From 36d22d82aa202bb199967e9512281e9a53db42c9 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Sun, 7 Apr 2024 21:33:14 +0200
Subject: Adding upstream version 115.7.0esr.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 .../web-platform/tests/animation-worklet/META.yml  |   4 +
 ...on-different-targets-via-main-thread.https.html |  65 ++++
 .../animate-non-accelerated-property.https.html    |  44 +++
 .../animation-worklet-inside-iframe.https.html     |  61 ++++
 .../animator-with-options.https.html               |  38 +++
 .../cancel-non-accelerated-property.https.html     |  45 +++
 .../web-platform/tests/animation-worklet/common.js |  61 ++++
 .../animation-worklet/current-time.https.html      |  64 ++++
 .../tests/animation-worklet/idlharness.any.js      |  18 ++
 .../animation-worklet/inactive-timeline.https.html | 139 +++++++++
 ...rget-driven-by-individual-local-time.https.html |  65 ++++
 .../animation-worklet/playback-rate.https.html     | 345 +++++++++++++++++++++
 .../references/not-translated-box-ref.html         |  12 +
 .../references/translated-box-ref.html             |  12 +
 .../resources/animator-iframe.html                 |  44 +++
 .../tests/animation-worklet/resources/iframe.html  |   9 +
 .../scroll-timeline-writing-modes.https.html       | 168 ++++++++++
 .../animation-worklet/stateful-animator.https.html | 216 +++++++++++++
 .../worklet-animation-animator-name.https.html     |  30 ++
 .../worklet-animation-cancel.https.html            |  45 +++
 .../worklet-animation-creation.https.html          | 141 +++++++++
 .../worklet-animation-duration.https.html          |  39 +++
 ...ed-timing-progress-on-worklet-thread.https.html |  87 ++++++
 ...imation-get-timing-on-worklet-thread.https.html |  54 ++++
 ...-animation-local-time-after-duration.https.html |  41 +++
 ...et-animation-local-time-before-start.https.html |  41 +++
 .../worklet-animation-local-time-null-1.https.html | 163 ++++++++++
 .../worklet-animation-local-time-null-2-ref.html   |  27 ++
 .../worklet-animation-local-time-null-2.https.html | 110 +++++++
 .../worklet-animation-pause-immediately.https.html |  37 +++
 .../worklet-animation-pause-resume.https.html      |  40 +++
 .../worklet-animation-pause.https.html             |  60 ++++
 .../worklet-animation-play.https.html              |  45 +++
 .../worklet-animation-set-keyframes.https.html     |  44 +++
 .../worklet-animation-set-timing.https.html        |  46 +++
 .../worklet-animation-start-delay-ref.html         |  12 +
 .../worklet-animation-start-delay.https.html       |  64 ++++
 ...n-with-effects-from-different-frames.https.html |  48 +++
 .../worklet-animation-with-fill-mode.https.html    | 147 +++++++++
 ...orklet-animation-with-invalid-effect.https.html |  36 +++
 .../worklet-animation-with-non-ascii-name-ref.html |  12 +
 ...orklet-animation-with-non-ascii-name.https.html |  59 ++++
 ...ith-scroll-timeline-and-display-none.https.html |  84 +++++
 ...th-scroll-timeline-and-overflow-hidden-ref.html |  45 +++
 ...-scroll-timeline-and-overflow-hidden.https.html |  68 ++++
 ...worklet-animation-with-scroll-timeline-ref.html |  51 +++
 ...ion-with-scroll-timeline-root-scroller-ref.html |  43 +++
 ...n-with-scroll-timeline-root-scroller.https.html |  69 +++++
 ...rklet-animation-with-scroll-timeline.https.html |  74 +++++
 .../worklet-animation-without-target.https.html    |  76 +++++
 50 files changed, 3348 insertions(+)
 create mode 100644 testing/web-platform/tests/animation-worklet/META.yml
 create mode 100644 testing/web-platform/tests/animation-worklet/animate-multiple-effects-on-different-targets-via-main-thread.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/animate-non-accelerated-property.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/animation-worklet-inside-iframe.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/animator-with-options.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/cancel-non-accelerated-property.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/common.js
 create mode 100644 testing/web-platform/tests/animation-worklet/current-time.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/idlharness.any.js
 create mode 100644 testing/web-platform/tests/animation-worklet/inactive-timeline.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/multiple-effects-on-same-target-driven-by-individual-local-time.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/playback-rate.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/references/not-translated-box-ref.html
 create mode 100644 testing/web-platform/tests/animation-worklet/references/translated-box-ref.html
 create mode 100644 testing/web-platform/tests/animation-worklet/resources/animator-iframe.html
 create mode 100644 testing/web-platform/tests/animation-worklet/resources/iframe.html
 create mode 100644 testing/web-platform/tests/animation-worklet/scroll-timeline-writing-modes.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/stateful-animator.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-animator-name.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-cancel.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-creation.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-duration.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-get-computed-timing-progress-on-worklet-thread.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-get-timing-on-worklet-thread.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-local-time-after-duration.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-local-time-before-start.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-local-time-null-1.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-local-time-null-2-ref.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-local-time-null-2.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-pause-immediately.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-pause-resume.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-pause.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-play.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-set-keyframes.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-set-timing.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-start-delay-ref.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-start-delay.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-with-effects-from-different-frames.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-with-fill-mode.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-with-invalid-effect.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-with-non-ascii-name-ref.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-with-non-ascii-name.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-and-display-none.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-and-overflow-hidden-ref.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-and-overflow-hidden.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-ref.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-root-scroller-ref.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-root-scroller.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline.https.html
 create mode 100644 testing/web-platform/tests/animation-worklet/worklet-animation-without-target.https.html

(limited to 'testing/web-platform/tests/animation-worklet')

diff --git a/testing/web-platform/tests/animation-worklet/META.yml b/testing/web-platform/tests/animation-worklet/META.yml
new file mode 100644
index 0000000000..88e7d924aa
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/META.yml
@@ -0,0 +1,4 @@
+spec: https://drafts.css-houdini.org/css-animationworklet/
+suggested_reviewers:
+  - flackr
+  - majido
diff --git a/testing/web-platform/tests/animation-worklet/animate-multiple-effects-on-different-targets-via-main-thread.https.html b/testing/web-platform/tests/animation-worklet/animate-multiple-effects-on-different-targets-via-main-thread.https.html
new file mode 100644
index 0000000000..d22ed4cd25
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/animate-multiple-effects-on-different-targets-via-main-thread.https.html
@@ -0,0 +1,65 @@
+<!DOCTYPE html>
+<title>Animate multiple effects on different targets via main thread</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<style>
+  #target {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+  #target2 {
+    width: 100px;
+    height: 100px;
+    background-color: blue;
+    box-shadow: 4px 4px 25px blue;
+  }
+</style>
+
+<div id="target"></div>
+<div id="target2"></div>
+
+<script id="simple_animate" type="text/worklet">
+  registerAnimator("test_animator", class {
+    animate(currentTime, effect) {
+      let effects = effect.getChildren();
+      effects[0].localTime = 1000;
+      effects[1].localTime = 1000;
+    }
+  });
+</script>
+
+<script>
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('simple_animate').textContent);
+
+    const effect = new KeyframeEffect(
+        document.getElementById("target"),
+        [
+          { background: 'green' },
+          { background: 'blue' },
+        ],
+        { duration: 2000 }
+    );
+    const effect2 = new KeyframeEffect(
+        document.getElementById("target2"),
+        [
+          { boxShadow: '4px 4px 25px blue' },
+          { boxShadow: '4px 4px 25px green' }
+        ],
+        { duration: 2000 }
+    );
+
+    const animation = new WorkletAnimation('test_animator', [effect, effect2]);
+    animation.play();
+    await waitForAsyncAnimationFrames(1);
+
+    assert_equals(getComputedStyle(document.getElementById('target')).backgroundColor, "rgb(0, 64, 128)");
+    assert_equals(getComputedStyle(document.getElementById('target2')).boxShadow, "rgb(0, 64, 128) 4px 4px 25px 0px");
+  }, 'Animating multiple effects on different targets via main thread should produce new output values accordingly');
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/animate-non-accelerated-property.https.html b/testing/web-platform/tests/animation-worklet/animate-non-accelerated-property.https.html
new file mode 100644
index 0000000000..8e30387530
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/animate-non-accelerated-property.https.html
@@ -0,0 +1,44 @@
+<!DOCTYPE html>
+<title>Animate non-accelerated property using worklet animation</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<div id="target"></div>
+<div id="target2"></div>
+
+<script>
+  promise_test(async t => {
+    await registerConstantLocalTimeAnimator(1000);
+    const target = document.getElementById("target");
+    const effect = new KeyframeEffect(
+        target,
+        [
+          { background: 'green' },
+          { background: 'blue' },
+        ],
+        { duration: 2000 }
+    );
+
+    const target2 = document.getElementById("target2");
+    const effect2 = new KeyframeEffect(
+        target2,
+        [
+          { boxShadow: '4px 4px 25px blue' },
+          { boxShadow: '4px 4px 25px green' }
+        ],
+        { duration: 2000 }
+    );
+    const animation = new WorkletAnimation('constant_time', effect);
+    animation.play();
+    const animation2 = new WorkletAnimation('constant_time', effect2);
+    animation2.play();
+
+    await waitForAsyncAnimationFrames(1);
+    assert_equals(getComputedStyle(target).backgroundColor, "rgb(0, 64, 128)");
+    assert_equals(getComputedStyle(target2).boxShadow, "rgb(0, 64, 128) 4px 4px 25px 0px");
+  }, "Individual worklet animation should output values at specified local time for corresponding targets and effects");
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/animation-worklet-inside-iframe.https.html b/testing/web-platform/tests/animation-worklet/animation-worklet-inside-iframe.https.html
new file mode 100644
index 0000000000..415f394401
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/animation-worklet-inside-iframe.https.html
@@ -0,0 +1,61 @@
+<!DOCTYPE html>
+<title>Test that AnimationWorklet inside frames with different origin causes new global scopes</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<style>
+.redbox {
+  width: 100px;
+  height: 100px;
+  background-color: #ff0000;
+}
+</style>
+
+<div id="target" class="redbox"></div>
+
+<script id="main_worklet" type="text/worklet">
+registerAnimator("duplicate_animator", class {
+  animate(currentTime, effect) {
+    effect.localTime = 500;
+  }
+});
+</script>
+
+<script>
+async_test(t => {
+  // Wait for iframe to load and start its animations.
+  window.onmessage = function(msg) {
+    window.requestAnimationFrame( _ => {
+      run_test(msg.data);
+    });
+  };
+
+  // Create and load the iframe to avoid racy cases.
+  var iframe = document.createElement('iframe');
+  iframe.src = 'resources/animator-iframe.html';
+  document.body.appendChild(iframe);
+
+  function run_test(data) {
+    runInAnimationWorklet(
+      document.getElementById('main_worklet').textContent
+    ).then(_ => {
+      // Create an animation for duplicate animator.
+      const target = document.getElementById('target');
+      const animation = new WorkletAnimation('duplicate_animator', new KeyframeEffect(target, [{ opacity: 0 }], { duration: 1000 }));
+      animation.play();
+
+      assert_equals(data, '0.4');
+
+      // wait until local times are synced back to the main thread.
+      waitForAnimationFrameWithCondition(_ => {
+        return getComputedStyle(target).opacity != '1';
+      }).then(t.step_func_done(() => {
+        assert_equals(getComputedStyle(target).opacity, '0.5');
+      }));
+    });
+  }
+}, 'Both main frame and iframe should update the opacity of their target');
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/animator-with-options.https.html b/testing/web-platform/tests/animation-worklet/animator-with-options.https.html
new file mode 100644
index 0000000000..975c57f038
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/animator-with-options.https.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<title>Worklet Animation with options</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<div id="target"></div>
+
+<script id="animate_with_options" type="text/worklet">
+  registerAnimator("test_animator", class {
+    constructor(options) {
+      this.time_ = options.time;
+    }
+    animate(currentTime, effect) {
+      effect.localTime = this.time_;
+    }
+  });
+</script>
+
+<script>
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('animate_with_options').textContent);
+    const target = document.getElementById('target');
+    const effect = new KeyframeEffect(target, [{ opacity: 0 }], { duration: 1000 });
+    const options = {'time': 500};
+    const animation = new WorkletAnimation('test_animator', effect, document.timeline, options);
+    animation.play();
+
+    // wait until local times are synced back to the main thread.
+    await waitForAnimationFrameWithCondition(_ => {
+      return getComputedStyle(target).opacity != '1';
+    });
+    assert_equals(getComputedStyle(target).opacity, "0.5");
+  }, "Animator should be able to use options to update the animation");
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/cancel-non-accelerated-property.https.html b/testing/web-platform/tests/animation-worklet/cancel-non-accelerated-property.https.html
new file mode 100644
index 0000000000..594da4c419
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/cancel-non-accelerated-property.https.html
@@ -0,0 +1,45 @@
+<!DOCTYPE html>
+<title>Cancel non accelerated property using worklet animation</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<style>
+#target {
+  background-color: red;
+}
+</style>
+
+<div id="target"></div>
+
+<script>
+  promise_test(async t => {
+    await registerConstantLocalTimeAnimator(1000);
+    const target = document.getElementById('target');
+    const effect = new KeyframeEffect(
+        target,
+        [
+          { background: 'green' },
+          { background: 'blue' },
+        ],
+        {
+          duration: 2000,
+          iteration: Infinity
+        }
+    );
+    const animation = new WorkletAnimation('constant_time', effect);
+    animation.play();
+
+    await waitForAsyncAnimationFrames(1);
+    // establish that the animation started
+    assert_equals(getComputedStyle(target).backgroundColor, "rgb(0, 64, 128)");
+    animation.cancel();
+
+    await waitForAsyncAnimationFrames(1);
+    // confirm the animation is cancelled
+    assert_equals(getComputedStyle(target).backgroundColor, "rgb(255, 0, 0)");
+  }, "Animation should update the outputs after starting and then return to pre-animated values after being cancelled");
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/common.js b/testing/web-platform/tests/animation-worklet/common.js
new file mode 100644
index 0000000000..ceb430b718
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/common.js
@@ -0,0 +1,61 @@
+'use strict';
+
+function registerPassthroughAnimator() {
+  return runInAnimationWorklet(`
+    registerAnimator('passthrough', class {
+      animate(currentTime, effect) {
+        effect.localTime = currentTime;
+      }
+    });
+  `);
+}
+
+function registerConstantLocalTimeAnimator(localTime) {
+  return runInAnimationWorklet(`
+    registerAnimator('constant_time', class {
+      animate(currentTime, effect) { effect.localTime = ${localTime}; }
+    });
+  `);
+}
+
+function runInAnimationWorklet(code) {
+  return CSS.animationWorklet.addModule(
+    URL.createObjectURL(new Blob([code], {type: 'text/javascript'}))
+  );
+}
+
+function approxEquals(actual, expected){
+  // precision in ms
+  const epsilon = 0.005;
+  const lowerBound = (expected - epsilon) < actual;
+  const upperBound = (expected + epsilon) > actual;
+  return lowerBound && upperBound;
+}
+
+function waitForAsyncAnimationFrames(count) {
+  // In Chrome, waiting for N+1 main thread frames guarantees that compositor has produced
+  // at least N frames.
+  // TODO(majidvp): re-evaluate this choice once other browsers have implemented
+  // AnimationWorklet.
+  return waitForAnimationFrames(count + 1);
+}
+
+async function waitForAnimationFrameWithCondition(condition) {
+  do {
+    await new Promise(window.requestAnimationFrame);
+  } while (!condition())
+}
+
+async function waitForDocumentTimelineAdvance() {
+  const timeAtStart = document.timeline.currentTime;
+  do {
+    await new Promise(window.requestAnimationFrame);
+  } while (timeAtStart === document.timeline.currentTime)
+}
+
+// Wait until animation's effect has a non-null localTime.
+async function waitForNotNullLocalTime(animation) {
+  await waitForAnimationFrameWithCondition(_ => {
+    return animation.effect.getComputedTiming().localTime !== null;
+  });
+}
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/current-time.https.html b/testing/web-platform/tests/animation-worklet/current-time.https.html
new file mode 100644
index 0000000000..a445d5b004
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/current-time.https.html
@@ -0,0 +1,64 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>The current time of a worklet animation</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<div id="box"></div>
+
+<script>
+'use strict';
+
+function CreateAnimation() {
+  const box = document.getElementById('box');
+  const effect = new KeyframeEffect(
+      box,
+      { height: ['100px', '50px'] },
+      10000);
+
+  return new WorkletAnimation('passthrough', effect);
+}
+
+setup(setupAndRegisterTests, {explicit_done: true});
+
+function setupAndRegisterTests() {
+  registerPassthroughAnimator().then(() => {
+    promise_test(async t => {
+      const animation = CreateAnimation();
+      animation.play();
+
+      assert_equals(animation.currentTime, 0,
+        'Current time returns the hold time set when entering the play-pending' +
+        'state');
+
+      animation.cancel();
+    }, 'The current time returns the hold time when set');
+
+    promise_test(async t => {
+      const animation = CreateAnimation();
+      animation.play();
+
+      // Allow one async animation frame to pass so that animation is running.
+      await waitForAsyncAnimationFrames(1);
+      assert_equals(animation.playState, "running");
+      // Allow time to advance so that we have a non-zero current time.
+      await waitForDocumentTimelineAdvance();
+      const timelineTime = document.timeline.currentTime;
+      assert_greater_than(animation.currentTime, 0);
+      assert_times_equal(animation.currentTime, (timelineTime - animation.startTime));
+
+      animation.cancel();
+    }, 'The current time is calculated from the timeline time and start time');
+
+    done();
+  });
+}
+
+// TODO(majidvp): Add tests for playbackRate and animations that are not
+// associated with a timeline once these are supported in  WorkletAnimation.
+// http://crbug.com/833846
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/idlharness.any.js b/testing/web-platform/tests/animation-worklet/idlharness.any.js
new file mode 100644
index 0000000000..a53ac739f3
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/idlharness.any.js
@@ -0,0 +1,18 @@
+// META: script=/resources/WebIDLParser.js
+// META: script=/resources/idlharness.js
+// META: timeout=long
+
+'use strict';
+
+// https://wicg.github.io/animation-worklet/
+
+idl_test(
+  ['css-animation-worklet'],
+  ['web-animations', 'html', 'cssom', 'dom'],
+  idl_array => {
+    idl_array.add_objects({
+      WorkletAnimation: ['new WorkletAnimation("name")'],
+      // TODO: WorkletGroupEffect
+    });
+  }
+);
diff --git a/testing/web-platform/tests/animation-worklet/inactive-timeline.https.html b/testing/web-platform/tests/animation-worklet/inactive-timeline.https.html
new file mode 100644
index 0000000000..3938cb3092
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/inactive-timeline.https.html
@@ -0,0 +1,139 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Correctness of worklet animation state when timeline becomes newly
+         active or inactive.</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+<style>
+  .scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+  }
+  .contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+function createScroller(test) {
+  var scroller = createDiv(test);
+  scroller.innerHTML = "<div class='contents'></div>";
+  scroller.classList.add('scroller');
+  return scroller;
+}
+
+function createScrollLinkedWorkletAnimation(test) {
+  const timeline = new ScrollTimeline({
+    scrollSource: createScroller(test),
+  });
+  const DURATION = 1000; // ms
+  const KEYFRAMES = { transform: ['translateY(100px)', 'translateY(200px)'] };
+  return new WorkletAnimation('passthrough', new KeyframeEffect(createDiv(test),
+        KEYFRAMES, DURATION), timeline);
+}
+
+setup(setupAndRegisterTests, {explicit_done: true});
+
+function setupAndRegisterTests() {
+  registerPassthroughAnimator().then(() => {
+
+    promise_test(async t => {
+      const animation = createScrollLinkedWorkletAnimation(t);
+      const scroller = animation.timeline.scrollSource;
+      const target = animation.effect.target;
+
+      // There is no direct way to control when local times of composited
+      // animations are synced to the main thread. This test uses another
+      // composited worklet animation with an always active timeline as an
+      // indicator of when the sync is ready. The sync is done when animation
+      // effect's output has changed as a result of advancing the timeline.
+      const animationRef = createScrollLinkedWorkletAnimation(t);
+      const scrollerRef = animationRef.timeline.scrollSource;
+      const targetRef = animationRef.effect.target;
+
+      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+      scroller.scrollTop = 0.2 * maxScroll;
+
+      // Make the timeline inactive.
+      scroller.style.display = "none"
+      // Force relayout.
+      scroller.scrollTop;
+
+      animation.play();
+      animationRef.play();
+      assert_equals(animation.currentTime, null,
+        'Initial current time must be unresolved in idle state.');
+      assert_equals(animation.startTime, null,
+        'Initial start time must be unresolved in idle state.');
+      waitForAnimationFrameWithCondition(_=> {
+        return animation.playState == "running"
+      });
+      assert_equals(animation.currentTime, null,
+        'Initial current time must be unresolved in playing state.');
+      assert_equals(animation.startTime, null,
+        'Initial start time must be unresolved in playing state.');
+
+      scrollerRef.scrollTop = 0.2 * maxScroll;
+
+      // Wait until local times are synced back to the main thread.
+      await waitForAnimationFrameWithCondition(_ => {
+        return animationRef.effect.getComputedTiming().localTime == 200;
+      });
+
+      assert_equals(animation.effect.getComputedTiming().localTime, null,
+        'The underlying effect local time must be undefined while the ' +
+        'timeline is inactive.');
+
+      // Make the timeline active.
+      scroller.style.display = "";
+      // Wait for new animation frame  which allows the timeline to compute new
+      // current time.
+      await waitForNextFrame();
+
+      assert_times_equal(animation.currentTime, 200,
+        'Current time must be initialized.');
+      assert_times_equal(animation.startTime, 0,
+        'Start time must be initialized.');
+
+      scrollerRef.scrollTop = 0.4 * maxScroll;
+      // Wait until local times are synced back to the main thread.
+      await waitForAnimationFrameWithCondition(_ => {
+        return animationRef.effect.getComputedTiming().localTime == 400;
+      });
+      assert_times_equal(animation.effect.getComputedTiming().localTime, 200,
+        'When the timeline becomes newly active, the underlying effect\'s ' +
+        'timing should be properly updated.');
+
+      // Make the timeline inactive again.
+      scroller.style.display = "none"
+      await waitForNextFrame();
+
+      assert_times_equal(animation.currentTime, 200,
+        'Current time must be the previous current time.');
+      assert_equals(animation.startTime, null,
+        'Initial start time must be unresolved.');
+
+      scrollerRef.scrollTop = 0.6 * maxScroll;
+      // Wait until local times are synced back to the main thread.
+      await waitForAnimationFrameWithCondition(_ => {
+        return animationRef.effect.getComputedTiming().localTime == 600;
+      });
+
+      assert_times_equal(animation.effect.getComputedTiming().localTime, 200,
+        'When the timeline becomes newly inactive, the underlying effect\'s ' +
+        'timing should stay unchanged.');
+    }, 'When timeline time becomes inactive previous current time must be ' +
+       'the current time and start time unresolved');
+    done();
+  });
+}
+</script>
+</body>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/multiple-effects-on-same-target-driven-by-individual-local-time.https.html b/testing/web-platform/tests/animation-worklet/multiple-effects-on-same-target-driven-by-individual-local-time.https.html
new file mode 100644
index 0000000000..edf8488ded
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/multiple-effects-on-same-target-driven-by-individual-local-time.https.html
@@ -0,0 +1,65 @@
+<!DOCTYPE html>
+<title>Multiple effects on same target driven by individual local time</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<style>
+  #target {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+  #target2 {
+    width: 100px;
+    height: 100px;
+    background-color: blue;
+    box-shadow: 4px 4px 25px blue;
+  }
+</style>
+
+<div id="target"></div>
+
+<script id="simple_animate" type="text/worklet">
+  registerAnimator("test_animator", class {
+    animate(currentTime, effect) {
+      let effects = effect.getChildren();
+      effects[0].localTime = 0;
+      effects[1].localTime = 1000;
+    }
+  });
+</script>
+
+<script>
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('simple_animate').textContent);
+
+    const effect = new KeyframeEffect(
+        document.getElementById("target"),
+        [
+          { background: 'green' },
+          { background: 'blue' },
+        ],
+        { duration: 2000 }
+    );
+    const effect2 = new KeyframeEffect(
+        document.getElementById("target"),
+        [
+          { width: '100px' },
+          { width: '200px' }
+        ],
+        { duration: 2000 }
+    );
+
+    const animation = new WorkletAnimation('test_animator', [effect, effect2]);
+    animation.play();
+    await waitForAsyncAnimationFrames(1);
+
+    assert_equals(getComputedStyle(document.getElementById('target')).backgroundColor, "rgb(0, 128, 0)");
+    assert_equals(getComputedStyle(document.getElementById('target')).width, "150px");
+  }, `Animating multiple effects on the same target using effect specific local time should output values
+  relative to each effects unique local time`);
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/playback-rate.https.html b/testing/web-platform/tests/animation-worklet/playback-rate.https.html
new file mode 100644
index 0000000000..2e2fb9a099
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/playback-rate.https.html
@@ -0,0 +1,345 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>The playback rate of a worklet animation</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+'use strict';
+// Presence of playback rate adds FP operations to calculating start_time
+// and current_time of animations. That's why it's needed to increase FP error
+// for comparing times in these tests.
+window.assert_times_equal = (actual, expected, description) => {
+  assert_approx_equals(actual, expected, 0.002, description);
+};
+</script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+<style>
+  .scroller {
+    overflow: auto;
+    height: 100px;
+    width: 100px;
+  }
+  .contents {
+    height: 1000px;
+    width: 100%;
+  }
+</style>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+function createWorkletAnimation(test) {
+  const DURATION = 10000; // ms
+  const KEYFRAMES = { transform: ['translateY(100px)', 'translateY(200px)'] };
+  return new WorkletAnimation('passthrough', new KeyframeEffect(createDiv(test),
+        KEYFRAMES, DURATION), document.timeline);
+}
+
+function createScroller(test) {
+  var scroller = createDiv(test);
+  scroller.innerHTML = "<div class='contents'></div>";
+  scroller.classList.add('scroller');
+  return scroller;
+}
+
+function createScrollLinkedWorkletAnimation(test) {
+  const timeline = new ScrollTimeline({
+    scrollSource: createScroller(test)
+  });
+  const DURATION = 10000; // ms
+  const KEYFRAMES = { transform: ['translateY(100px)', 'translateY(200px)'] };
+  return new WorkletAnimation('passthrough', new KeyframeEffect(createDiv(test),
+        KEYFRAMES, DURATION), timeline);
+}
+
+setup(setupAndRegisterTests, {explicit_done: true});
+
+function setupAndRegisterTests() {
+  registerPassthroughAnimator().then(() => {
+
+    promise_test(async t => {
+      const animation = createWorkletAnimation(t);
+
+      animation.playbackRate = 0.5;
+      animation.play();
+      assert_equals(animation.currentTime, 0,
+        'Zero current time is not affected by playbackRate.');
+    }, 'Zero current time is not affected by playbackRate set while the ' +
+       'animation is in idle state.');
+
+    promise_test(async t => {
+      const animation = createWorkletAnimation(t);
+
+      animation.play();
+      animation.playbackRate = 0.5;
+      assert_equals(animation.currentTime, 0,
+        'Zero current time is not affected by playbackRate.');
+    }, 'Zero current time is not affected by playbackRate set while the ' +
+       'animation is in play-pending state.');
+
+    promise_test(async t => {
+      const animation = createWorkletAnimation(t);
+      const playbackRate = 2;
+
+      animation.play();
+
+      await waitForAnimationFrameWithCondition(_=> {
+        return animation.playState == "running"
+      });
+      // Make sure the current time is not Zero.
+      await waitForDocumentTimelineAdvance();
+
+      // Set playback rate while the animation is playing.
+      const prevCurrentTime = animation.currentTime;
+      animation.playbackRate = playbackRate;
+
+      assert_times_equal(animation.currentTime, prevCurrentTime,
+        'The current time should stay unaffected by setting playback rate.');
+    }, 'Non zero current time is not affected by playbackRate set while the ' +
+       'animation is in play state.');
+
+    promise_test(async t => {
+      const animation = createWorkletAnimation(t);
+      const playbackRate = 0.2;
+
+      animation.play();
+
+      await waitForAnimationFrameWithCondition(_=> {
+        return animation.playState == "running"
+      });
+
+      // Set playback rate while the animation is playing.
+      const prevCurrentTime = animation.currentTime;
+      const prevTimelineTime = document.timeline.currentTime;
+      animation.playbackRate = playbackRate;
+
+      // Play the animation some more.
+      await waitForDocumentTimelineAdvance();
+
+      const currentTime = animation.currentTime;
+      const currentTimelineTime = document.timeline.currentTime;
+
+      assert_times_equal(
+        currentTime - prevCurrentTime,
+        (currentTimelineTime - prevTimelineTime) * playbackRate,
+        'The current time should increase 0.2 times faster than timeline.');
+    }, 'The playback rate affects the rate of progress of the current time.');
+
+    promise_test(async t => {
+      const animation = createWorkletAnimation(t);
+      const playbackRate = 2;
+
+      // Set playback rate while the animation is in 'idle' state.
+      animation.playbackRate = playbackRate;
+      const prevTimelineTime = document.timeline.currentTime;
+      animation.play();
+
+      await waitForAnimationFrameWithCondition(_=> {
+        return animation.playState == "running"
+      });
+      await waitForDocumentTimelineAdvance();
+
+      const currentTime = animation.currentTime;
+      const timelineTime = document.timeline.currentTime;
+      assert_times_equal(
+        currentTime,
+        (timelineTime - prevTimelineTime) * playbackRate,
+        'The current time should increase two times faster than timeline.');
+    }, 'The playback rate set before the animation started playing affects ' +
+       'the rate of progress of the current time');
+
+    promise_test(async t => {
+      const timing = { duration: 100,
+                      easing: 'linear',
+                      fill: 'none',
+                      iterations: 1
+                     };
+      // TODO(crbug.com/937382): Currently composited
+      // workletAnimation.currentTime and the corresponding
+      // effect.getComputedTiming().localTime are computed by main and
+      // compositing threads respectively and, as a result, don't match.
+      // To workaround this limitation we compare the output of two identical
+      // animations that only differ in playback rate. The expectation is that
+      // their output matches after taking their playback rates into
+      // consideration. This works since these two animations start at the same
+      // time on the same thread.
+      // Once the issue is fixed, this test needs to change so expected
+      // effect.getComputedTiming().localTime is compared against
+      // workletAnimation.currentTime.
+      const target = createDiv(t);
+      const targetRef = createDiv(t);
+      const keyframeEffect = new KeyframeEffect(
+        target, { opacity: [1, 0] }, timing);
+      const keyframeEffectRef = new KeyframeEffect(
+        targetRef, { opacity: [1, 0] }, timing);
+      const animation = new WorkletAnimation(
+        'passthrough', keyframeEffect, document.timeline);
+      const animationRef = new WorkletAnimation(
+        'passthrough', keyframeEffectRef, document.timeline);
+      const playbackRate = 2;
+      animation.playbackRate = playbackRate;
+      animation.play();
+      animationRef.play();
+
+      // wait until local times are synced back to the main thread.
+      await waitForAnimationFrameWithCondition(_ => {
+        return getComputedStyle(target).opacity != '1';
+      });
+
+      assert_times_equal(
+        keyframeEffect.getComputedTiming().localTime,
+        keyframeEffectRef.getComputedTiming().localTime * playbackRate,
+        'When playback rate is set on WorkletAnimation, the underlying ' +
+        'effect\'s timing should be properly updated.');
+
+      assert_approx_equals(
+        1 - Number(getComputedStyle(target).opacity),
+        (1 - Number(getComputedStyle(targetRef).opacity)) * playbackRate,
+        0.001,
+        'When playback rate is set on WorkletAnimation, the underlying effect' +
+        ' should produce correct visual result.');
+    }, 'When playback rate is updated, the underlying effect is properly ' +
+       'updated with the current time of its WorkletAnimation and produces ' +
+       'correct visual result.');
+
+    promise_test(async t => {
+      const animation = createScrollLinkedWorkletAnimation(t);
+      const scroller = animation.timeline.scrollSource;
+      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+      scroller.scrollTop = 0.2 * maxScroll;
+
+      animation.playbackRate = 0.5;
+      animation.play();
+      await waitForAnimationFrameWithCondition(_=> {
+        return animation.playState == "running"
+      });
+      assert_percents_equal(animation.currentTime, 10,
+        'Initial current time is scaled by playbackRate.');
+    }, 'Initial current time is scaled by playbackRate set while ' +
+       'scroll-linked animation is in idle state.');
+
+    promise_test(async t => {
+      const animation = createScrollLinkedWorkletAnimation(t);
+      const scroller = animation.timeline.scrollSource;
+      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+      scroller.scrollTop = 0.2 * maxScroll;
+
+      animation.play();
+      animation.playbackRate = 0.5;
+
+      assert_percents_equal(animation.currentTime, 20,
+        'Initial current time is not affected by playbackRate.');
+    }, 'Initial current time is not affected by playbackRate set while '+
+       'scroll-linked animation is in play-pending state.');
+
+    promise_test(async t => {
+      const animation = createScrollLinkedWorkletAnimation(t);
+      const scroller = animation.timeline.scrollSource;
+      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+      const playbackRate = 2;
+
+      animation.play();
+      scroller.scrollTop = 0.2 * maxScroll;
+      await waitForAnimationFrameWithCondition(_=> {
+        return animation.playState == "running"
+      });
+      // Set playback rate while the animation is playing.
+      animation.playbackRate = playbackRate;
+      assert_percents_equal(animation.currentTime, 20,
+        'The current time should stay unaffected by setting playback rate.');
+    }, 'The current time is not affected by playbackRate set while the ' +
+       'scroll-linked animation is in play state.');
+
+    promise_test(async t => {
+      const animation = createScrollLinkedWorkletAnimation(t);
+      const scroller = animation.timeline.scrollSource;
+      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+      const playbackRate = 2;
+
+      animation.play();
+      await waitForAnimationFrameWithCondition(_=> {
+        return animation.playState == "running"
+      });
+      scroller.scrollTop = 0.1 * maxScroll;
+
+      // Set playback rate while the animation is playing.
+      animation.playbackRate = playbackRate;
+
+      scroller.scrollTop = 0.2 * maxScroll;
+
+      assert_equals(
+        animation.currentTime.value - 10, 10 * playbackRate,
+        'The current time should increase twice faster than scroll timeline.');
+    }, 'Scroll-linked animation playback rate affects the rate of progress ' +
+       'of the current time.');
+
+    promise_test(async t => {
+      const animation = createScrollLinkedWorkletAnimation(t);
+      const scroller = animation.timeline.scrollSource;
+      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+      const playbackRate = 2;
+
+      // Set playback rate while the animation is in 'idle' state.
+      animation.playbackRate = playbackRate;
+      animation.play();
+      await waitForAnimationFrameWithCondition(_=> {
+        return animation.playState == "running"
+      });
+      scroller.scrollTop = 0.2 * maxScroll;
+
+      assert_percents_equal(animation.currentTime, 20 * playbackRate,
+        'The current time should increase two times faster than timeline.');
+    }, 'The playback rate set before scroll-linked animation started playing ' +
+       'affects the rate of progress of the current time');
+
+    promise_test(async t => {
+      const scroller = createScroller(t);
+      const timeline = new ScrollTimeline({
+        scrollSource: scroller
+      });
+      const timing = { duration: 1000,
+                      easing: 'linear',
+                      fill: 'none',
+                      iterations: 1
+                    };
+      const target = createDiv(t);
+      const keyframeEffect = new KeyframeEffect(
+        target, { opacity: [1, 0] }, timing);
+      const animation = new WorkletAnimation(
+        'passthrough', keyframeEffect, timeline);
+      const playbackRate = 2;
+      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+
+      animation.play();
+      animation.playbackRate = playbackRate;
+      await waitForAnimationFrameWithCondition(_=> {
+        return animation.playState == "running"
+      });
+
+      scroller.scrollTop = 0.2 * maxScroll;
+      // wait until local times are synced back to the main thread.
+      await waitForAnimationFrameWithCondition(_ => {
+        return getComputedStyle(target).opacity != '1';
+      });
+
+      assert_percents_equal(
+        keyframeEffect.getComputedTiming().localTime,
+        20 * playbackRate,
+        'When playback rate is set on WorkletAnimation, the underlying ' +
+        'effect\'s timing should be properly updated.');
+      assert_approx_equals(
+        Number(getComputedStyle(target).opacity),
+        1 - 20 * playbackRate / 1000, 0.001,
+        'When playback rate is set on WorkletAnimation, the underlying ' +
+        'effect should produce correct visual result.');
+    }, 'When playback rate is updated, the underlying effect is properly ' +
+       'updated with the current time of its scroll-linked WorkletAnimation ' +
+       'and produces correct visual result.');
+    done();
+  });
+}
+</script>
+</body>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/references/not-translated-box-ref.html b/testing/web-platform/tests/animation-worklet/references/not-translated-box-ref.html
new file mode 100644
index 0000000000..96acf1ad96
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/references/not-translated-box-ref.html
@@ -0,0 +1,12 @@
+<!DOCTYPE html>
+<title>Reference for Animation Worklet local time set after duration</title>
+<style>
+#box {
+  width: 100px;
+  height: 100px;
+  background-color: green;
+  will-change: transform;
+}
+</style>
+
+<div id="box"></div>
diff --git a/testing/web-platform/tests/animation-worklet/references/translated-box-ref.html b/testing/web-platform/tests/animation-worklet/references/translated-box-ref.html
new file mode 100644
index 0000000000..f1dde2e19b
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/references/translated-box-ref.html
@@ -0,0 +1,12 @@
+<!DOCTYPE html>
+<style>
+#box {
+  width: 100px;
+  height: 100px;
+  transform: translateY(100px);
+  background-color: green;
+  will-change: transform;
+}
+</style>
+
+<div id="box"></div>
diff --git a/testing/web-platform/tests/animation-worklet/resources/animator-iframe.html b/testing/web-platform/tests/animation-worklet/resources/animator-iframe.html
new file mode 100644
index 0000000000..f9a5fab9b7
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/resources/animator-iframe.html
@@ -0,0 +1,44 @@
+<!DOCTYPE html>
+<style>
+.greenbox {
+  width: 100px;
+  height: 100px;
+  background-color: #00ff00;
+}
+</style>
+<script src="/web-animations/testcommon.js"></script>
+<script src="../common.js"></script>
+
+<script id="iframe_worklet" type="text/worklet">
+registerAnimator("iframe_animator", class {
+  animate(currentTime, effect) {
+    effect.localTime = 600;
+  }
+});
+registerAnimator("duplicate_animator", class {
+  animate(currentTime, effect) {
+    effect.localTime = 800;
+  }
+});
+</script>
+
+<div id="iframe_target" class="greenbox"></div>
+
+<script>
+runInAnimationWorklet(
+  document.getElementById('iframe_worklet').textContent
+).then(_ => {
+  const target = document.getElementById('iframe_target');
+  // Only create an animation for iframe_animator.
+  const effect = new KeyframeEffect(target, [{ opacity: 0 }], { duration: 1000 });
+  const animation = new WorkletAnimation('iframe_animator', effect);
+  animation.play();
+
+  // wait until local times are synced back to the main thread.
+  waitForAnimationFrameWithCondition(_ => {
+    return getComputedStyle(target).opacity != '1';
+  }).then(_ => {
+    window.parent.postMessage(getComputedStyle(target).opacity, '*');
+  });
+ });
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/resources/iframe.html b/testing/web-platform/tests/animation-worklet/resources/iframe.html
new file mode 100644
index 0000000000..e128fa53e4
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/resources/iframe.html
@@ -0,0 +1,9 @@
+<!DOCTYPE html>
+<style>
+#iframe_box {
+  width: 100px;
+  height: 100px;
+  background-color: green;
+}
+</style>
+<div id='iframe_box'></div>
diff --git a/testing/web-platform/tests/animation-worklet/scroll-timeline-writing-modes.https.html b/testing/web-platform/tests/animation-worklet/scroll-timeline-writing-modes.https.html
new file mode 100644
index 0000000000..2bd17a89da
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/scroll-timeline-writing-modes.https.html
@@ -0,0 +1,168 @@
+<!DOCTYPE html>
+<title>Tests that ScrollTimeline works properly with writing mode and directionality</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+
+<script>
+// Creates a DOM structure like:
+//   - container
+//     - box {100x100}
+//     - scroller {100x100, writing-mode, direction}
+//       - contents
+function createTestDOM(x_scroll_axis, writing_mode, direction) {
+  const elements = {};
+
+  elements.container = document.createElement('div');
+
+  elements.box = document.createElement('div');
+  elements.box.style.height = '100px';
+  elements.box.style.width = '100px';
+
+  elements.scroller = document.createElement('div');
+  elements.scroller.style.height = '100px';
+  elements.scroller.style.width = '100px';
+  if (x_scroll_axis)
+    elements.scroller.style.overflowX = 'scroll';
+  else
+    elements.scroller.style.overflowY = 'scroll';
+  elements.scroller.style.direction = direction;
+  elements.scroller.style.writingMode = writing_mode;
+
+  // Callers don't need access to this.
+  const contents = document.createElement('div');
+  contents.style.height = x_scroll_axis ? '100%' : '1000px';
+  contents.style.width = x_scroll_axis ? '1000px' : '100%';
+
+  elements.scroller.appendChild(contents);
+  elements.container.appendChild(elements.box);
+  elements.container.appendChild(elements.scroller);
+  document.body.appendChild(elements.container);
+
+  return elements;
+}
+
+function createAndPlayTestAnimation(elements, timeline_orientation) {
+  const effect = new KeyframeEffect(
+      elements.box,
+      [{transform: 'translateY(0)'}, {transform: 'translateY(200px)'}], {
+        duration: 1000,
+      });
+
+  const timeline = new ScrollTimeline({
+    scrollSource: elements.scroller,
+    orientation: timeline_orientation
+  });
+  const animation = new WorkletAnimation('passthrough', effect, timeline);
+  animation.play();
+  return animation;
+}
+
+setup(setupAndRegisterTests, {explicit_done: true});
+
+function setupAndRegisterTests() {
+  registerPassthroughAnimator().then(() => {
+    // Note that block horizontal-tb is tested implicitly in the basic
+    // ScrollTimeline tests (as it is the default).
+    async_test(
+        block_vertical_lr,
+        'A block ScrollTimeline should produce the correct current time for vertical-lr');
+    async_test(
+        block_vertical_rl,
+        'A block ScrollTimeline should produce the correct current time for vertical-rl');
+    // Again, inline for horizontal-tb and direction: ltr is the default
+    // inline mode and so is tested elsewhere.
+    async_test(
+        inline_horizontal_tb_rtl,
+        'An inline ScrollTimeline should produce the correct current time for horizontal-tb and direction: rtl');
+    async_test(
+        inline_vertical_writing_mode_ltr,
+        'An inline ScrollTimeline should produce the correct current time for vertical writing mode');
+    async_test(
+        inline_vertical_writing_mode_rtl,
+        'An inline ScrollTimeline should produce the correct current time for vertical writing mode and direction: rtl');
+    done();
+  });
+}
+
+function block_vertical_lr(t) {
+  const elements = createTestDOM(true, 'vertical-lr', 'ltr');
+  const animation = createAndPlayTestAnimation(elements, 'block');
+
+  // Move the scroller to the 25% point.
+  const maxScroll =
+      elements.scroller.scrollWidth - elements.scroller.clientWidth;
+  elements.scroller.scrollLeft = 0.25 * maxScroll;
+
+  waitForNotNullLocalTime(animation).then(t.step_func_done(() => {
+    assert_equals(
+        getComputedStyle(elements.box).transform, 'matrix(1, 0, 0, 1, 0, 50)');
+  }));
+}
+
+function block_vertical_rl(t) {
+  const elements = createTestDOM(true, 'vertical-rl', 'ltr');
+  const animation = createAndPlayTestAnimation(elements, 'block');
+
+  // Move the scroller to the left 25% point, since it is vertical-rl,
+  // i.e leftwards overflow direction, scrollLeft is -25% point.
+  const maxScroll =
+      elements.scroller.scrollWidth - elements.scroller.clientWidth;
+  elements.scroller.scrollLeft = -0.25 * maxScroll;
+
+  waitForNotNullLocalTime(animation).then(t.step_func_done(() => {
+    assert_equals(
+        getComputedStyle(elements.box).transform, 'matrix(1, 0, 0, 1, 0, 50)');
+  }));
+}
+
+function inline_horizontal_tb_rtl(t) {
+  const elements = createTestDOM(true, 'horizontal-tb', 'rtl');
+  const animation = createAndPlayTestAnimation(elements, 'inline');
+
+  // Move the scroller to the left 25% point, since it is direction: rtl,
+  // i.e leftwards overflow direction, scrollLeft is -25% point.
+  const maxScroll =
+      elements.scroller.scrollWidth - elements.scroller.clientWidth;
+  elements.scroller.scrollLeft = -0.25 * maxScroll;
+
+  waitForNotNullLocalTime(animation).then(t.step_func_done(() => {
+    assert_equals(
+        getComputedStyle(elements.box).transform, 'matrix(1, 0, 0, 1, 0, 50)');
+  }));
+}
+
+function inline_vertical_writing_mode_ltr(t) {
+  const elements = createTestDOM(false, 'vertical-lr', 'ltr');
+  const animation = createAndPlayTestAnimation(elements, 'inline');
+
+  // Move the scroller to the 25% point.
+  const maxScroll =
+      elements.scroller.scrollHeight - elements.scroller.clientHeight;
+  elements.scroller.scrollTop = 0.25 * maxScroll;
+
+  waitForNotNullLocalTime(animation).then(t.step_func_done(() => {
+    assert_equals(
+        getComputedStyle(elements.box).transform, 'matrix(1, 0, 0, 1, 0, 50)');
+  }));
+}
+
+function inline_vertical_writing_mode_rtl(t) {
+  const elements = createTestDOM(false, 'vertical-lr', 'rtl');
+  const animation = createAndPlayTestAnimation(elements, 'inline');
+
+  // Move the scroller to the top 25% point, since it is a vertical-lr writing mode
+  // and direction: rtl, i.e upwards overflow direction, scrollTop is -25% point.
+  const maxScroll =
+      elements.scroller.scrollHeight - elements.scroller.clientHeight;
+  elements.scroller.scrollTop = -0.25 * maxScroll;
+
+  waitForNotNullLocalTime(animation).then(t.step_func_done(() => {
+    assert_equals(
+        getComputedStyle(elements.box).transform, 'matrix(1, 0, 0, 1, 0, 50)');
+  }));
+}
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/stateful-animator.https.html b/testing/web-platform/tests/animation-worklet/stateful-animator.https.html
new file mode 100644
index 0000000000..be29fa109c
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/stateful-animator.https.html
@@ -0,0 +1,216 @@
+<!DOCTYPE html>
+<title>Basic use of stateful animator</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<div id="target"></div>
+
+<script id="stateful_animator_basic" type="text/worklet">
+  registerAnimator("stateful_animator_basic", class {
+    constructor(options, state = { test_local_time: 0 }) {
+      this.test_local_time = state.test_local_time;
+    }
+    animate(currentTime, effect) {
+      effect.localTime = this.test_local_time++;
+    }
+    state() {
+      return {
+        test_local_time: this.test_local_time
+      };
+    }
+  });
+</script>
+
+<script id="stateless_animator_basic" type="text/worklet">
+  registerAnimator("stateless_animator_basic", class {
+    constructor(options, state = { test_local_time: 0 }) {
+      this.test_local_time = state.test_local_time;
+    }
+    animate(currentTime, effect) {
+      effect.localTime = this.test_local_time++;
+    }
+    // Unless a valid state function is provided, the animator is considered
+    // stateless. e.g. animator with incorrect state function name.
+    State() {
+      return {
+        test_local_time: this.test_local_time
+      };
+    }
+  });
+</script>
+
+<script id="stateless_animator_preserves_effect_local_time" type="text/worklet">
+  registerAnimator("stateless_animator_preserves_effect_local_time", class {
+    animate(currentTime, effect) {
+      // The local time will be carried over to the new global scope.
+      effect.localTime = effect.localTime ? effect.localTime + 1 : 1;
+    }
+  });
+</script>
+
+<script id="stateless_animator_does_not_copy_effect_object" type="text/worklet">
+  registerAnimator("stateless_animator_does_not_copy_effect_object", class {
+    animate(currentTime, effect) {
+      effect.localTime = effect.localTime ? effect.localTime + 1 : 1;
+      effect.foo = effect.foo ? effect.foo + 1 : 1;
+      // This condition becomes true once we switch global scope and only preserve local time
+      // otherwise these values keep increasing in lock step.
+      if (effect.localTime > effect.foo) {
+        // This works as long as we switch global scope before 10000 frames.
+        // which is a safe assumption.
+        effect.localTime = 10000;
+      }
+    }
+  });
+</script>
+
+<script id="state_function_returns_empty" type="text/worklet">
+  registerAnimator("state_function_returns_empty", class {
+    constructor(options, state = { test_local_time: 0 }) {
+      this.test_local_time = state.test_local_time;
+    }
+    animate(currentTime, effect) {
+      effect.localTime = this.test_local_time++;
+    }
+    state() {}
+  });
+</script>
+
+<script id="state_function_returns_not_serializable" type="text/worklet">
+  registerAnimator("state_function_returns_not_serializable", class {
+    constructor(options) {
+      this.test_local_time = 0;
+    }
+    animate(currentTime, effect) {
+      effect.localTime = this.test_local_time++;
+    }
+    state() {
+      return new Symbol('foo');
+    }
+  });
+</script>
+
+<script>
+  const EXPECTED_FRAMES_TO_A_SCOPE_SWITCH = 15;
+  async function localTimeDoesNotUpdate(animation) {
+    // The local time stops increasing after the animator instance being dropped.
+    // e.g. 0, 1, 2, .., n, n, n, n, .. where n is the frame that the global
+    // scope switches at.
+    let last_local_time = animation.effect.getComputedTiming().localTime;
+    let frame_count = 0;
+    const FRAMES_WITHOUT_CHANGE = 10;
+    do {
+      await new Promise(window.requestAnimationFrame);
+      let current_local_time = animation.effect.getComputedTiming().localTime;
+      if (approxEquals(last_local_time, current_local_time))
+        ++frame_count;
+      else
+        frame_count = 0;
+      last_local_time = current_local_time;
+    } while (frame_count < FRAMES_WITHOUT_CHANGE);
+  }
+
+  async function localTimeResetsToZero(animation) {
+    // The local time is reset upon global scope switching. e.g.
+    // 0, 1, 2, .., 0, 1, 2, .., 0, 1, 2, .., 0, 1, 2, ...
+    let reset_count = 0;
+    const LOCAL_TIME_RESET_CHECK = 3;
+    do {
+      await new Promise(window.requestAnimationFrame);
+      if (approxEquals(0, animation.effect.getComputedTiming().localTime))
+        ++reset_count;
+    } while (reset_count < LOCAL_TIME_RESET_CHECK);
+  }
+
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('stateful_animator_basic').textContent);
+    const target = document.getElementById('target');
+    const effect = new KeyframeEffect(target, [{ opacity: 0 }], { duration: 1000 });
+    const animation = new WorkletAnimation('stateful_animator_basic', effect);
+    animation.play();
+
+    // effect.localTime should be correctly increased upon global scope
+    // switches for stateful animators.
+    await waitForAnimationFrameWithCondition(_ => {
+      return approxEquals(animation.effect.getComputedTiming().localTime,
+          EXPECTED_FRAMES_TO_A_SCOPE_SWITCH);
+    });
+
+    animation.cancel();
+  }, "Stateful animator can use its state to update the animation. Pass if test does not timeout");
+
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('stateless_animator_basic').textContent);
+    const target = document.getElementById('target');
+    const effect = new KeyframeEffect(target, [{ opacity: 0 }], { duration: 1000 });
+    const animation = new WorkletAnimation('stateless_animator_basic', effect);
+    animation.play();
+
+    // The local time should be reset to 0 upon global scope switching for
+    // stateless animators.
+    await localTimeResetsToZero(animation);
+
+    animation.cancel();
+  }, "Stateless animator gets reecreated with 'undefined' state.");
+
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('stateless_animator_preserves_effect_local_time').textContent);
+    const target = document.getElementById('target');
+    const effect = new KeyframeEffect(target, [{ opacity: 0 }], { duration: 1000 });
+    const animation = new WorkletAnimation('stateless_animator_preserves_effect_local_time', effect);
+    animation.play();
+
+    await waitForAnimationFrameWithCondition(_ => {
+        return approxEquals(animation.effect.getComputedTiming().localTime,
+            EXPECTED_FRAMES_TO_A_SCOPE_SWITCH);
+    });
+
+    animation.cancel();
+  }, "Stateless animator should preserve the local time of its effect.");
+
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('stateless_animator_does_not_copy_effect_object').textContent);
+    const target = document.getElementById('target');
+    const effect = new KeyframeEffect(target, [{ opacity: 0 }], { duration: 1000 });
+    const animation = new WorkletAnimation('stateless_animator_does_not_copy_effect_object', effect);
+    animation.play();
+
+    await waitForAnimationFrameWithCondition(_ => {
+        return approxEquals(animation.effect.getComputedTiming().localTime, 10000);
+    });
+
+    animation.cancel();
+  }, "Stateless animator should not copy the effect object.");
+
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('state_function_returns_empty').textContent);
+    const target = document.getElementById('target');
+    const effect = new KeyframeEffect(target, [{ opacity: 0 }], { duration: 1000 });
+    const animation = new WorkletAnimation('state_function_returns_empty', effect);
+    animation.play();
+
+    // The local time should be reset to 0 upon global scope switching for
+    // stateless animators.
+    await localTimeResetsToZero(animation);
+
+    animation.cancel();
+  }, "Stateful animator gets recreated with 'undefined' state if state function returns undefined.");
+
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('state_function_returns_not_serializable').textContent);
+    const target = document.getElementById('target');
+    const effect = new KeyframeEffect(target, [{ opacity: 0 }], { duration: 1000, iteration: Infinity });
+    const animation = new WorkletAnimation('state_function_returns_not_serializable', effect);
+    animation.play();
+
+    // The local time of an animation increases until the registered animator
+    // gets removed.
+    await localTimeDoesNotUpdate(animation);
+
+    animation.cancel();
+  }, "Stateful Animator instance gets dropped (does not get migrated) if state function is not serializable.");
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-animator-name.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-animator-name.https.html
new file mode 100644
index 0000000000..bd886ccd02
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-animator-name.https.html
@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+
+<title>Worklet Animation's animator name should be accessible via animatorName property</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="common.js"></script>
+
+<script id="test_animator" type="text/worklet">
+  class NoopAnimator {
+    animate(currentTime, effect) {}
+  }
+  registerAnimator('Tokyo', NoopAnimator);
+  registerAnimator('دزفول', NoopAnimator);
+</script>
+
+<body></body>
+
+<script>
+promise_test(async t => {
+  await runInAnimationWorklet(document.getElementById('test_animator').textContent);
+
+  // An ascii name and a non-ascii one.
+  for (let name of ['Tokyo', 'دزفول']) {
+    const animation = new WorkletAnimation(name, new KeyframeEffect(document.body, {}));
+    assert_equals(name, animation.animatorName);
+  }
+}, 'Verify that animatorName matches passed name');
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-cancel.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-cancel.https.html
new file mode 100644
index 0000000000..3b664ecddb
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-cancel.https.html
@@ -0,0 +1,45 @@
+<html class="reftest-wait">
+<title>Canceling a playing WorkletAnimation should remove the effect</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<meta name="assert" content="Canceling a playing animation should remove the effect">
+<link rel="match" href="references/not-translated-box-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+<script src="common.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+</style>
+
+<div id="box"></div>
+
+<script>
+  registerConstantLocalTimeAnimator(500).then(_ => {
+    const box = document.getElementById('box');
+    const effect = new KeyframeEffect(box,
+      [
+        {transform: 'translateY(0)'},
+        {transform: 'translateY(200px)'}
+      ], {
+        duration: 1000,
+        iterations: Infinity
+      }
+    );
+
+    const animation = new WorkletAnimation('constant_time', effect);
+    animation.play();
+
+    waitForAsyncAnimationFrames(1).then(_ => {
+      // Canceling a playing animation should remove the effect.
+      animation.cancel();
+      waitForAsyncAnimationFrames(1).then(_ => {
+        takeScreenshot();
+      });
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-creation.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-creation.https.html
new file mode 100644
index 0000000000..b7d1a43721
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-creation.https.html
@@ -0,0 +1,141 @@
+<!DOCTYPE html>
+<title>Verify that WorkletAnimation is correctly created</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<style>
+  .scroller {
+    height: 100px;
+    width: 100px;
+    overflow: scroll;
+  }
+  .content {
+    height: 500px;
+    width: 500px;
+  }
+</style>
+
+<script>
+function CreateKeyframeEffect(element) {
+  return new KeyframeEffect(
+      element,
+      [
+        { transform: 'translateY(0%)' },
+        { transform: 'translateY(100%)' }
+      ],
+      { duration: 3000, fill: 'forwards' }
+  );
+}
+</script>
+<script id="simple_animate" type="text/worklet">
+  registerAnimator("test-animator", class {
+    animate(currentTime, effect) {}
+  });
+</script>
+
+<div id='element'></div>
+<div id='element2'></div>
+<div class='scroller'>
+  <div class='content'></div>
+</div>
+
+<script>
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('simple_animate').textContent);
+    let effect = CreateKeyframeEffect(document.querySelector('#element'));
+    let workletAnimation = new WorkletAnimation('test-animator', effect);
+    assert_equals(workletAnimation.playState, 'idle');
+    assert_equals(workletAnimation.timeline, document.timeline);
+  }, 'WorkletAnimation creation without timeline should use default documentation timeline');
+
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('simple_animate').textContent);
+    let effect = CreateKeyframeEffect(document.querySelector('#element'));
+    let workletAnimation = new WorkletAnimation('test-animator', effect);
+    assert_equals(workletAnimation.playState, 'idle');
+  }, 'WorkletAnimation creation with timeline should work');
+
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('simple_animate').textContent);
+    let iframe = document.createElement('iframe');
+    iframe.src = 'resources/iframe.html';
+    document.body.appendChild(iframe);
+
+    await waitForAnimationFrameWithCondition(_ => {
+      return iframe.contentDocument.getElementById('iframe_box') != null;
+    });
+    let iframe_document = iframe.contentDocument;
+    let effect = CreateKeyframeEffect(iframe_document.getElementById('iframe_box'));
+
+    let animation_with_main_frame_timeline =
+        new WorkletAnimation('test-animator', effect, document.timeline);
+    assert_equals(animation_with_main_frame_timeline.timeline, document.timeline);
+
+    let animation_with_iframe_timeline =
+        new WorkletAnimation('test-animator', effect, iframe_document.timeline);
+    assert_equals(animation_with_iframe_timeline.timeline, iframe_document.timeline);
+
+    let animation_with_default_timeline = new WorkletAnimation('test-animator', effect);
+    // The spec says that the default timeline is taken from 'the Document that is
+    // associated with the window that is the current global object'. In this case
+    // that is the main document's timeline, not the iframe (despite the target
+    // being in the iframe).
+    assert_equals(animation_with_default_timeline.timeline, document.timeline);
+
+    iframe.remove();
+  }, 'WorkletAnimation creation should choose the correct timeline based on the current global object');
+
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('simple_animate').textContent);
+    let effect = CreateKeyframeEffect(document.querySelector('#element'));
+    let options = { my_param: 'foo', my_other_param: true };
+    let workletAnimation = new WorkletAnimation(
+        'test-animator', effect, document.timeline, options);
+    assert_equals(workletAnimation.playState, 'idle');
+  }, 'WorkletAnimation creation with timeline and options should work');
+
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('simple_animate').textContent);
+    let effect = CreateKeyframeEffect(document.querySelector('#element'));
+    let scroller = document.querySelector('.scroller');
+    let scrollTimeline = new ScrollTimeline(
+        { scrollSource: scroller, orientation: 'inline' });
+    let workletAnimation = new WorkletAnimation(
+        'test-animator', effect, scrollTimeline);
+    assert_equals(workletAnimation.playState, 'idle');
+  }, 'ScrollTimeline is a valid timeline for a WorkletAnimation');
+
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('simple_animate').textContent);
+    let constructorFunc = function() { new WorkletAnimation(
+        'test-animator', []); };
+    assert_throws_dom('NotSupportedError', constructorFunc);
+  }, 'If there are no effects specified, object construction should fail');
+
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('simple_animate').textContent);
+    let effect = CreateKeyframeEffect(document.querySelector('#element'));
+
+    let otherDoc = document.implementation.createHTMLDocument();
+    let otherElement = otherDoc.createElement('div');
+    otherDoc.body.appendChild(otherElement);
+    let otherEffect = CreateKeyframeEffect(otherElement);
+
+    let workletAnimation = new WorkletAnimation(
+        'test-animator', [ effect, otherEffect ]);
+    assert_equals(workletAnimation.playState, 'idle');
+  }, 'Creating animation with effects from different documents is allowed');
+
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('simple_animate').textContent);
+    let effect = CreateKeyframeEffect(document.querySelector('#element'));
+    let constructorFunc = function() {
+        new WorkletAnimation('unregistered-animator', effect);
+    };
+    assert_throws_dom('InvalidStateError', constructorFunc);
+  }, 'Constructing worklet animation for unregisested animator should throw');
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-duration.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-duration.https.html
new file mode 100644
index 0000000000..1a8afc1e89
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-duration.https.html
@@ -0,0 +1,39 @@
+<html>
+<title>WorkletAnimation should continue to be in effect forever, even if its duration is passed</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<div id="box"></div>
+
+<script>
+  promise_test(async t => {
+    await registerConstantLocalTimeAnimator(0.5);
+
+    const box = document.getElementById('box');
+    const effect = new KeyframeEffect(box,
+      [
+        {transform: 'translateY(0px)' },
+        {transform: 'translateY(200px)' }
+      ], {
+        duration: 1,
+      }
+    );
+
+    const animation = new WorkletAnimation('constant_time', effect);
+    animation.play();
+
+    let expected_transform = "matrix(1, 0, 0, 1, 0, 100)";
+    await waitForAnimationFrameWithCondition(_ => {
+      return getComputedStyle(box).transform == expected_transform;
+    });
+
+    // The animation is specified to last for 1 millisecond
+    await new Promise(resolve => step_timeout(resolve, 500));
+
+    assert_equals(getComputedStyle(document.getElementById("box")).transform, expected_transform);
+  }, "WorkletAnimation should continue to be in effect forever, even if its duration is passed");
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-get-computed-timing-progress-on-worklet-thread.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-get-computed-timing-progress-on-worklet-thread.https.html
new file mode 100644
index 0000000000..a66a4b9156
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-get-computed-timing-progress-on-worklet-thread.https.html
@@ -0,0 +1,87 @@
+<html>
+<title>Animation Worklet should update calculated timing whenever localTime changes</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<div id="box"></div>
+
+<script id="get_computed_timing_animator" type="text/worklet">
+  registerAnimator('get_computed_timing', class {
+    constructor(options, state) {
+      this.step = state ? state.step : 0;
+    }
+    state() {
+      return {
+        step: 0
+      }
+    }
+    animate(currentTime, effect){
+      if (this.step === 0){
+        // check calculated timing values before ever setting effect.localTime
+        effect.localTime = (effect.getComputedTiming().currentIteration * 100) + (effect.getComputedTiming().progress * 100);
+        this.step = 1;
+      }
+      else if (this.step === 1){
+        // set effect.localTime, this should be the first time calculated timing values are computed
+        effect.localTime = 420; // 20% of the way through the last iteration
+
+        // using the calculated timing of effect, set effect.localTime.
+        effect.localTime = (effect.getComputedTiming().currentIteration * 100) + (effect.getComputedTiming().progress * 100);
+        this.step = 2;
+      }
+      else if (this.step === 2){
+        // set effect.localTime to null
+        effect.localTime = null;
+        effect.localTime = (effect.getComputedTiming().currentIteration * 100) + (effect.getComputedTiming().progress * 100);
+        this.step = 3;
+      }
+      else if (this.step === 3){
+        // Check to make sure we can go from null to a valid localTime and that calculated timing values are computed
+        effect.localTime = 350; // 50% of the way through second iteration
+        effect.localTime = (effect.getComputedTiming().currentIteration * 100) + (effect.getComputedTiming().progress * 100);
+        this.step = 4;
+      }
+    }
+  });
+</script>
+
+<script>
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('get_computed_timing_animator').textContent);
+
+    const box = document.getElementById("box");
+    const effect = new KeyframeEffect(
+      box,
+      [
+        { opacity: 0 },
+        { opacity: 1 }
+      ], {
+        delay: 200,
+        duration: 100,
+        iterations: 3
+      }
+    );
+
+    const animation = new WorkletAnimation('get_computed_timing', effect);
+    animation.play();
+
+    // check calculated timing values before ever setting effect.localTime
+    await waitForAnimationFrameWithCondition(() => {return approxEquals(effect.getComputedTiming().localTime, 0)});
+
+    // Check to make sure initial values can be set for computed timing
+    await waitForAnimationFrameWithCondition(() => {return approxEquals(effect.getComputedTiming().localTime, 220)});
+
+    // Make sure setting effect.localTime to null causes calculated timing values to be computed
+    await waitForAnimationFrameWithCondition(() => {return approxEquals(effect.getComputedTiming().localTime, 0)});
+
+    // Make sure we can go from null to a valid localTime and that calculated timing values are computed
+    await waitForAnimationFrameWithCondition(() => {return approxEquals(effect.getComputedTiming().localTime, 150)});
+
+    // Passes if it doesn't timeout
+    animation.cancel();
+  }, "WorkletAnimation effect should recompute its calculated timing if its local time changes");
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-get-timing-on-worklet-thread.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-get-timing-on-worklet-thread.https.html
new file mode 100644
index 0000000000..4ba68d79e4
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-get-timing-on-worklet-thread.https.html
@@ -0,0 +1,54 @@
+<html class="reftest-wait">
+<title>Animation Worklet should have access to effect timing from within the worklet thread</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<meta name="assert" content="Animation Worklet should have access to effect timing from within the worklet thread">
+<link rel="match" href="references/translated-box-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+<script src="common.js"></script>
+
+<style>
+  #box{
+    height: 100px;
+    width: 100px;
+    background-color: green;
+  }
+</style>
+
+<div id="box"></div>
+
+<script id="get_timing_animator"  type="text/worklet">
+  registerAnimator('get_timing', class {
+    animate(currentTime, effect){
+      effect.localTime = effect.getTiming().delay + (effect.getTiming().duration / 2);
+    }
+  });
+</script>
+
+<script>
+  runInAnimationWorklet(
+    document.getElementById('get_timing_animator').textContent
+  ).then(() => {
+    const box = document.getElementById("box");
+    const effect = new KeyframeEffect(
+      box,
+      [
+        {transform: 'translateY(0)'},
+        {transform: 'translateY(200px)'}
+      ],
+      {
+        delay: 2000,
+        duration: 1000
+      }
+    );
+
+    const animation = new WorkletAnimation('get_timing', effect);
+    animation.play();
+
+    waitForAsyncAnimationFrames(1).then(_ => {
+      takeScreenshot();
+    });
+  });
+</script>
+</html>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-after-duration.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-after-duration.https.html
new file mode 100644
index 0000000000..21293bc09b
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-after-duration.https.html
@@ -0,0 +1,41 @@
+<html class="reftest-wait">
+<title>Animation Worklet local time set after duration</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<meta name="assert" content="If an effect doesn't have fill-mode specified, setting its local time beyond its duration makes the animation inactive.">
+<link rel="match" href="references/not-translated-box-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+<script src="common.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+</style>
+
+<div id="box"></div>
+
+<script>
+  registerConstantLocalTimeAnimator(2000).then(() => {
+    const box = document.getElementById('box');
+    const effect = new KeyframeEffect(box,
+      [
+      { transform: 'translateY(100px)' },
+      { transform: 'translateY(200px)' }
+      ], {
+        duration: 1000,
+        delay: 1000
+      }
+    );
+
+    const animation = new WorkletAnimation('constant_time', effect);
+    animation.play();
+
+    waitForAsyncAnimationFrames(1).then(_ => {
+      takeScreenshot();
+    });
+  });
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-before-start.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-before-start.https.html
new file mode 100644
index 0000000000..a959b73c08
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-before-start.https.html
@@ -0,0 +1,41 @@
+<html class="reftest-wait">
+<title>Animation Worklet local time set before start</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<meta name="assert" content="The local time should be trimmed by the duration, e.g. this is equivalent to effect.localTime = 0">
+<link rel="match" href="references/translated-box-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+<script src="common.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+</style>
+
+<div id="box"></div>
+
+<script>
+  registerConstantLocalTimeAnimator(-500).then(() => {
+    const box = document.getElementById('box');
+    const effect = new KeyframeEffect(box,
+      [
+      { transform: 'translateY(100px)' },
+      { transform: 'translateY(0px)' }
+      ], {
+        duration: 1000,
+        fill: 'backwards'
+      }
+    );
+
+    const animation = new WorkletAnimation('constant_time', effect);
+    animation.play();
+
+    waitForAsyncAnimationFrames(1).then(_ => {
+      takeScreenshot();
+    });
+  });
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-null-1.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-null-1.https.html
new file mode 100644
index 0000000000..52727fa6ea
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-null-1.https.html
@@ -0,0 +1,163 @@
+<!DOCTYPE html>
+<title>Setting localTime to null means effect does not apply</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<style>
+.box {
+  width: 100px;
+  height: 100px;
+  background-color: green;
+  display: inline-block;
+}
+</style>
+
+<div>
+<div class="box" id="target1"></div>
+<div class="box" id="target2"></div>
+<div class="box" id="target3"></div>
+<div class="box" id="target4"></div>
+</div>
+
+
+<script>
+promise_test(async t => {
+  await runInAnimationWorklet(`
+    registerAnimator("blank_animator", class {
+      animate(currentTime, effect) {
+        // Unset effect.localTime is equivalent to 'null'
+      }
+    });
+  `);
+  const target = document.getElementById('target1');
+
+  const animation = new WorkletAnimation('blank_animator',
+    new KeyframeEffect(target,
+      [
+        { transform: 'translateY(100px)' },
+        { transform: 'translateY(200px)' }
+      ], {
+        duration: 1000,
+      }
+    )
+  );
+  animation.play();
+  await waitForAsyncAnimationFrames(1);
+  assert_equals(getComputedStyle(target).transform, "none");
+}, "A worklet which never sets localTime has no effect.");
+
+promise_test(async t => {
+  await runInAnimationWorklet(`
+    registerAnimator("null_animator", class {
+      animate(currentTime, effect) {
+        effect.localTime = null;
+      }
+    });
+  `);
+  const target = document.getElementById('target2');
+
+  const animation = new WorkletAnimation('null_animator',
+    new KeyframeEffect(target,
+      [
+        { transform: 'translateY(100px)' },
+        { transform: 'translateY(200px)' }
+      ], {
+        duration: 1000,
+      }
+    )
+  );
+  animation.play();
+  await waitForAsyncAnimationFrames(1);
+  assert_equals(getComputedStyle(target).transform, "none");
+}, "A worklet which sets localTime to null has no effect.");
+
+promise_test(async t => {
+  await runInAnimationWorklet(`
+    registerAnimator("drop_animator", class {
+      animate(currentTime, effect) {
+        if (currentTime < 500)
+          effect.localTime = 500;
+        else if (currentTime < 1000)
+          effect.localTime = 0;
+        else
+          effect.localTime = null;
+      }
+    });
+  `);
+  const target = document.getElementById('target3');
+
+  const animation = new WorkletAnimation('drop_animator',
+    new KeyframeEffect(target,
+      [
+        { transform: 'translateY(100px)' },
+        { transform: 'translateY(200px)' }
+      ], {
+        duration: 1000,
+      }
+    )
+  );
+  animation.play();
+  await waitForAsyncAnimationFrames(5);
+  assert_equals(getComputedStyle(target).transform, "matrix(1, 0, 0, 1, 0, 150)",
+      "The animation has an effect at first");
+
+  await waitForAnimationFrameWithCondition(() => animation.currentTime > 500);
+  await waitForAsyncAnimationFrames(1);
+  assert_equals(getComputedStyle(target).transform, "matrix(1, 0, 0, 1, 0, 100)",
+      "The effect correctly changes");
+
+  await waitForAnimationFrameWithCondition(() => animation.currentTime > 1000);
+  await waitForAsyncAnimationFrames(1);
+  assert_equals(getComputedStyle(target).transform, "none",
+      "The effect stops on nulling of localTime");
+
+}, "A worklet which changes localTime to from a number to null has no effect on transform.");
+
+promise_test(async t => {
+  await runInAnimationWorklet(`
+    registerAnimator("drop2_animator", class {
+      animate(currentTime, effect) {
+        if (currentTime < 500)
+          effect.localTime = 500;
+        else if (currentTime < 1000)
+          effect.localTime = 0;
+        else
+          effect.localTime = null;
+      }
+    });
+  `);
+  const target = document.getElementById('target4');
+
+  const animation = new WorkletAnimation('drop2_animator',
+    new KeyframeEffect(target,
+      [
+        { backgroundColor: 'red' },
+        { backgroundColor: 'blue' }
+      ], {
+        duration: 1000,
+      }
+    )
+  );
+  animation.play();
+  await waitForAsyncAnimationFrames(5);
+  assert_equals(getComputedStyle(target).backgroundColor, "rgb(128, 0, 128)",
+      "The animation has an effect at first");
+
+  await waitForAnimationFrameWithCondition(() => animation.currentTime > 500);
+  await waitForAsyncAnimationFrames(1);
+  assert_equals(getComputedStyle(target).backgroundColor, "rgb(255, 0, 0)",
+      "The effect correctly changes");
+
+  await waitForAnimationFrameWithCondition(() => animation.currentTime > 1000);
+  await waitForAsyncAnimationFrames(1);
+  assert_equals(getComputedStyle(target).backgroundColor, "rgb(0, 128, 0)",
+      "The effect stops on nulling of localTime");
+
+}, "A worklet which changes localTime to from a number to null has no effect on backgroundColor.");
+
+
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-null-2-ref.html b/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-null-2-ref.html
new file mode 100644
index 0000000000..3b7a2b9258
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-null-2-ref.html
@@ -0,0 +1,27 @@
+<!DOCTYPE html>
+<title>Setting localTime to null means effect does not apply (reftest)</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<style>
+.box {
+  width: 100px;
+  height: 100px;
+  background-color: green;
+  display: inline-block;
+}
+
+#control {
+  background-color: red;
+  transform: translateY(100px);
+}
+</style>
+
+<div>
+<div class="box" id="target1"></div>
+<div class="box" id="target2"></div>
+<div class="box" id="target3"></div>
+<div class="box" id="target4"></div>
+<div class="box" id="control"></div>
+</div>
+
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-null-2.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-null-2.https.html
new file mode 100644
index 0000000000..9c499bac0e
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-null-2.https.html
@@ -0,0 +1,110 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<title>Setting localTime to null means effect does not apply (reftest)</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<link rel="match" href="worklet-animation-local-time-null-2-ref.html">
+<meta name="timeout" content="long">
+
+<script src="/common/reftest-wait.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<style>
+.box {
+  width: 100px;
+  height: 100px;
+  background-color: green;
+  display: inline-block;
+}
+</style>
+
+<div>
+<div class="box" id="target1"></div>
+<div class="box" id="target2"></div>
+<div class="box" id="target3"></div>
+<div class="box" id="target4"></div>
+<div class="box" id="control"></div>
+</div>
+
+
+<script>
+runInAnimationWorklet(`
+  registerAnimator("blank_animator", class {
+    animate(currentTime, effect) {
+      // Unset effect.localTime is equivalent to 'null'
+    }
+  });
+
+  registerAnimator("null_animator", class {
+    animate(currentTime, effect) {
+      effect.localTime = null;
+    }
+  });
+
+  registerAnimator("drop_animator", class {
+    animate(currentTime, effect) {
+      if (currentTime < 500)
+        effect.localTime = 500;
+      else if (currentTime < 1000)
+        effect.localTime = 0;
+      else
+        effect.localTime = null;
+    }
+  });
+
+  registerAnimator("add_animator", class {
+    animate(currentTime, effect) {
+      if (currentTime < 1000)
+        effect.localTime = 500;
+      else
+        effect.localTime = 0;
+    }
+  });
+`).then(() => {
+
+  const start_animation = (animator, targetId, keyframes) => {
+    const animation = new WorkletAnimation(animator,
+      new KeyframeEffect(
+        document.getElementById(targetId),
+        keyframes,
+        {duration: 1000}
+      )
+    );
+    animation.play();
+    return animation;
+  };
+
+  start_animation('blank_animator','target1', [
+    { transform: 'translateY(100px)' },
+    { transform: 'translateY(200px)' }
+  ]);
+
+  start_animation('null_animator','target2', [
+    { transform: 'translateY(100px)' },
+    { transform: 'translateY(200px)' }
+  ]);
+
+  start_animation('drop_animator','target3', [
+    { transform: 'translateY(100px)' },
+    { transform: 'translateY(200px)' }
+  ]);
+
+  start_animation('drop_animator','target4', [
+    { backgroundColor: 'red' },
+    { backgroundColor: 'blue' }
+  ]);
+
+  // check that animation worklets are running to stop accidental pass
+  const control_anim = start_animation('add_animator','control', [
+    { backgroundColor: 'red', transform: 'translateY(100px)' },
+    { backgroundColor: 'blue', transform: 'translateY(200px)' }
+  ]);
+
+  waitForAnimationFrameWithCondition(() => control_anim.currentTime > 1000)
+    // long timeout due to laggy compositor thread on debug build.
+    .then(() => waitForAsyncAnimationFrames(120))
+    .then(takeScreenshot);
+});
+
+
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-pause-immediately.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-pause-immediately.https.html
new file mode 100644
index 0000000000..f9dcf30bc9
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-pause-immediately.https.html
@@ -0,0 +1,37 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<title>Verify that calling pause immediately after playing works as expected</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<link rel="match" href="references/translated-box-ref.html">
+
+<script src="/common/reftest-wait.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+<style>
+#box {
+  width: 100px;
+  height: 100px;
+  background-color: green;
+}
+</style>
+
+<div id="box"></div>
+
+<script>
+registerPassthroughAnimator().then(async _ => {
+  const box = document.getElementById('box');
+  const effect = new KeyframeEffect(box,
+    { transform: ['translateY(100px)', 'translateY(200px)'] },
+    { duration: 100, iterations: 1 }
+  );
+
+  const animation = new WorkletAnimation('passthrough', effect);
+  animation.play();
+  // Immediately pausing animation should freeze the current time at 0.
+  animation.pause();
+  // Wait at least one frame to ensure a paused animation actually freezes.
+  await waitForAsyncAnimationFrames(1);
+  takeScreenshot();
+});
+</script>
+</html>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-pause-resume.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-pause-resume.https.html
new file mode 100644
index 0000000000..f26a93468c
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-pause-resume.https.html
@@ -0,0 +1,40 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<title>Verify that calling pause immediately after playing works as expected</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<link rel="match" href="references/translated-box-ref.html">
+
+<script src="/common/reftest-wait.js"></script>
+<script src="common.js"></script>
+<style>
+#box {
+  width: 100px;
+  height: 100px;
+  background-color: green;
+}
+</style>
+
+<div id="box"></div>
+
+<script>
+registerPassthroughAnimator().then(async _  => {
+  const duration = 18; // a bit longer than a frame
+  const box = document.getElementById('box');
+  const effect = new KeyframeEffect(box,
+    { transform: ['translateY(0px)', 'translateY(100px)'] },
+    { duration: duration, iterations: 1, fill: 'forwards'}
+  );
+
+  const animation = new WorkletAnimation('passthrough', effect);
+  // Immediately pausing animation should freeze the current time at 0.
+  animation.pause();
+  // Playing should cause animation to resume.
+  animation.play();
+  // Wait until we ensure animation has reached completion.
+  await waitForAnimationFrameWithCondition( _ => {
+    return animation.currentTime >= duration;
+  });
+  takeScreenshot();
+});
+</script>
+</html>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-pause.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-pause.https.html
new file mode 100644
index 0000000000..417db9e37a
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-pause.https.html
@@ -0,0 +1,60 @@
+<!DOCTYPE html>
+<title>Verify that currentTime and playState are correct when animation is paused</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<div id="box"></div>
+
+<script>
+
+setup(setupAndRegisterTests, {explicit_done: true});
+
+function createAnimation() {
+  const box = document.getElementById('box');
+  const effect = new KeyframeEffect(box,
+    { transform: ['translateY(100px)', 'translateY(200px)'] },
+    { duration: 100, iterations: 1 }
+  );
+
+  return new WorkletAnimation('passthrough', effect);
+}
+
+async function setupAndRegisterTests() {
+  await registerPassthroughAnimator();
+
+  promise_test(async t => {
+    const animation = createAnimation();
+    animation.play();
+    // Immediately pausing animation should freeze the current time at 0.
+    animation.pause();
+    assert_equals(animation.currentTime, 0);
+    assert_equals(animation.playState, "paused");
+    // Wait some time to ensure a paused animation actually freezes.
+   await waitForNextFrame();
+    assert_equals(animation.currentTime, 0);
+    assert_equals(animation.playState, "paused");
+  }, 'pausing an animation freezes its current time');
+
+  promise_test(async t => {
+    const animation = createAnimation();
+    animation.pause();
+    animation.play();
+    // Allow one async animation frame to pass so that animation is running.
+    await waitForAsyncAnimationFrames(1);
+    assert_equals(animation.playState, "running");
+    // Allow time to advance so that we have a non-zero current time.
+    await waitForDocumentTimelineAdvance();
+    const timelineTime = document.timeline.currentTime;
+    assert_greater_than(animation.currentTime, 0);
+    assert_times_equal(animation.currentTime, (timelineTime - animation.startTime));
+  }, 'playing a paused animation should resume it');
+
+  done();
+}
+
+</script>
+
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-play.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-play.https.html
new file mode 100644
index 0000000000..038cd74aab
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-play.https.html
@@ -0,0 +1,45 @@
+<!DOCTYPE html>
+<title>Basic use of Worklet Animation</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<div id="target"></div>
+
+<script>
+  promise_test(async t => {
+    await registerConstantLocalTimeAnimator(500);
+    const effect = new KeyframeEffect(target, [{ opacity: 0 }], { duration: 1000 });
+    const animation = new WorkletAnimation('constant_time', effect);
+    animation.play();
+
+    // wait until local times are synced back to the main thread.
+    await waitForAnimationFrameWithCondition(_ => {
+      return getComputedStyle(target).opacity != '1';
+    });
+    assert_equals(getComputedStyle(target).opacity, "0.5");
+
+    animation.cancel();
+  }, "A running worklet animation should output values at specified local time.");
+
+  promise_test(async t => {
+    await registerConstantLocalTimeAnimator(500);
+    const effect = new KeyframeEffect(target, [{ opacity: 0 }], { duration: 1000 });
+    const animation = new WorkletAnimation('constant_time', effect);
+    animation.play();
+
+    await waitForAnimationFrameWithCondition(_=> {
+        return animation.playState == "running"
+    });
+
+    const prevCurrentTime = animation.currentTime;
+    animation.play();
+    assert_equals(animation.playState, "running");
+    assert_equals(animation.currentTime, prevCurrentTime)
+
+    animation.cancel();
+  }, "Playing a running animation should be a no-op.");
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-set-keyframes.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-set-keyframes.https.html
new file mode 100644
index 0000000000..d3d02898db
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-set-keyframes.https.html
@@ -0,0 +1,44 @@
+<html class="reftest-wait">
+<title>Worklet Animation sets keyframes</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<meta name="assert" content="Can update the keyframes for an effect while the animation is running">
+<link rel="match" href="references/translated-box-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+<script src="common.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+</style>
+
+<div id="box"></div>
+
+<script>
+  registerConstantLocalTimeAnimator(500).then(()=>{
+    const keyframes_before = [
+      { transform: 'translateX(0)' },
+      { transform: 'translateX(200px)' }
+    ];
+    const keyframes_after = [
+      { transform: 'translateY(0)' },
+      { transform: 'translateY(200px)' }
+    ];
+
+    const box = document.getElementById('box');
+    const effect = new KeyframeEffect(box, keyframes_before, {duration: 1000});
+    const animation = new WorkletAnimation('constant_time', effect);
+    animation.play();
+
+    waitForAsyncAnimationFrames(1).then(_ => {
+      effect.setKeyframes(keyframes_after);
+      waitForAsyncAnimationFrames(1).then(_ => {
+        takeScreenshot();
+      });
+    });
+  });
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-set-timing.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-set-timing.https.html
new file mode 100644
index 0000000000..6c5cd51300
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-set-timing.https.html
@@ -0,0 +1,46 @@
+<html class="reftest-wait">
+<title>Worklet Animation sets timing</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<meta name="assert" content="Can update the timing for an effect while the animation is running">
+<link rel="match" href="references/translated-box-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+<script src="common.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+</style>
+
+<div id="box"></div>
+
+<script>
+  registerConstantLocalTimeAnimator(500).then(()=>{
+    const keyframes = [
+      { transform: 'translateY(0)' },
+      { transform: 'translateY(400px)' }
+    ];
+    const options_before = {
+      duration: 1000
+    };
+    const options_after = {
+      duration: 2000
+    };
+
+    const box = document.getElementById('box');
+    const effect = new KeyframeEffect(box, keyframes, options_before);
+    const animation = new WorkletAnimation('constant_time', effect);
+    animation.play();
+
+    waitForAsyncAnimationFrames(1).then(_ => {
+      effect.updateTiming(options_after);
+      waitForAsyncAnimationFrames(1).then(_ => {
+        takeScreenshot();
+      });
+    });
+  });
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-start-delay-ref.html b/testing/web-platform/tests/animation-worklet/worklet-animation-start-delay-ref.html
new file mode 100644
index 0000000000..efef6f842b
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-start-delay-ref.html
@@ -0,0 +1,12 @@
+<!DOCTYPE html>
+<title>Reference for WorkletAnimation should respect delay given in options</title>
+<style>
+.box {
+  width: 100px;
+  height: 100px;
+  background-color: green;
+}
+</style>
+
+<div class="box"></div>
+<div style="transform: translateX(100px);" class="box"></div>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-start-delay.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-start-delay.https.html
new file mode 100644
index 0000000000..c8683f7dac
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-start-delay.https.html
@@ -0,0 +1,64 @@
+<html class="reftest-wait">
+<title>WorkletAnimation should respect delay given in options</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<meta name="assert" content="Worklet Animation should respect delay given in options">
+<link rel="match" href="worklet-animation-start-delay-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+<script src="common.js"></script>
+
+<style>
+  .box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+</style>
+
+<div id="t0" class="box"></div>
+<div id="t1" class="box"></div>
+<div id="out"></div>
+<script id="visual_update"  type="text/worklet">
+  registerAnimator("t0_animator", class {
+    animate(currentTime, effect) {
+      effect.localTime = 500;
+    }
+  });
+
+  registerAnimator("t1_animator", class {
+    animate(currentTime, effect) {
+      effect.localTime = 5500;
+    }
+  });
+</script>
+
+<script>
+  runInAnimationWorklet(
+    document.getElementById('visual_update').textContent
+  ).then(()=>{
+    const keyframes = [
+      {transform: 'translateX(0)' },
+      {transform: 'translateX(200px)' }
+    ];
+    const options = {
+      duration: 1000,
+      delay: 5000,
+    };
+
+    const $t0 = document.getElementById('t0');
+    const $t0_effect = new KeyframeEffect($t0, keyframes, options);
+    const $t0_animation = new WorkletAnimation('t0_animator', $t0_effect);
+
+    const $t1 = document.getElementById('t1');
+    const $t1_effect = new KeyframeEffect($t1, keyframes, options);
+    const $t1_animation = new WorkletAnimation('t1_animator', $t1_effect);
+
+    $t0_animation.play();
+    $t1_animation.play();
+
+    waitForAsyncAnimationFrames(1).then(_ => {
+      takeScreenshot();
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-with-effects-from-different-frames.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-with-effects-from-different-frames.https.html
new file mode 100644
index 0000000000..152b13839c
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-with-effects-from-different-frames.https.html
@@ -0,0 +1,48 @@
+<!DOCTYPE html>
+<title>Worklet animation can animate effects from different frames</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="common.js"></script>
+
+<div id="box"></div>
+
+<script id="simple_animate" type="text/worklet">
+  registerAnimator("test_animator", class {
+    animate(currentTime, effect) {
+      let effects = effect.getChildren();
+      effects[0].localTime = 500;
+      effects[1].localTime = 750;
+    }
+  });
+</script>
+
+<script>
+  promise_test(async t => {
+    await runInAnimationWorklet(document.getElementById('simple_animate').textContent);
+    const effect = new KeyframeEffect(box, [{ opacity: 0 }], { duration: 1000 });
+
+    let iframe = document.createElement('iframe');
+    iframe.src = 'resources/iframe.html';
+    document.body.appendChild(iframe);
+
+    await waitForAnimationFrameWithCondition(_ => {
+      return iframe.contentDocument.getElementById('iframe_box') != null;
+    });
+    let iframe_box = iframe.contentDocument.getElementById('iframe_box');
+    let iframe_effect = new KeyframeEffect(
+        iframe_box, [{ opacity: 0 }], { duration: 1000 }
+    );
+
+    const animation = new WorkletAnimation('test_animator', [effect, iframe_effect]);
+    animation.play();
+
+    await waitForNotNullLocalTime(animation);
+    assert_equals(getComputedStyle(box).opacity, '0.5');
+    assert_equals(getComputedStyle(iframe_box).opacity, '0.25');
+
+    iframe.remove();
+    animation.cancel();
+  }, "Effects from different documents can be animated within one worklet animation");
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-with-fill-mode.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-with-fill-mode.https.html
new file mode 100644
index 0000000000..725d10de43
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-with-fill-mode.https.html
@@ -0,0 +1,147 @@
+<!DOCTYPE html>
+<title>Test that worklet animation works with different fill modes</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<style>
+.target {
+  width: 100px;
+  height: 100px;
+  background-color: green;
+}
+</style>
+
+<div id="target" class='target'></div>
+
+<script>
+setup(setupAndRegisterTests, {explicit_done: true});
+
+function setupAndRegisterTests() {
+  registerConstantLocalTimeAnimator(2000).then(() => {
+    promise_test(
+        effect_with_fill_mode_forwards,
+        "Effect with fill mode forwards in after phase produces output that is equivalent to effect's end value.");
+
+    promise_test(
+        effect_without_fill_mode_forwards,
+        'Effect without fill mode forwards in after phase (local time beyond end) should deactivate the animation.');
+
+    promise_test(
+        effect_without_fill_forwards_at_end,
+        'Effect without fill mode in after phase (local time at end) should deactivate the animation.');
+
+    promise_test(
+        effect_with_fill_backwards,
+        "Effect with fill mode backwards in before phase produces output that is equivalent to effect's start value.");
+
+    promise_test(
+        effect_without_fill_backwards,
+        'Effect without fill mode backwards in before phase (local time before start) should deactivate the animation.');
+
+    promise_test(
+        effect_without_fill_backwards_at_start,
+        'Effect with local time at start point is in active phase.');
+
+    done();
+  });
+}
+
+async function effect_with_fill_mode_forwards(t) {
+  const effect_with_fill_forwards = new KeyframeEffect(
+      target,
+      { opacity: [0.5, 0] },
+      { duration: 1000, fill: 'forwards' });
+  const animation = new WorkletAnimation(
+      'constant_time',
+      effect_with_fill_forwards);
+  animation.play();
+  await waitForNotNullLocalTime(animation);
+
+  assert_equals(getComputedStyle(target).opacity, '0');
+
+  animation.cancel();
+}
+
+async function effect_without_fill_mode_forwards(t) {
+  const effect_without_fill_forwards = new KeyframeEffect(
+      target,
+      { opacity: [0.5, 0] },
+      { duration: 1000 });
+  const animation = new WorkletAnimation(
+      'constant_time',
+      effect_without_fill_forwards);
+  animation.play();
+  await waitForNotNullLocalTime(animation);
+
+  assert_equals(getComputedStyle(target).opacity, '1');
+
+  animation.cancel();
+}
+
+async function effect_without_fill_forwards_at_end(t) {
+  const effect_without_fill_forwards_at_end = new KeyframeEffect(
+      target,
+      { opacity: [0.5, 0] },
+      { duration: 2000 });
+  const animation = new WorkletAnimation(
+      'constant_time',
+      effect_without_fill_forwards_at_end);
+  animation.play();
+  await waitForNotNullLocalTime(animation);
+
+  assert_equals(getComputedStyle(target).opacity, '1');
+
+  animation.cancel();
+}
+
+async function effect_with_fill_backwards(t) {
+  const effect_with_fill_backwards = new KeyframeEffect(
+      target,
+      { opacity: [0.5, 0] },
+      { duration: 1000, delay: 2001, fill: 'backwards' });
+  const animation = new WorkletAnimation(
+      'constant_time',
+      effect_with_fill_backwards);
+  animation.play();
+  await waitForNotNullLocalTime(animation);
+
+  assert_equals(getComputedStyle(target).opacity, '0.5');
+
+  animation.cancel();
+}
+
+async function effect_without_fill_backwards(t) {
+  const effect_without_fill_backwards = new KeyframeEffect(
+      target,
+      { opacity: [0.5, 0] },
+      { duration: 1000, delay: 2001 });
+  const animation = new WorkletAnimation(
+      'constant_time',
+      effect_without_fill_backwards);
+  animation.play();
+  waitForNotNullLocalTime(animation);
+
+  assert_equals(getComputedStyle(target).opacity, '1');
+
+  animation.cancel();
+}
+
+async function effect_without_fill_backwards_at_start(t) {
+  const effect_without_fill_backwards_at_start = new KeyframeEffect(
+      target,
+      { opacity: [0.5, 0] },
+      { duration: 1000, delay: 2000 });
+  const animation = new WorkletAnimation(
+      'constant_time',
+      effect_without_fill_backwards_at_start);
+  animation.play();
+  await waitForNotNullLocalTime(animation);
+
+  assert_equals(getComputedStyle(target).opacity, '0.5');
+
+  animation.cancel();
+}
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-with-invalid-effect.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-with-invalid-effect.https.html
new file mode 100644
index 0000000000..75261d251c
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-with-invalid-effect.https.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<title>Test that worklet animation with invalid effect cannot be played</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<style>
+#target {
+  width: 100px;
+  height: 100px;
+}
+</style>
+
+<div id="target"></div>
+
+<script>
+'use strict';
+
+promise_test(async function() {
+  await registerPassthroughAnimator();
+  let playFunc = function() {
+    let effect = new KeyframeEffect(
+        document.getElementById('target'),
+        [
+          // No keyframe.
+        ],
+        { duration: 1000 }
+    );
+    let animation = new WorkletAnimation('passthrough', effect);
+    animation.play();
+  }
+  assert_throws_dom('InvalidStateError', playFunc);
+}, 'Trying to play invalid worklet animation should throw an exception.');
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-with-non-ascii-name-ref.html b/testing/web-platform/tests/animation-worklet/worklet-animation-with-non-ascii-name-ref.html
new file mode 100644
index 0000000000..012f6f9d51
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-with-non-ascii-name-ref.html
@@ -0,0 +1,12 @@
+<!DOCTYPE html>
+<title>Reference for WorkletAnimation name should accept non-ASCII characters</title>
+<style>
+.box {
+  width: 100px;
+  height: 100px;
+  background-color: green;
+}
+</style>
+
+<div style="transform: translateX(50px);" class="box"></div>
+<div style="transform: translateX(150px);" class="box"></div>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-with-non-ascii-name.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-with-non-ascii-name.https.html
new file mode 100644
index 0000000000..d3a3f4ad35
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-with-non-ascii-name.https.html
@@ -0,0 +1,59 @@
+<html class="reftest-wait">
+<title>WorkletAnimation name should accept non-ASCII characters</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<meta name="assert" content="Worklet Animation name should accept non-ASCII characters">
+<link rel="match" href="worklet-animation-with-non-ascii-name-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+<script src="common.js"></script>
+
+<style>
+  .box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+</style>
+
+<div id="t0" class="box"></div>
+<div id="t1" class="box"></div>
+<script id="visual_update"  type="text/worklet">
+  registerAnimator('bob', class {
+    animate(currentTime, effect) {
+      effect.localTime = 250;
+    }
+  });
+  registerAnimator('東京', class {
+    animate(currentTime, effect) {
+      effect.localTime = 750;
+    }
+  });
+</script>
+<script>
+  runInAnimationWorklet(
+    document.getElementById('visual_update').textContent
+  ).then(() => {
+    const keyframes = [
+      {transform: 'translateX(0)' },
+      {transform: 'translateX(200px)' }
+    ];
+    const options = {
+      duration: 1000
+    };
+    const $t0 = document.getElementById('t0');
+    const $t0_effect = new KeyframeEffect($t0, keyframes, options);
+    const $t0_animation = new WorkletAnimation('bob', $t0_effect);
+
+    const $t1 = document.getElementById('t1');
+    const $t1_effect = new KeyframeEffect($t1, keyframes, options);
+    const $t1_animation = new WorkletAnimation('東京', $t1_effect);
+
+    $t0_animation.play();
+    $t1_animation.play();
+
+    waitForAsyncAnimationFrames(1).then(_ => {
+      takeScreenshot();
+    });
+  });
+</script>
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-and-display-none.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-and-display-none.https.html
new file mode 100644
index 0000000000..0bba0039da
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-and-display-none.https.html
@@ -0,0 +1,84 @@
+<html class="reftest-wait">
+<title>Scroll timeline with WorkletAnimation and transition from display:none to display:block</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<meta name="assert" content="Scroll timeline should properly handle going from display:none to display:block">
+<link rel="match" href="worklet-animation-with-scroll-timeline-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+<script src="common.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  /* Hide scrollbars to avoid unnecessary visual issues related to them */
+  #scroller::-webkit-scrollbar {
+    display: none;
+  }
+
+  #scroller {
+    scrollbar-width: none;
+    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"></div>
+</div>
+
+<script>
+  registerPassthroughAnimator().then(()=>{
+    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({ scrollSource: scroller, orientation: 'block' });
+    const animation = new WorkletAnimation('passthrough', effect, timeline);
+    animation.play();
+
+    // Ensure that the WorkletAnimation will have been started on the compositor.
+    waitForAsyncAnimationFrames(1).then(_ => {
+      // Now return the scroller to the world, which will cause it to be composited
+      // and the animation should update on the compositor side.
+      scroller.classList.remove('removed');
+      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+      scroller.scrollTop = 0.5 * maxScroll;
+
+      waitForAsyncAnimationFrames(1).then(_ => {
+        takeScreenshot();
+      });
+    });
+  });
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-and-overflow-hidden-ref.html b/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-and-overflow-hidden-ref.html
new file mode 100644
index 0000000000..c6d7314e39
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-and-overflow-hidden-ref.html
@@ -0,0 +1,45 @@
+<!DOCTYPE html>
+<title>Scroll timeline with WorkletAnimation 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>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-and-overflow-hidden.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-and-overflow-hidden.https.html
new file mode 100644
index 0000000000..c2332bd6ce
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-and-overflow-hidden.https.html
@@ -0,0 +1,68 @@
+<html class="reftest-wait">
+<title>Scroll timeline with WorkletAnimation using a scroller with overflow hidden</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<meta name="assert" content="Worklet animation correctly updates values when using a overflow: hidden on the scroller being used as the source for the ScrollTimeline">
+<link rel="match" href="worklet-animation-with-scroll-timeline-and-overflow-hidden-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+<script src="common.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>
+  registerPassthroughAnimator().then(_ => {
+    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({ scrollSource: scroller, orientation: 'block' });
+    const animation = new WorkletAnimation('passthrough', effect, timeline);
+    animation.play();
+
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+    waitForAnimationFrameWithCondition(_ => {
+      return getComputedStyle(box).transform != 'matrix(1, 0, 0, 1, 0, 0)';
+    }).then(_ => {
+      takeScreenshot();
+    });
+  });
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-ref.html b/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-ref.html
new file mode 100644
index 0000000000..1316d69a42
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-ref.html
@@ -0,0 +1,51 @@
+<!DOCTYPE html>
+<title>Reference for Animation Worklet 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;
+  }
+
+  /* Hide scrollbars to avoid unnecessary visual issues related to them */
+  #scroller::-webkit-scrollbar {
+    display: none;
+  }
+
+  #scroller {
+    scrollbar-width: none;
+    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>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-root-scroller-ref.html b/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-root-scroller-ref.html
new file mode 100644
index 0000000000..917b044841
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-root-scroller-ref.html
@@ -0,0 +1,43 @@
+<!DOCTYPE html>
+<title>Reference for Scroll timeline with WorkletAnimation using the root scroller</title>
+<style>
+  /* Hide scrollbars to avoid unnecessary visual issues related to them */
+  html::-webkit-scrollbar {
+    display: none;
+  }
+
+  html {
+    scrollbar-width: none;
+    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"></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>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-root-scroller.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-root-scroller.https.html
new file mode 100644
index 0000000000..60560a938a
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline-root-scroller.https.html
@@ -0,0 +1,69 @@
+<html class="reftest-wait">
+<title>Scroll timeline with WorkletAnimation using the root scroller</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<meta name="assert"
+  content="Worklet animation correctly updates values when using the root scroller as the source for the ScrollTimeline">
+<link rel="match" href="worklet-animation-with-scroll-timeline-root-scroller-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+<script src="common.js"></script>
+
+<style>
+  /* Hide scrollbars to avoid unnecessary visual issues related to them */
+  html::-webkit-scrollbar {
+    display: none;
+  }
+
+  html {
+    scrollbar-width: none;
+    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"></div>
+
+<script>
+  registerPassthroughAnimator().then(() => {
+    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({ scrollSource: scroller, orientation: 'block' });
+    const animation = new WorkletAnimation('passthrough', effect, timeline);
+    animation.play();
+
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+
+    waitForAnimationFrameWithCondition(_ => {
+      return getComputedStyle(box).transform != 'matrix(1, 0, 0, 1, 0, 0)';
+    }).then(_ => {
+      takeScreenshot();
+    });
+  });
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline.https.html
new file mode 100644
index 0000000000..99bd171d92
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-with-scroll-timeline.https.html
@@ -0,0 +1,74 @@
+<html class="reftest-wait">
+<title>Basic use of scroll timeline with WorkletAnimation</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+<meta name="assert" content="Should be able to use the scroll timeline to drive the worklet animation timing">
+<link rel="match" href="worklet-animation-with-scroll-timeline-ref.html">
+
+<script src="/web-animations/testcommon.js"></script>
+<script src="/common/reftest-wait.js"></script>
+<script src="common.js"></script>
+
+<style>
+  #box {
+    width: 100px;
+    height: 100px;
+    background-color: green;
+  }
+
+  #covered {
+    width: 100px;
+    height: 100px;
+    background-color: red;
+  }
+
+  /* Hide scrollbars to avoid unnecessary visual issues related to them */
+  #scroller::-webkit-scrollbar {
+    display: none;
+  }
+
+  #scroller {
+    scrollbar-width: none;
+    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>
+  registerPassthroughAnimator().then(() => {
+    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({ scrollSource: scroller, orientation: 'block' });
+    const animation = new WorkletAnimation('passthrough', effect, timeline);
+    animation.play();
+
+    // Move the scroller to the halfway point.
+    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
+    scroller.scrollTop = 0.5 * maxScroll;
+
+    waitForAsyncAnimationFrames(1).then(_ => {
+      takeScreenshot();
+    });
+  });
+</script>
\ No newline at end of file
diff --git a/testing/web-platform/tests/animation-worklet/worklet-animation-without-target.https.html b/testing/web-platform/tests/animation-worklet/worklet-animation-without-target.https.html
new file mode 100644
index 0000000000..bfb6b4faee
--- /dev/null
+++ b/testing/web-platform/tests/animation-worklet/worklet-animation-without-target.https.html
@@ -0,0 +1,76 @@
+<!DOCTYPE html>
+<title>Verify that effect without target is supported</title>
+<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="common.js"></script>
+
+<div id="box"></div>
+
+<script>
+
+setup(setupAndRegisterTests, {explicit_done: true});
+
+async function setupAndRegisterTests() {
+  await registerPassthroughAnimator();
+
+  promise_test(async t => {
+    const effect = new KeyframeEffect(null,
+      { transform: ['translateY(100px)', 'translateY(200px)'] },
+      { duration: Infinity, iterations: 1 }
+    );
+    const animation = new WorkletAnimation('passthrough', effect);
+    animation.play();
+
+    // Allow one async animation frame to pass so that animation is running.
+    await waitForAsyncAnimationFrames(1);
+    assert_equals(animation.playState, "running");
+    // Allow time to advance so that we have a non-zero current time.
+    await waitForDocumentTimelineAdvance();
+    const t0 = document.timeline.currentTime;
+    assert_greater_than(animation.currentTime, 0);
+    assert_times_equal(animation.currentTime, (t0 - animation.startTime));
+    assert_equals(animation.playState, "running");
+
+    animation.cancel();
+  }, 'Animating effect with no target should work.');
+
+  promise_test(async t => {
+    const effect = new KeyframeEffect(document.getElementById('box'),
+      { transform: ['translateY(100px)', 'translateY(200px)'] },
+      { duration: Infinity, iterations: 1 }
+    );
+
+    const animation = new WorkletAnimation('passthrough', effect);
+    animation.play();
+
+    // Allow one async animation frame to pass so that animation is running.
+    await waitForAsyncAnimationFrames(1);
+    assert_equals(animation.playState, "running");
+    // Allow time to advance so that we have a non-zero current time.
+    await waitForDocumentTimelineAdvance();
+    const t0 = document.timeline.currentTime;
+    assert_greater_than(animation.currentTime, 0);
+    assert_times_equal(animation.currentTime, (t0 - animation.startTime));
+    assert_equals(animation.playState, "running");
+
+    await waitForDocumentTimelineAdvance();
+    animation.effect.target = null;
+    const t1 = document.timeline.currentTime;
+    assert_times_equal(animation.currentTime, (t1 - animation.startTime));
+    assert_equals(animation.playState, "running");
+
+    await waitForDocumentTimelineAdvance();
+    animation.effect.target = document.getElementById('box');
+    const t2 = document.timeline.currentTime;
+    assert_times_equal(animation.currentTime, (t2 - animation.startTime));
+    assert_equals(animation.playState, "running");
+
+    animation.cancel();
+  }, 'The existence of a target does not affect the animation.');
+
+  done();
+}
+</script>
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