeStart: 620,
+      rangeEnd: 880
+    }).then(anim => verifyTimelineOffsets(anim, 620, 880));
+  }, 'View timeline with percent based inset.');
+
+  promise_test(async t => {
+    t.add_cleanup(() => {
+      container.classList.remove('scroll-padded');
+    });
+    const anim = await runTimelineInsetTest(t, {
+      inset: [ "auto", "auto" ],
+      rangeStart: 600,
+      rangeEnd: 900
+    });
+    verifyTimelineOffsets(anim, 600, 900);
+    container.classList.add('scroll-padded');
+    await runTimelineRangeTest(t, {
+      anim: anim,
+      rangeStart: 620,
+      rangeEnd: 890,
+    }, 'Adjust for scroll-padding')
+        .then(anim => verifyTimelineOffsets(anim, 620, 890));
+  }, 'view timeline with inset auto.');
+
+promise_test(async t => {
+  t.add_cleanup(() => {
+    target.classList.remove('big-font');
+  });
+  const anim = await runTimelineInsetTest(t, {
+    inset: [ CSS.em(1), CSS.em(2) ],
+    rangeStart: 632,
+    rangeEnd: 884
+  });
+  verifyTimelineOffsets(anim, 632, 884);
+  target.classList.add('big-font');
+  await runTimelineRangeTest(t, {
+    anim: anim,
+    rangeStart: 640,
+    rangeEnd: 880,
+  }, 'Adjust for font size increase')
+      .then(anim => verifyTimelineOffsets(anim, 640, 880));
+}, 'view timeline with font relative inset.');
+
+promise_test(async t => {
+  const vw = window.innerWidth;
+  const vh = window.innerHeight;
+  const vmin = Math.min(vw, vh);
+  await runTimelineInsetTest(t, {
+    inset: [ CSS.vw(10), CSS.vw(20) ],
+    rangeStart: 600 + 0.2 * vw,
+    rangeEnd: 900 - 0.1 * vw
+  });
+  await runTimelineInsetTest(t, {
+    inset: [ CSS.vmin(10), CSS.vmin(20) ],
+    rangeStart: 600 + 0.2 * vmin,
+    rangeEnd: 900 - 0.1 * vmin
+  });
+}, 'view timeline with viewport relative insets.');
+
+promise_test(async t => {
+  await runTimelineInsetTest(t, {
+    inset: "10px",
+    rangeStart: 610,
+    rangeEnd: 890
+  });
+  await runTimelineInsetTest(t, {
+    inset: "10px 20px",
+    rangeStart: 620,
+    rangeEnd: 890
+  });
+  await runTimelineInsetTest(t, {
+    inset: "10%",
+    rangeStart: 620,
+    rangeEnd: 880
+  });
+  await runTimelineInsetTest(t, {
+    inset: "10% 20%",
+    rangeStart: 640,
+    rangeEnd: 880
+  });
+  await runTimelineInsetTest(t, {
+    inset: "auto",
+    rangeStart: 600,
+    rangeEnd: 900
+  });
+  await runTimelineInsetTest(t, {
+    inset: "1em 2em",
+    rangeStart: 632,
+    rangeEnd: 884
+  });
+  assert_throws_js(TypeError, () => {
+    new ViewTimeline({
+      subject: target,
+      inset: "go fish"
+    });
+  });
+
+  assert_throws_js(TypeError, () => {
+    new ViewTimeline({
+      subject: target,
+      inset: "1 2"
+    });
+  });
+
+}, 'view timeline inset as string');
+
+promise_test(async t => {
+  assert_throws_js(TypeError, () => {
+    new ViewTimeline({
+      subject: target,
+      inset: [ CSS.rad(1) ]
+    });
+  });
+
+  assert_throws_js(TypeError, () => {
+    new ViewTimeline({
+      subject: target,
+      inset: [ CSS.px(10), CSS.px(10), CSS.px(10) ]
+    });
+  });
+
+
+}, 'view timeline with invalid inset');
+
+</script>
diff --git a/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-snapport.html b/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-snapport.html
new file mode 100644
index 0000000000..5d68d37037
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-snapport.html
@@ -0,0 +1,58 @@
+<!DOCTYPE html>
+<title>ViewTimeline vs. scroll-padding-*</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#view-timelines">
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#view-progress-visibility-range">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="/scroll-animations/view-timelines/testcommon.js"></script>
+<style>
+  #container {
+    border: 10px solid lightgray;
+    overflow-y: scroll;
+    height: 200px;
+    width: 200px;
+    scroll-padding: 40px;
+  }
+  .spacer {
+    height: 800px;
+  }
+  #target {
+    background-color: green;
+    height: 200px;
+    width: 100px;
+  }
+</style>
+<body>
+  <div id="container">
+    <div id="leading-space" class="spacer"></div>
+    <div id="target"></div>
+    <div id="trailing-space" class="spacer"></div>
+  </div>
+</body>
+<script>
+  promise_test(async t => {
+    container.scrollTop = 0;
+    await waitForNextFrame();
+
+    const anim = CreateViewTimelineOpacityAnimation(t, target);
+    await anim.ready;
+
+    // 0%
+    container.scrollTop = 600;
+    await waitForNextFrame();
+    assert_percents_equal(anim.currentTime, 0);
+
+    // 50%
+    container.scrollTop = 800;
+    await waitForNextFrame();
+    assert_percents_equal(anim.currentTime, 50);
+
+    // 100%
+    container.scrollTop = 1000;
+    await waitForNextFrame();
+    assert_percents_equal(anim.currentTime, 100);
+  }, 'Default ViewTimeline is not affected by scroll-padding');
+</script>
+</html>
diff --git a/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-source.tentative.html b/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-source.tentative.html
new file mode 100644
index 0000000000..f8aabc8bdd
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-source.tentative.html
@@ -0,0 +1,94 @@
+<!DOCTYPE html>
+<html id="top">
+<meta charset="utf-8">
+<title>View timeline source</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#viewtimeline-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<style>
+#outer {
+  height: 400px;
+  width: 400px;
+  overflow: clip;
+}
+
+#inner {
+  height: 300px;
+  width: 300px;
+  overflow: clip;
+}
+
+#outer.scroller,
+#inner.scroller {
+  overflow: scroll;
+}
+
+#spacer {
+  height: 1000px;
+}
+
+#target {
+  background: green;
+  height: 40px;
+  width: 40px;
+}
+</style>
+<body>
+  <div id="outer" class="scroller">
+    <div id="inner" class="scroller">
+      <div id="target"></div>
+      <div id="spacer"></div>
+    </div>
+  </div>
+</body>
+<script>
+'use strict';
+
+function resetScrollers() {
+  inner.classList.add('scroller');
+  outer.classList.add('scroller');
+}
+
+function assert_source_id(viewTimeline, expected) {
+  const source = viewTimeline.source;
+  assert_true(!!source, 'No source');
+  assert_equals(source.id, expected);
+}
+
+promise_test(async t => {
+  t.add_cleanup(resetScrollers);
+  const viewTimeline = new ViewTimeline({ subject: target });
+  assert_equals(viewTimeline.subject, target);
+  assert_source_id(viewTimeline, 'inner');
+
+  inner.classList.remove('scroller');
+  assert_source_id(viewTimeline, 'outer');
+
+  outer.classList.remove('scroller');
+  assert_source_id(viewTimeline, 'top');
+}, 'Default source for a View timeline is the nearest scroll ' +
+   'ancestor to the subject');
+
+promise_test(async t => {
+  t.add_cleanup(resetScrollers);
+  const viewTimeline =
+      new ViewTimeline({ source: outer, subject: target });
+  assert_equals(viewTimeline.subject, target);
+  assert_source_id(viewTimeline, 'inner');
+}, 'View timeline ignores explicitly set source');
+
+promise_test(async t => {
+  t.add_cleanup(resetScrollers);
+  const viewTimeline =
+      new ViewTimeline({ subject: target });
+  assert_equals(viewTimeline.subject, target);
+  assert_source_id(viewTimeline, 'inner');
+
+  target.style = "display: none";
+  assert_equals(viewTimeline.source, null);
+
+}, 'View timeline source is null when display:none');
+
+</script>
+</html>
diff --git a/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-subject-size-changes.html b/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-subject-size-changes.html
new file mode 100644
index 0000000000..b438317f7c
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/view-timelines/view-timeline-subject-size-changes.html
@@ -0,0 +1,79 @@
+<!DOCTYPE html>
+<html id="top">
+<meta charset="utf-8">
+<title>View timeline Subject size changes after creation of Animation</title>
+<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#viewtimeline-interface">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="/scroll-animations/scroll-timelines/testcommon.js"></script>
+<script src="/scroll-animations/view-timelines/testcommon.js"></script>
+<style>
+  #container {
+    border:  10px solid lightgray;
+    overflow-y: scroll;
+    height:  400px;
+    width: 400px;
+  }
+  .spacer {
+    height:  500px;
+  }
+  #target {
+    background-color:  green;
+    height:  100px;
+    width:  100px;
+  }
+</style>
+<body>
+  <div id="container">
+    <div class="spacer"></div>
+    <div id="target"></div>
+    <div class="spacer"></div>
+  </div>
+</body>
+
+<script type="text/javascript">
+promise_test(async t => {
+  const options = {
+    axis: 'vertical',
+    timing: {
+      delay: { phase: 'enter', percent: CSS.percent(0) },
+      endDelay: { phase: 'enter', percent: CSS.percent(100) },
+      // Set fill to accommodate floating point precision errors at the endpoints.
+      fill: 'both'
+    }
+  };
+
+  container.scrollTop = 0;
+  await waitForNextFrame();
+
+  const anim = CreateViewTimelineOpacityAnimation(t, target, options);
+  anim.effect.updateTiming(options.timing);
+  await anim.ready;
+
+  // Advance to the start offset, which triggers entry to the active phase.
+  container.scrollTop = 100;
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).opacity, '0.3',
+                `Effect at the start of the active phase`);
+
+  // Advance to the midpoint of the animation.
+  container.scrollTop = 150;
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).opacity,'0.5',
+                `Effect at the midpoint of the active range`);
+
+  // Since the height of the target is cut in half, the animation should be at the end now.
+  target.style.height = '50px';
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).opacity, '0.7',
+                `Effect at the end of the active range`);
+
+  // Advance to the midpoint of the animation again.
+  container.scrollTop = 125;
+  await waitForNextFrame();
+  assert_equals(getComputedStyle(target).opacity,'0.5',
+                `Effect at the midpoint of the active range again`);
+
+  }, 'View timeline with subject size change after the creation of the animation');
+</script>
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