From 0ebf5bdf043a27fd3dfb7f92e0cb63d88954c44d Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Fri, 19 Apr 2024 03:47:29 +0200
Subject: Adding upstream version 115.8.0esr.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 .../animation-effects/active-time.html             |  141 +++
 .../animation-effects/current-iteration.html       |  620 ++++++++++++
 .../timing-model/animation-effects/local-time.html |   29 +
 .../animation-effects/phases-and-states.html       |  149 +++
 .../simple-iteration-progress.html                 |  600 ++++++++++++
 .../animations/canceling-an-animation.html         |  127 +++
 .../document-timeline-animation-ref.html           |   19 +
 .../animations/document-timeline-animation.html    |   63 ++
 .../animations/finishing-an-animation.html         |  330 +++++++
 .../infinite-duration-animation-ref.html           |   19 +
 .../animations/infinite-duration-animation.html    |   64 ++
 .../animations/pausing-an-animation.html           |  119 +++
 .../timing-model/animations/play-states.html       |  185 ++++
 .../animations/playing-an-animation.html           |  177 ++++
 .../animations/reverse-running-animation-ref.html  |   17 +
 .../animations/reverse-running-animation.html      |   50 +
 .../animations/reversing-an-animation.html         |  266 +++++
 ...updating-the-playback-rate-of-an-animation.html |  171 ++++
 .../setting-the-current-time-of-an-animation.html  |  167 ++++
 .../setting-the-playback-rate-of-an-animation.html |  110 +++
 .../setting-the-start-time-of-an-animation.html    |  329 +++++++
 .../setting-the-target-effect-of-an-animation.html |  129 +++
 .../setting-the-timeline-of-an-animation.html      |  255 +++++
 .../animations/sync-start-times-ref.html           |   20 +
 .../timing-model/animations/sync-start-times.html  |   72 ++
 .../the-current-time-of-an-animation.html          |   75 ++
 .../animations/update-playback-rate-fast-ref.html  |   17 +
 .../animations/update-playback-rate-fast.html      |   52 +
 .../animations/update-playback-rate-zero-ref.html  |   16 +
 .../animations/update-playback-rate-zero.html      |   46 +
 .../animations/updating-the-finished-state.html    |  457 +++++++++
 .../time-transformations/transformed-progress.html |  391 ++++++++
 .../timing-model/timelines/document-timelines.html |   50 +
 .../timelines/resources/target-frame.html          |   76 ++
 .../timelines/resources/timeline-frame.html        |   20 +
 .../timelines/sibling-iframe-timeline.html         |   58 ++
 .../timing-model/timelines/timelines.html          |  112 +++
 .../update-and-send-events-replacement.html        | 1017 ++++++++++++++++++++
 .../timelines/update-and-send-events.html          |  257 +++++
 39 files changed, 6872 insertions(+)
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animation-effects/active-time.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animation-effects/current-iteration.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animation-effects/local-time.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animation-effects/phases-and-states.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animation-effects/simple-iteration-progress.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/canceling-an-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/document-timeline-animation-ref.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/document-timeline-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/finishing-an-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/infinite-duration-animation-ref.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/infinite-duration-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/pausing-an-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/play-states.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/playing-an-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/reverse-running-animation-ref.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/reverse-running-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/reversing-an-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/seamlessly-updating-the-playback-rate-of-an-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/setting-the-current-time-of-an-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/setting-the-playback-rate-of-an-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/setting-the-start-time-of-an-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/setting-the-target-effect-of-an-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/setting-the-timeline-of-an-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/sync-start-times-ref.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/sync-start-times.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/the-current-time-of-an-animation.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-fast-ref.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-fast.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-zero-ref.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-zero.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/animations/updating-the-finished-state.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/time-transformations/transformed-progress.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/timelines/document-timelines.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/timelines/resources/target-frame.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/timelines/resources/timeline-frame.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/timelines/sibling-iframe-timeline.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/timelines/timelines.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/timelines/update-and-send-events-replacement.html
 create mode 100644 testing/web-platform/tests/web-animations/timing-model/timelines/update-and-send-events.html

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

diff --git a/testing/web-platform/tests/web-animations/timing-model/animation-effects/active-time.html b/testing/web-platform/tests/web-animations/timing-model/animation-effects/active-time.html
new file mode 100644
index 0000000000..a2feb2323c
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animation-effects/active-time.html
@@ -0,0 +1,141 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Active time</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#calculating-the-active-time">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+test(t => {
+  const tests = [ { fill: 'none', progress: null },
+                  { fill: 'backwards', progress: 0 },
+                  { fill: 'forwards', progress: null },
+                  { fill: 'both', progress: 0 } ];
+  for (const test of tests) {
+    const anim = createDiv(t).animate(null, { delay: 1, fill: test.fill });
+    assert_equals(anim.effect.getComputedTiming().progress, test.progress,
+                  `Progress in before phase when using '${test.fill}' fill`);
+  }
+}, 'Active time in before phase');
+
+test(t => {
+  const anim = createDiv(t).animate(null, 1000);
+  anim.currentTime = 500;
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.5);
+}, 'Active time in active phase and no start delay is the local time');
+
+test(t => {
+  const anim = createDiv(t).animate(null, { duration: 1000, delay: 500 });
+  anim.currentTime = 1000;
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.5);
+}, 'Active time in active phase and positive start delay is the local time'
+   + ' minus the start delay');
+
+test(t => {
+  const anim = createDiv(t).animate(null, { duration: 1000, delay: -500 });
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.5);
+}, 'Active time in active phase and negative start delay is the local time'
+   + ' minus the start delay');
+
+test(t => {
+  const anim = createDiv(t).animate(null);
+  assert_equals(anim.effect.getComputedTiming().progress, null);
+}, 'Active time in after phase with no fill is unresolved');
+
+test(t => {
+  const anim = createDiv(t).animate(null, { fill: 'backwards' });
+  assert_equals(anim.effect.getComputedTiming().progress, null);
+}, 'Active time in after phase with backwards-only fill is unresolved');
+
+test(t => {
+  const anim = createDiv(t).animate(null, { duration: 1000,
+                                            iterations: 2.3,
+                                            delay: 500, // Should have no effect
+                                            fill: 'forwards' });
+  anim.finish();
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 2);
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.3);
+}, 'Active time in after phase with forwards fill is the active duration');
+
+test(t => {
+  const anim = createDiv(t).animate(null, { duration: 0,
+                                            iterations: Infinity,
+                                            fill: 'forwards' });
+  anim.finish();
+  assert_equals(anim.effect.getComputedTiming().currentIteration, Infinity);
+  assert_equals(anim.effect.getComputedTiming().progress, 1);
+}, 'Active time in after phase with forwards fill, zero-duration, and '
+   + ' infinite iteration count is the active duration');
+
+test(t => {
+  const anim = createDiv(t).animate(null, { duration: 1000,
+                                            iterations: 2.3,
+                                            delay: 500,
+                                            endDelay: 4000,
+                                            fill: 'forwards' });
+  anim.finish();
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 2);
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.3);
+}, 'Active time in after phase with forwards fill and positive end delay'
+   + ' is the active duration');
+
+test(t => {
+  const anim = createDiv(t).animate(null, { duration: 1000,
+                                            iterations: 2.3,
+                                            delay: 500,
+                                            endDelay: -800,
+                                            fill: 'forwards' });
+  anim.finish();
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 1);
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.5);
+}, 'Active time in after phase with forwards fill and negative end delay'
+   + ' is the active duration + end delay');
+
+test(t => {
+  const anim = createDiv(t).animate(null, { duration: 1000,
+                                            iterations: 2.3,
+                                            delay: 500,
+                                            endDelay: -2500,
+                                            fill: 'forwards' });
+  anim.finish();
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 0);
+  assert_equals(anim.effect.getComputedTiming().progress, 0);
+}, 'Active time in after phase with forwards fill and negative end delay'
+   + ' greater in magnitude than the active duration is zero');
+
+test(t => {
+  const anim = createDiv(t).animate(null, { duration: 1000,
+                                            iterations: 2.3,
+                                            delay: 500,
+                                            endDelay: -4000,
+                                            fill: 'forwards' });
+  anim.finish();
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 0);
+  assert_equals(anim.effect.getComputedTiming().progress, 0);
+}, 'Active time in after phase with forwards fill and negative end delay'
+   + ' greater in magnitude than the sum of the active duration and start delay'
+   + ' is zero');
+
+test(t => {
+  const anim = createDiv(t).animate(null, { duration: 1000,
+                                            iterations: 2.3,
+                                            delay: 500,
+                                            fill: 'both' });
+  anim.finish();
+  assert_equals(anim.effect.getComputedTiming().currentIteration, 2);
+  assert_time_equals_literal(anim.effect.getComputedTiming().progress, 0.3);
+}, 'Active time in after phase with \'both\' fill is the active duration');
+
+test(t => {
+  // Create an effect with a non-zero duration so we ensure we're not just
+  // testing the after-phase behavior.
+  const effect = new KeyframeEffect(null, null, 1);
+  assert_equals(effect.getComputedTiming().progress, null);
+}, 'Active time when the local time is unresolved, is unresolved');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animation-effects/current-iteration.html b/testing/web-platform/tests/web-animations/timing-model/animation-effects/current-iteration.html
new file mode 100644
index 0000000000..24464ce05f
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animation-effects/current-iteration.html
@@ -0,0 +1,620 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Current iteration</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#current-iteration">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<script src="../../resources/effect-tests.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+function runTests(tests, description) {
+  for (const currentTest of tests) {
+    let testParams = Object.entries(currentTest.input)
+                           .map(([attr, value]) => `${attr}:${value}`)
+                           .join(' ');
+    if (currentTest.playbackRate !== undefined) {
+      testParams += ` playbackRate:${currentTest.playbackRate}`;
+    }
+
+    test(t => {
+      const div = createDiv(t);
+      const anim = div.animate({}, currentTest.input);
+      if (currentTest.playbackRate !== undefined) {
+        anim.playbackRate = currentTest.playbackRate;
+      }
+
+      assert_computed_timing_for_each_phase(
+        anim,
+        'currentIteration',
+        { before: currentTest.before,
+          activeBoundary: currentTest.active,
+          after: currentTest.after },
+      );
+    }, `${description}: ${testParams}`);
+  }
+}
+
+async_test(t => {
+  const div = createDiv(t);
+  const anim = div.animate({ opacity: [ 0, 1 ] }, { delay: 1 });
+  assert_equals(anim.effect.getComputedTiming().currentIteration, null);
+  anim.finished.then(t.step_func(() => {
+    assert_equals(anim.effect.getComputedTiming().currentIteration, null);
+    t.done();
+  }));
+}, 'Test currentIteration during before and after phase when fill is none');
+
+
+// --------------------------------------------------------------------
+//
+// Zero iteration duration tests
+//
+// --------------------------------------------------------------------
+
+runTests([
+  {
+    input:    { iterations: 0,
+                iterationStart: 0,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 0
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 0,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 0
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 0,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 0
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 2.5,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 2,
+    after: 2
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 2.5,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 2,
+    after: 2
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 2.5,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 2,
+    after: 2
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 3,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 3,
+    after: 3
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 3,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 3,
+    after: 3
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 3,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 3,
+    after: 3
+  }
+], 'Test zero iterations');
+
+
+// --------------------------------------------------------------------
+//
+// Tests where the iteration count is an integer
+//
+// --------------------------------------------------------------------
+
+runTests([
+  {
+    input:    { iterations: 3,
+                iterationStart: 0,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 2
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 0,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0,
+    after: 2
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 0,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 2.5,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 2,
+    after: 5
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 2.5,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 2,
+    active: 2,
+    after: 5
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 2.5,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 2,
+    active: 2
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 3,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 3,
+    after: 5
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 3,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 3,
+    active: 3,
+    after: 5
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 3,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 3,
+    active: 3
+  }
+], 'Test integer iterations');
+
+
+// --------------------------------------------------------------------
+//
+// Tests where the iteration count is a fraction
+//
+// --------------------------------------------------------------------
+
+runTests([
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 0,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 3
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 0,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0,
+    after: 3
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 0,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 2.5,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 2,
+    after: 5
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 2.5,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 2,
+    active: 2,
+    after: 5
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 2.5,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 2,
+    active: 2
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 3,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 3,
+    after: 6
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 3,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 3,
+    active: 3,
+    after: 6
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 3,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 3,
+    active: 3
+  }
+], 'Test fractional iterations');
+
+
+// --------------------------------------------------------------------
+//
+// Tests where the iteration count is Infinity
+//
+// --------------------------------------------------------------------
+
+runTests([
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 0,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: Infinity
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 0,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 0,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 2.5,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 2,
+    after: Infinity
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 2.5,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 2,
+    active: 2
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 2.5,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 2,
+    active: 2
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 3,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 3,
+    after: Infinity
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 3,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 3,
+    active: 3
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 3,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 3,
+    active: 3
+  }
+], 'Test infinity iterations');
+
+
+// --------------------------------------------------------------------
+//
+// End delay tests
+//
+// --------------------------------------------------------------------
+
+runTests([
+  {
+    input:    { duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: 50 },
+    before: 0,
+    active: 0,
+    after: 0
+  },
+
+  {
+    input:    { duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -50 },
+    before: 0,
+    active: 0,
+    after: 0
+  },
+
+  {
+    input:    { duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -100 },
+    before: 0,
+    active: 0,
+    after: 0
+  },
+
+  {
+    input:    { duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -200 },
+    before: 0,
+    active: 0,
+    after: 0
+  },
+
+  {
+    input:    { iterationStart: 0.5,
+                duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: 50 },
+    before: 0,
+    active: 0,
+    after: 1
+  },
+
+  {
+    input:    { iterationStart: 0.5,
+                duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -50 },
+    before: 0,
+    active: 0,
+    after: 1
+  },
+
+  {
+    input:    { iterationStart: 0.5,
+                duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -100 },
+    before: 0,
+    active: 0,
+    after: 0
+  },
+
+  {
+    input:    { iterations: 2,
+                duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -100 },
+    before: 0,
+    active: 0,
+    after: 1
+  },
+
+  {
+    input:    { iterations: 1,
+                iterationStart: 2,
+                duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -50 },
+    before: 2,
+    active: 2,
+    after: 2
+  },
+
+  {
+    input:    { iterations: 1,
+                iterationStart: 2,
+                duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -100 },
+    before: 2,
+    active: 2,
+    after: 2
+  },
+], 'Test end delay');
+
+
+// --------------------------------------------------------------------
+//
+// Negative playback rate tests
+//
+// --------------------------------------------------------------------
+
+runTests([
+  {
+    input:    { duration: 1,
+                delay: 1,
+                fill: 'both' },
+    playbackRate: -1,
+    before: 0,
+    active: 0,
+    after: 0
+  },
+
+  {
+    input:    { duration: 1,
+                delay: 1,
+                iterations: 2,
+                fill: 'both' },
+    playbackRate: -1,
+    before: 0,
+    active: 1,
+    after: 1
+  },
+
+  {
+    input:    { duration: 0,
+                delay: 1,
+                fill: 'both' },
+    playbackRate: -1,
+    before: 0,
+    after: 0
+  },
+
+  {
+    input:    { duration: 0,
+                iterations: 0,
+                delay: 1,
+                fill: 'both' },
+    playbackRate: -1,
+    before: 0,
+    after: 0
+  },
+], 'Test negative playback rate');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animation-effects/local-time.html b/testing/web-platform/tests/web-animations/timing-model/animation-effects/local-time.html
new file mode 100644
index 0000000000..79437d9f54
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animation-effects/local-time.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Local time</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#local-time">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<body>
+<script>
+'use strict';
+
+test(t => {
+  const anim = createDiv(t).animate(null, 10 * MS_PER_SEC);
+  for (const seconds of [-1, 0, 5, 10, 20]) {
+    anim.currentTime = seconds * MS_PER_SEC;
+    assert_equals(
+      anim.effect.getComputedTiming().localTime,
+      seconds * MS_PER_SEC
+    );
+  }
+}, 'Local time is current time for animation effects associated with an animation');
+
+test(t => {
+  const effect = new KeyframeEffect(createDiv(t), null, 10 * MS_PER_SEC);
+  assert_equals(effect.getComputedTiming().localTime, null);
+}, 'Local time is unresolved for animation effects not associated with an animation');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animation-effects/phases-and-states.html b/testing/web-platform/tests/web-animations/timing-model/animation-effects/phases-and-states.html
new file mode 100644
index 0000000000..a33dbf517e
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animation-effects/phases-and-states.html
@@ -0,0 +1,149 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>Phases and states</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#animation-effect-phases-and-states">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+// --------------------------------------------------------------------
+//
+// Phases
+//
+// --------------------------------------------------------------------
+
+test(t => {
+  const animation = createDiv(t).animate(null, 1);
+
+  for (const test of [{ currentTime: -1, phase: 'before' },
+                      { currentTime:  0, phase: 'active' },
+                      { currentTime:  1, phase: 'after'  }]) {
+    assert_phase_at_time(animation, test.phase, test.currentTime);
+  }
+}, 'Phase calculation for a simple animation effect');
+
+test(t => {
+  const animation = createDiv(t).animate(null, { duration: 1, delay: 1 });
+
+  for (const test of [{ currentTime: 0, phase: 'before' },
+                      { currentTime: 1, phase: 'active' },
+                      { currentTime: 2, phase: 'after'  }]) {
+    assert_phase_at_time(animation, test.phase, test.currentTime);
+  }
+}, 'Phase calculation for an animation effect with a positive start delay');
+
+test(t => {
+  const animation = createDiv(t).animate(null, { duration: 1, delay: -1 });
+
+  for (const test of [{ currentTime: -2, phase: 'before' },
+                      { currentTime: -1, phase: 'before' },
+                      { currentTime:  0, phase: 'after'  }]) {
+    assert_phase_at_time(animation, test.phase, test.currentTime);
+  }
+}, 'Phase calculation for an animation effect with a negative start delay');
+
+test(t => {
+  const animation = createDiv(t).animate(null, { duration: 1, endDelay: 1 });
+
+  for (const test of [{ currentTime: -1, phase: 'before' },
+                      { currentTime:  0, phase: 'active' },
+                      { currentTime:  1, phase: 'after'  },
+                      { currentTime:  2, phase: 'after'  }]) {
+    assert_phase_at_time(animation, test.phase, test.currentTime);
+  }
+}, 'Phase calculation for an animation effect with a positive end delay');
+
+test(t => {
+  const animation = createDiv(t).animate(null, { duration: 2, endDelay: -1 });
+
+  for (const test of [{ currentTime: -1,   phase: 'before' },
+                      { currentTime:  0,   phase: 'active' },
+                      { currentTime:  0.9, phase: 'active' },
+                      { currentTime:  1,   phase: 'after'  }]) {
+    assert_phase_at_time(animation, test.phase, test.currentTime);
+  }
+}, 'Phase calculation for an animation effect with a negative end delay lesser'
+   + ' in magnitude than the active duration');
+
+test(t => {
+  const animation = createDiv(t).animate(null, { duration: 1, endDelay: -1 });
+
+  for (const test of [{ currentTime: -1, phase: 'before' },
+                      { currentTime:  0, phase: 'after'  },
+                      { currentTime:  1, phase: 'after'  }]) {
+    assert_phase_at_time(animation, test.phase, test.currentTime);
+  }
+}, 'Phase calculation for an animation effect with a negative end delay equal'
+   + ' in magnitude to the active duration');
+
+test(t => {
+  const animation = createDiv(t).animate(null, { duration: 1, endDelay: -2 });
+
+  for (const test of [{ currentTime: -2, phase: 'before' },
+                      { currentTime: -1, phase: 'before' },
+                      { currentTime:  0, phase: 'after'  }]) {
+    assert_phase_at_time(animation, test.phase, test.currentTime);
+  }
+}, 'Phase calculation for an animation effect with a negative end delay'
+   + ' greater in magnitude than the active duration');
+
+test(t => {
+  const animation = createDiv(t).animate(null, { duration: 2,
+                                                 delay: 1,
+                                                 endDelay: -1 });
+
+  for (const test of [{ currentTime: 0, phase: 'before' },
+                      { currentTime: 1, phase: 'active' },
+                      { currentTime: 2, phase: 'after'  }]) {
+    assert_phase_at_time(animation, test.phase, test.currentTime);
+  }
+}, 'Phase calculation for an animation effect with a positive start delay'
+   + ' and a negative end delay lesser in magnitude than the active duration');
+
+test(t => {
+  const animation = createDiv(t).animate(null, { duration: 1,
+                                                 delay: -1,
+                                                 endDelay: -1 });
+
+  for (const test of [{ currentTime: -2, phase: 'before' },
+                      { currentTime: -1, phase: 'before' },
+                      { currentTime:  0, phase: 'after'  }]) {
+    assert_phase_at_time(animation, test.phase, test.currentTime);
+  }
+}, 'Phase calculation for an animation effect with a negative start delay'
+   + ' and a negative end delay equal in magnitude to the active duration');
+
+test(t => {
+  const animation = createDiv(t).animate(null, { duration: 1,
+                                                 delay: -1,
+                                                 endDelay: -2 });
+
+  for (const test of [{ currentTime: -3, phase: 'before' },
+                      { currentTime: -2, phase: 'before' },
+                      { currentTime: -1, phase: 'before' },
+                      { currentTime:  0, phase: 'after'  }]) {
+    assert_phase_at_time(animation, test.phase, test.currentTime);
+  }
+}, 'Phase calculation for an animation effect with a negative start delay'
+   + ' and a negative end delay equal greater in magnitude than the active'
+   + ' duration');
+
+test(t => {
+  const animation = createDiv(t).animate(null, 1);
+  animation.playbackRate = -1;
+
+  for (const test of [{ currentTime: -1, phase: 'before' },
+                      { currentTime:  0, phase: 'before' },
+                      { currentTime:  1, phase: 'active' },
+                      { currentTime:  2, phase: 'after'  }]) {
+    assert_phase_at_time(animation, test.phase, test.currentTime);
+  }
+}, 'Phase calculation for a simple animation effect with negative playback'
+   + ' rate');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animation-effects/simple-iteration-progress.html b/testing/web-platform/tests/web-animations/timing-model/animation-effects/simple-iteration-progress.html
new file mode 100644
index 0000000000..3c42f79a71
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animation-effects/simple-iteration-progress.html
@@ -0,0 +1,600 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Simple iteration progress</title>
+<link rel="help"
+      href="https://drafts.csswg.org/web-animations/#simple-iteration-progress">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<script src="../../resources/effect-tests.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+function runTests(tests, description) {
+  for (const currentTest of tests) {
+    let testParams = Object.entries(currentTest.input)
+                           .map(([attr, value]) => `${attr}:${value}`)
+                           .join(' ');
+    if (currentTest.playbackRate !== undefined) {
+      testParams += ` playbackRate:${currentTest.playbackRate}`;
+    }
+
+    test(t => {
+      const div = createDiv(t);
+      const anim = div.animate({}, currentTest.input);
+      if (currentTest.playbackRate !== undefined) {
+        anim.playbackRate = currentTest.playbackRate;
+      }
+
+      assert_computed_timing_for_each_phase(
+        anim,
+        'progress',
+        { before: currentTest.before,
+          activeBoundary: currentTest.active,
+          after: currentTest.after },
+      );
+    }, `${description}: ${testParams}`);
+  }
+}
+
+
+// --------------------------------------------------------------------
+//
+// Zero iteration duration tests
+//
+// --------------------------------------------------------------------
+
+runTests([
+  {
+    input:    { iterations: 0,
+                iterationStart: 0,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 0
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 0,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 0
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 0,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 0
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 2.5,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0.5,
+    after: 0.5
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 2.5,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0.5,
+    after: 0.5
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 2.5,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0.5,
+    after: 0.5
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 3,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 0
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 3,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 0
+  },
+
+  {
+    input:    { iterations: 0,
+                iterationStart: 3,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 0
+  }
+], 'Test zero iterations');
+
+
+// --------------------------------------------------------------------
+//
+// Tests where the iteration count is an integer
+//
+// --------------------------------------------------------------------
+
+runTests([
+  {
+    input:    { iterations: 3,
+                iterationStart: 0,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 1
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 0,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0,
+    after: 1
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 0,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 2.5,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0.5,
+    after: 0.5
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 2.5,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0.5,
+    active: 0.5,
+    after: 0.5
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 2.5,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0.5,
+    active: 0.5
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 3,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 1
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 3,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0,
+    after: 1
+  },
+
+  {
+    input:    { iterations: 3,
+                iterationStart: 3,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0
+  }
+], 'Test integer iterations');
+
+
+// --------------------------------------------------------------------
+//
+// Tests where the iteration count is a fraction
+//
+// --------------------------------------------------------------------
+
+runTests([
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 0,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 0.5
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 0,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0,
+    after: 0.5
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 0,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 2.5,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0.5,
+    after: 1
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 2.5,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0.5,
+    active: 0.5,
+    after: 1
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 2.5,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0.5,
+    active: 0.5
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 3,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 0.5
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 3,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0,
+    after: 0.5
+  },
+
+  {
+    input:    { iterations: 3.5,
+                iterationStart: 3,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0
+  }
+], 'Test fractional iterations');
+
+
+// --------------------------------------------------------------------
+//
+// Tests where the iteration count is Infinity
+//
+// --------------------------------------------------------------------
+
+runTests([
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 0,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 1
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 0,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 0,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 2.5,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0.5,
+    after: 0.5
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 2.5,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0.5,
+    active: 0.5
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 2.5,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0.5,
+    active: 0.5
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 3,
+                duration: 0,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    after: 1
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 3,
+                duration: 100,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0
+  },
+
+  {
+    input:    { iterations: Infinity,
+                iterationStart: 3,
+                duration: Infinity,
+                delay: 1,
+                fill: 'both' },
+    before: 0,
+    active: 0
+  }
+], 'Test infinity iterations');
+
+
+// --------------------------------------------------------------------
+//
+// End delay tests
+//
+// --------------------------------------------------------------------
+
+runTests([
+  {
+    input:    { duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: 50 },
+    before: 0,
+    active: 0,
+    after: 1
+  },
+
+  {
+    input:    { duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -50 },
+    before: 0,
+    active: 0,
+    after: 0.5
+  },
+
+  {
+    input:    { duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -100 },
+    before: 0,
+    active: 0,
+    after: 0
+  },
+
+  {
+    input:    { duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -200 },
+    before: 0,
+    active: 0,
+    after: 0
+  },
+
+  {
+    input:    { iterationStart: 0.5,
+                duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: 50 },
+    before: 0.5,
+    active: 0.5,
+    after: 0.5
+  },
+
+  {
+    input:    { iterationStart: 0.5,
+                duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -50 },
+    before: 0.5,
+    active: 0.5,
+    after: 0
+  },
+
+  {
+    input:    { iterationStart: 0.5,
+                duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -100 },
+    before: 0.5,
+    active: 0.5,
+    after: 0.5
+  },
+
+  {
+    input:    { iterations: 2,
+                duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -100 },
+    before: 0,
+    active: 0,
+    after: 0
+  },
+
+  {
+    input:    { iterations: 1,
+                iterationStart: 2,
+                duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -50 },
+    before: 0,
+    active: 0,
+    after: 0.5
+  },
+
+  {
+    input:    { iterations: 1,
+                iterationStart: 2,
+                duration: 100,
+                delay: 1,
+                fill: 'both',
+                endDelay: -100 },
+    before: 0,
+    active: 0,
+    after: 0
+  },
+], 'Test end delay');
+
+
+// --------------------------------------------------------------------
+//
+// Negative playback rate tests
+//
+// --------------------------------------------------------------------
+
+runTests([
+  {
+    input:    { duration: 1,
+                delay: 1,
+                fill: 'both' },
+    playbackRate: -1,
+    before: 0,
+    active: 1,
+    after: 1
+  },
+
+  {
+    input:    { duration: 0,
+                delay: 1,
+                fill: 'both' },
+    playbackRate: -1,
+    before: 0,
+    after: 1
+  },
+
+  {
+    input:    { duration: 0,
+                iterations: 0,
+                delay: 1,
+                fill: 'both' },
+    playbackRate: -1,
+    before: 0,
+    after: 0
+  },
+], 'Test negative playback rate');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/canceling-an-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/canceling-an-animation.html
new file mode 100644
index 0000000000..f296ac4da7
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/canceling-an-animation.html
@@ -0,0 +1,127 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Canceling an animation</title>
+<link rel="help"
+    href="https://drafts.csswg.org/web-animations/#canceling-an-animation-section">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+test(t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.cancel();
+
+  assert_equals(animation.startTime, null,
+                'The start time of a canceled animation should be unresolved');
+  assert_equals(animation.currentTime, null,
+                'The hold time of a canceled animation should be unresolved');
+}, 'Canceling an animation should cause its start time and hold time to be'
+   + ' unresolved');
+
+promise_test(t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  const retPromise = animation.ready.then(() => {
+    assert_unreached('ready promise was fulfilled');
+  }).catch(err => {
+    assert_equals(err.name, 'AbortError',
+                  'ready promise is rejected with AbortError');
+  });
+
+  animation.cancel();
+
+  return retPromise;
+}, 'A play-pending ready promise should be rejected when the animation is'
+   + ' canceled');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await animation.ready;
+
+  // Make it pause-pending
+  animation.pause();
+
+  // We need to store the original ready promise since cancel() will
+  // replace it
+  const originalPromise = animation.ready;
+  animation.cancel();
+
+  await promise_rejects_dom(t, 'AbortError', originalPromise,
+                        'Cancel should abort ready promise');
+}, 'A pause-pending ready promise should be rejected when the animation is'
+   + ' canceled');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null);
+  animation.cancel();
+  const promiseResult = await animation.ready;
+  assert_equals(promiseResult, animation);
+}, 'When an animation is canceled, it should create a resolved Promise');
+
+test(t => {
+  const animation = createDiv(t).animate(null);
+  const promise = animation.ready;
+  animation.cancel();
+  assert_not_equals(animation.ready, promise);
+  promise_rejects_dom(t, 'AbortError', promise, 'Cancel should abort ready promise');
+}, 'The ready promise should be replaced when the animation is canceled');
+
+promise_test(t => {
+  const animation = new Animation(
+    new KeyframeEffect(null, null, { duration: 1000 }),
+    null
+  );
+  assert_equals(animation.playState, 'idle',
+                'The animation should be initially idle');
+
+  animation.finished.then(t.step_func(() => {
+    assert_unreached('Finished promise should not resolve');
+  }), t.step_func(() => {
+    assert_unreached('Finished promise should not reject');
+  }));
+
+  animation.cancel();
+
+  return waitForAnimationFrames(3);
+}, 'The finished promise should NOT be rejected if the animation is already'
+   + ' idle');
+
+promise_test(t => {
+  const animation = new Animation(
+    new KeyframeEffect(null, null, { duration: 1000 }),
+    null
+  );
+  assert_equals(animation.playState, 'idle',
+                'The animation should be initially idle');
+
+  animation.oncancel = t.step_func(() => {
+    assert_unreached('Cancel event should not be fired');
+  });
+
+  animation.cancel();
+
+  return waitForAnimationFrames(3);
+}, 'The cancel event should NOT be fired if the animation is already'
+   + ' idle');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, 100 * MS_PER_SEC);
+  div.remove();
+
+  const eventWatcher = new EventWatcher(t, animation, 'cancel');
+
+  await animation.ready;
+  animation.cancel();
+
+  await eventWatcher.wait_for('cancel');
+
+  assert_equals(animation.effect.target.parentNode, null,
+    'cancel event should be fired for the animation on an orphaned element');
+}, 'Canceling an animation should fire cancel event on orphaned element');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/document-timeline-animation-ref.html b/testing/web-platform/tests/web-animations/timing-model/animations/document-timeline-animation-ref.html
new file mode 100644
index 0000000000..d1ee52a553
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/document-timeline-animation-ref.html
@@ -0,0 +1,19 @@
+<!DOCTYPE html>
+<title>Reference for document timeline animation</title>
+<style>
+  #notes {
+    position: absolute;
+    left: 0px;
+    top: 100px;
+  }
+  body {
+    background: white;
+  }
+</style>
+<body>
+  <div id="box"></div>
+  <p id="notes">
+    This test creates a document timeline animation. If any blue pixels appear
+    in the screenshot, the test fails.
+  </p>
+</body>
\ No newline at end of file
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/document-timeline-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/document-timeline-animation.html
new file mode 100644
index 0000000000..7d4dc76849
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/document-timeline-animation.html
@@ -0,0 +1,63 @@
+
+<!DOCTYPE html>
+<html class="reftest-wait">
+<meta charset="UTF-8">
+<title>document timeline animation</title>
+<link rel="match" href="document-timeline-animation-ref.html">
+<script src="/common/reftest-wait.js"></script>
+<script src="../../testcommon.js"></script>
+<style>
+  #box-1, #box-2 {
+    position: absolute;
+    top: 0px;
+    width: 40px;
+    height: 40px;
+  }
+  #box-1 {
+    background: blue;
+    z-index: 1;
+    left: 0px;
+  }
+  #box-2 {
+    background: white;
+    z-index: 2;
+    left: 100px;
+  }
+  #notes {
+    position: absolute;
+    left: 0px;
+    top: 100px;
+  }
+  body {
+    background: white;
+  }
+</style>
+
+<body>
+  <div id="box-1"></div>
+  <div id="box-2"></div>
+  <p id="notes">
+    This test creates a document timeline animation. If any blue pixels appear
+    in the screenshot, the test fails.
+  </p>
+</body>
+<script>
+  onload = async function() {
+    const elem = document.getElementById('box-1');
+    const keyframes = [
+      { transform: 'none' },
+      { transform: 'translateX(100px)' }
+    ];
+    const effect =
+        new KeyframeEffect(elem, keyframes,
+                           {iterations: 1, duration: 10000, fill: 'forwards'});
+    const timeline = new DocumentTimeline();
+    const animation = new Animation(effect, timeline);
+    animation.play();
+    await animation.ready;
+    animation.finish();
+    await animation.finished;
+    await waitForAnimationFrames(2);
+    takeScreenshot();
+  };
+</script>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/finishing-an-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/finishing-an-animation.html
new file mode 100644
index 0000000000..fbf6558f78
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/finishing-an-animation.html
@@ -0,0 +1,330 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Finishing an animation</title>
+<link rel="help"
+  href="https://drafts.csswg.org/web-animations/#finishing-an-animation-section">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<script src="../../resources/timing-override.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate(null, 100 * MS_PER_SEC);
+  animation.playbackRate = 0;
+
+  assert_throws_dom('InvalidStateError', () => {
+    animation.finish();
+  });
+}, 'Finishing an animation with a zero playback rate throws');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate(null,
+                                { duration : 100 * MS_PER_SEC,
+                                  iterations : Infinity });
+
+  assert_throws_dom('InvalidStateError', () => {
+    animation.finish();
+  });
+}, 'Finishing an infinite animation throws');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate(null, 100 * MS_PER_SEC);
+  animation.finish();
+
+  assert_time_equals_literal(animation.currentTime, 100 * MS_PER_SEC,
+    'After finishing, the currentTime should be set to the end of the'
+    + ' active duration');
+}, 'Finishing an animation seeks to the end time');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate(null, 100 * MS_PER_SEC);
+   // 1s past effect end
+  animation.currentTime =
+    animation.effect.getComputedTiming().endTime + 1 * MS_PER_SEC;
+  animation.finish();
+
+  assert_time_equals_literal(animation.currentTime, 100 * MS_PER_SEC,
+    'After finishing, the currentTime should be set back to the end of the'
+    + ' active duration');
+}, 'Finishing an animation with a current time past the effect end jumps'
+   + ' back to the end');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animation = div.animate(null, 100 * MS_PER_SEC);
+  animation.currentTime = 100 * MS_PER_SEC;
+  await animation.finished;
+
+  animation.playbackRate = -1;
+  animation.finish();
+
+  assert_equals(animation.currentTime, 0,
+                'After finishing a reversed animation the currentTime ' +
+                'should be set to zero');
+}, 'Finishing a reversed animation jumps to zero time');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animation = div.animate(null, 100 * MS_PER_SEC);
+  animation.currentTime = 100 * MS_PER_SEC;
+  await animation.finished;
+
+  animation.playbackRate = -1;
+  animation.currentTime = -1000;
+  animation.finish();
+
+  assert_equals(animation.currentTime, 0,
+                'After finishing a reversed animation the currentTime ' +
+                'should be set back to zero');
+}, 'Finishing a reversed animation with a current time less than zero'
+   + ' makes it jump back to zero');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animation = div.animate(null, 100 * MS_PER_SEC);
+  animation.pause();
+  await animation.ready;
+
+  animation.finish();
+
+  assert_equals(animation.playState, 'finished',
+                'The play state of a paused animation should become ' +
+                '"finished"');
+  assert_times_equal(animation.startTime,
+                     animation.timeline.currentTime - 100 * MS_PER_SEC,
+                     'The start time of a paused animation should be set');
+}, 'Finishing a paused animation resolves the start time');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate(null, 100 * MS_PER_SEC);
+  // Update playbackRate so we can test that the calculated startTime
+  // respects it
+  animation.playbackRate = 2;
+  animation.pause();
+  // While animation is still pause-pending call finish()
+  animation.finish();
+
+  assert_false(animation.pending);
+  assert_equals(animation.playState, 'finished',
+                'The play state of a pause-pending animation should become ' +
+                '"finished"');
+  assert_times_equal(animation.startTime,
+                     animation.timeline.currentTime - 100 * MS_PER_SEC / 2,
+                     'The start time of a pause-pending animation should ' +
+                     'be set');
+}, 'Finishing a pause-pending animation resolves the pending task'
+   + ' immediately and update the start time');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate(null, 100 * MS_PER_SEC);
+  animation.playbackRate = -2;
+  animation.pause();
+  animation.finish();
+
+  assert_false(animation.pending);
+  assert_equals(animation.playState, 'finished',
+                'The play state of a pause-pending animation should become ' +
+                '"finished"');
+  assert_times_equal(animation.startTime, animation.timeline.currentTime,
+                     'The start time of a pause-pending animation should be ' +
+                     'set');
+}, 'Finishing a pause-pending animation with negative playback rate'
+   + ' resolves the pending task immediately');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate(null, 100 * MS_PER_SEC);
+  animation.playbackRate = 0.5;
+  animation.finish();
+
+  assert_false(animation.pending);
+  assert_equals(animation.playState, 'finished',
+                'The play state of a play-pending animation should become ' +
+                '"finished"');
+  assert_times_equal(animation.startTime,
+                     animation.timeline.currentTime - 100 * MS_PER_SEC / 0.5,
+                     'The start time of a play-pending animation should ' +
+                     'be set');
+}, 'Finishing an animation while play-pending resolves the pending'
+   + ' task immediately');
+
+// FIXME: Add a test for when we are play-pending without an active timeline.
+// - In that case even after calling finish() we should still be pending but
+//   the current time should be updated
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animation = div.animate(null, 100 * MS_PER_SEC);
+  await animation.ready;
+
+  animation.pause();
+  animation.play();
+  // We are now in the unusual situation of being play-pending whilst having
+  // a resolved start time. Check that finish() still triggers a transition
+  // to the finished state immediately.
+  animation.finish();
+
+  assert_equals(animation.playState, 'finished',
+                'After aborting a pause then finishing an animation its play ' +
+                'state should become "finished" immediately');
+}, 'Finishing an animation during an aborted pause makes it finished'
+   + ' immediately');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animation = div.animate(null, 100 * MS_PER_SEC);
+  let resolvedFinished = false;
+  animation.finished.then(() => {
+    resolvedFinished = true;
+  });
+
+  await animation.ready;
+
+  animation.finish();
+  await Promise.resolve();
+
+  assert_true(resolvedFinished, 'finished promise should be resolved');
+}, 'Finishing an animation resolves the finished promise synchronously');
+
+promise_test(async t => {
+  const effect = new KeyframeEffect(null, null, 100 * MS_PER_SEC);
+  const animation = new Animation(effect, document.timeline);
+  let resolvedFinished = false;
+  animation.finished.then(() => {
+    resolvedFinished = true;
+  });
+
+  await animation.ready;
+
+  animation.finish();
+  await Promise.resolve();
+
+  assert_true(resolvedFinished, 'finished promise should be resolved');
+}, 'Finishing an animation without a target resolves the finished promise'
+   + ' synchronously');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  const promise = animation.ready;
+  let readyResolved = false;
+
+  animation.finish();
+  animation.ready.then(() => { readyResolved = true; });
+
+  const promiseResult = await animation.finished;
+
+  assert_equals(promiseResult, animation);
+  assert_equals(animation.ready, promise);
+  assert_true(readyResolved);
+}, 'A pending ready promise is resolved and not replaced when the animation'
+   + ' is finished');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await animation.ready;
+
+  animation.updatePlaybackRate(2);
+  assert_true(animation.pending);
+
+  animation.finish();
+  assert_false(animation.pending);
+  assert_equals(animation.playbackRate, 2);
+  assert_time_equals_literal(animation.currentTime, 100 * MS_PER_SEC);
+}, 'A pending playback rate should be applied immediately when an animation'
+   + ' is finished');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await animation.ready;
+
+  animation.updatePlaybackRate(0);
+
+  assert_throws_dom('InvalidStateError', () => {
+    animation.finish();
+  });
+}, 'An exception should be thrown if the effective playback rate is zero');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, {
+    duration: 100 * MS_PER_SEC,
+    iterations: Infinity
+  });
+  animation.currentTime = 50 * MS_PER_SEC;
+  animation.playbackRate = -1;
+  await animation.ready;
+
+  animation.updatePlaybackRate(1);
+
+  assert_throws_dom('InvalidStateError', () => {
+    animation.finish();
+  });
+}, 'An exception should be thrown when finishing if the effective playback rate'
+   + ' is positive and the target effect end is infinity');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, {
+    duration: 100 * MS_PER_SEC,
+    iterations: Infinity
+  });
+  await animation.ready;
+
+  animation.updatePlaybackRate(-1);
+
+  animation.finish();
+  // Should not have thrown
+}, 'An exception is NOT thrown when finishing if the effective playback rate'
+   + ' is negative and the target effect end is infinity');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, 100 * MS_PER_SEC);
+  div.remove();
+
+  const eventWatcher = new EventWatcher(t, animation, 'finish');
+
+  await animation.ready;
+  animation.finish();
+
+  await eventWatcher.wait_for('finish');
+  assert_equals(animation.effect.target.parentNode, null,
+    'finish event should be fired for the animation on an orphaned element');
+}, 'Finishing an animation fires finish event on orphaned element');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await animation.ready;
+
+  const originalFinishPromise = animation.finished;
+
+  animation.cancel();
+  assert_equals(animation.startTime, null);
+  assert_equals(animation.currentTime, null);
+
+  const resolvedFinishPromise = animation.finished;
+  assert_not_equals(originalFinishPromise, resolvedFinishPromise,
+               'Canceling an animation should create a new finished promise');
+
+  animation.finish();
+  assert_equals(animation.playState, 'finished',
+                'The play state of a canceled animation should become ' +
+                '"finished"');
+  assert_times_equal(animation.startTime,
+                     animation.timeline.currentTime - 100 * MS_PER_SEC,
+                     'The start time of a finished animation should be set');
+  assert_times_equal(animation.currentTime, 100000,
+                     'Hold time should be set to end boundary of the animation');
+
+}, 'Finishing a canceled animation sets the current and start times');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/infinite-duration-animation-ref.html b/testing/web-platform/tests/web-animations/timing-model/animations/infinite-duration-animation-ref.html
new file mode 100644
index 0000000000..6b358bd4e7
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/infinite-duration-animation-ref.html
@@ -0,0 +1,19 @@
+<!DOCTYPE html>
+<title>Reference for infinite duration animation</title>
+<style>
+  #notes {
+    position: absolute;
+    left: 0px;
+    top: 100px;
+  }
+  body {
+    background: white;
+  }
+</style>
+<body>
+  <p id="notes">
+    This test creates an infinite duration animations, which should be stuck at
+    a progress of 0. If any blue pixels appear in the screenshot, the test
+    fails.
+  </p>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/infinite-duration-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/infinite-duration-animation.html
new file mode 100644
index 0000000000..c641e5afa2
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/infinite-duration-animation.html
@@ -0,0 +1,64 @@
+
+<!DOCTYPE html>
+<html class="reftest-wait">
+<meta charset="UTF-8">
+<title>Infinite duration animation</title>
+<link rel="match" href="infinite-duration-animation-ref.html">
+<script src="/common/reftest-wait.js"></script>
+<script src="../../testcommon.js"></script>
+<style>
+  #box-1, #box-2 {
+    border: 1px solid white;
+    height: 40px;
+    position: absolute;
+    top: 40px;
+    width: 40px;
+  }
+  #box-1 {
+    background: blue;
+    z-index: 1;
+  }
+  #box-2 {
+    background: white;
+    z-index: 2;
+  }
+  #notes {
+    position: absolute;
+    left: 0px;
+    top: 100px;
+  }
+  body {
+    background: white;
+  }
+</style>
+
+<body>
+  <div id="box-1"></div>
+  <div id="box-2"></div>
+  <p id="notes">
+    This test creates an infinite duration animations, which should be stuck at
+    a progress of 0. If any blue pixels appear in the screenshot, the test
+    fails.
+  </p>
+</body>
+<script>
+  onload = async function() {
+    // Double rAF to ensure that we are not bogged down during initialization
+    // and the compositor is ready.
+    waitForAnimationFrames(2).then(() => {
+      const elem = document.getElementById('box-1');
+      const keyframes = [
+        { transform: 'translateX(0px)' },
+        { transform: 'translateX(100px)' }
+      ];
+      const effect =
+          new KeyframeEffect(elem, keyframes,
+                             {iterations: 3, duration: Infinity});
+      const animation = new Animation(effect);
+      animation.play();
+      animation.ready.then(() => {
+        takeScreenshotDelayed(100);
+      });
+    });
+  };
+</script>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/pausing-an-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/pausing-an-animation.html
new file mode 100644
index 0000000000..dd9522cb35
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/pausing-an-animation.html
@@ -0,0 +1,119 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Pausing an animation</title>
+<link rel="help"
+  href="https://drafts.csswg.org/web-animations/#pausing-an-animation-section">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await animation.ready;
+
+  const startTimeBeforePausing = animation.startTime;
+
+  animation.pause();
+  assert_equals(animation.startTime, startTimeBeforePausing,
+                'The start time does not change when pausing-pending');
+
+  await animation.ready;
+
+  assert_equals(animation.startTime, null,
+                'The start time is unresolved when paused');
+}, 'Pausing clears the start time');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await animation.ready;
+
+  animation.pause();
+  assert_not_equals(animation.startTime, null,
+                    'The start time is resolved when pause-pending');
+
+  animation.play();
+  assert_not_equals(animation.startTime, null,
+                    'The start time is preserved when a pause is aborted');
+}, 'Aborting a pause preserves the start time');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  const promise = animation.ready;
+  animation.pause();
+
+  const promiseResult = await promise;
+
+  assert_equals(promiseResult, animation);
+  assert_equals(animation.ready, promise);
+  assert_false(animation.pending, 'No longer pause-pending');
+}, 'A pending ready promise should be resolved and not replaced when the'
+   + ' animation is paused');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  // Let animation start roughly half-way through
+  animation.currentTime = 50 * MS_PER_SEC;
+  await animation.ready;
+
+  // Go pause-pending and also set a pending playback rate
+  animation.pause();
+  animation.updatePlaybackRate(0.5);
+
+  await animation.ready;
+  // If the current time was updated using the new playback rate it will jump
+  // back to 25s but if we correctly used the old playback rate the current time
+  // will be >= 50s.
+  assert_greater_than_equal(animation.currentTime, 50 * MS_PER_SEC);
+}, 'A pause-pending animation maintains the current time when applying a'
+   + ' pending playback rate');
+
+promise_test(async t => {
+  // This test does not cover a specific step in the algorithm but serves as a
+  // high-level sanity check that pausing does, in fact, freeze the current
+  // time.
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await animation.ready;
+
+  animation.pause();
+  await animation.ready;
+
+  const currentTimeAfterPausing = animation.currentTime;
+
+  await waitForNextFrame();
+
+  assert_equals(animation.currentTime, currentTimeAfterPausing,
+    'Animation.currentTime is unchanged after pausing');
+}, 'The animation\'s current time remains fixed after pausing');
+
+
+promise_test(async t => {
+
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+
+  const originalReadyPromise = animation.ready;
+  animation.cancel();
+  assert_equals(animation.startTime, null);
+  assert_equals(animation.currentTime, null);
+
+  const readyPromise = animation.ready;
+  assert_not_equals(originalReadyPromise, readyPromise,
+              'Canceling an animation should create a new ready promise');
+
+  animation.pause();
+  assert_equals(animation.playState, 'paused',
+                'Pausing a canceled animation should update the play state');
+  assert_true(animation.pending, 'animation should be pause-pending');
+  await animation.ready;
+  assert_false(animation.pending,
+               'animation should no longer be pause-pending');
+  assert_equals(animation.startTime, null, 'start time should be unresolved');
+  assert_equals(animation.currentTime, 0, 'current time should be set to zero');
+
+}, 'Pausing a canceled animation sets the current time');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/play-states.html b/testing/web-platform/tests/web-animations/timing-model/animations/play-states.html
new file mode 100644
index 0000000000..ec7d8c842f
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/play-states.html
@@ -0,0 +1,185 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Play states</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#play-states">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+test(t => {
+  const animation = new Animation(
+    new KeyframeEffect(null, {}, 100 * MS_PER_SEC)
+  );
+  assert_equals(animation.currentTime, null,
+                'Current time should be initially unresolved');
+
+  assert_equals(animation.playState, 'idle');
+}, 'reports \'idle\' for an animation with an unresolved current time'
+   + ' and no pending tasks')
+
+test(t => {
+  const animation = createDiv(t).animate({}, 100 * MS_PER_SEC);
+
+  animation.pause();
+
+  assert_equals(animation.playState, 'paused');
+}, 'reports \'paused\' for an animation with a pending pause task');
+
+test(t => {
+  const animation = new Animation(
+    new KeyframeEffect(null, {}, 100 * MS_PER_SEC)
+  );
+
+  animation.currentTime = 0;
+  assert_equals(animation.startTime, null,
+                'Start time should still be unresolved after setting current'
+                + ' time');
+
+  assert_equals(animation.playState, 'paused');
+}, 'reports \'paused\' for an animation with a resolved current time and'
+   + ' unresolved start time')
+
+test(t => {
+  const animation = new Animation(
+    new KeyframeEffect(null, {}, 100 * MS_PER_SEC)
+  );
+
+  animation.startTime = document.timeline.currentTime;
+  assert_not_equals(animation.currentTime, null,
+                    'Current time should be resolved after setting start time');
+
+  assert_equals(animation.playState, 'running');
+}, 'reports \'running\' for an animation with a resolved start time and'
+   + ' current time');
+
+test(t => {
+  const animation = new Animation(
+    new KeyframeEffect(null, {}, 100 * MS_PER_SEC)
+  );
+  animation.startTime = document.timeline.currentTime;
+
+  animation.currentTime = 100 * MS_PER_SEC;
+
+  assert_equals(animation.playState, 'finished');
+}, 'reports \'finished\' when playback rate > 0 and'
+   + ' current time = target effect end');
+
+test(t => {
+  const animation = new Animation(
+    new KeyframeEffect(null, {}, 100 * MS_PER_SEC)
+  );
+  animation.startTime = document.timeline.currentTime;
+
+  animation.playbackRate = 0;
+  animation.currentTime = 100 * MS_PER_SEC;
+
+  assert_equals(animation.playState, 'running');
+}, 'reports \'running\' when playback rate = 0 and'
+   + ' current time = target effect end');
+
+test(t => {
+  const animation = new Animation(
+    new KeyframeEffect(null, {}, 100 * MS_PER_SEC)
+  );
+  animation.startTime = document.timeline.currentTime;
+
+  animation.playbackRate = -1;
+  animation.currentTime = 100 * MS_PER_SEC;
+
+  assert_equals(animation.playState, 'running');
+}, 'reports \'running\' when playback rate < 0 and'
+   + ' current time = target effect end');
+
+test(t => {
+  const animation = new Animation(
+    new KeyframeEffect(null, {}, 100 * MS_PER_SEC)
+  );
+  animation.startTime = document.timeline.currentTime;
+
+  animation.currentTime = 0;
+
+  assert_equals(animation.playState, 'running');
+}, 'reports \'running\' when playback rate > 0 and'
+   + ' current time = 0');
+
+test(t => {
+  const animation = new Animation(
+    new KeyframeEffect(null, {}, 100 * MS_PER_SEC)
+  );
+  animation.startTime = document.timeline.currentTime;
+
+  animation.playbackRate = 0;
+  animation.currentTime = 0;
+
+  assert_equals(animation.playState, 'running');
+}, 'reports \'running\' when playback rate = 0 and'
+   + ' current time = 0');
+
+test(t => {
+  const animation = new Animation(
+    new KeyframeEffect(null, {}, 100 * MS_PER_SEC)
+  );
+  animation.startTime = document.timeline.currentTime;
+
+  animation.playbackRate = -1;
+  animation.currentTime = 0;
+
+  assert_equals(animation.playState, 'finished');
+}, 'reports \'finished\' when playback rate < 0 and'
+   + ' current time = 0');
+
+test(t => {
+  const animation = createDiv(t).animate({}, 0);
+  assert_equals(animation.startTime, null,
+                'Sanity check: start time should be unresolved');
+
+  assert_equals(animation.playState, 'finished');
+}, 'reports \'finished\' when playback rate > 0 and'
+   + ' current time = target effect end and there is a pending play task');
+
+test(t => {
+  const animation = createDiv(t).animate({}, 100 * MS_PER_SEC);
+  assert_equals(animation.startTime, null,
+                'Sanity check: start time should be unresolved');
+
+  assert_equals(animation.playState, 'running');
+}, 'reports \'running\' when playback rate > 0 and'
+   + ' current time < target effect end and there is a pending play task');
+
+test(t => {
+  const animation = createDiv(t).animate({}, 100 * MS_PER_SEC);
+  assert_equals(animation.playState, 'running');
+  assert_true(animation.pending);
+}, 'reports \'running\' for a play-pending animation');
+
+test(t => {
+  const animation = createDiv(t).animate({}, 100 * MS_PER_SEC);
+  animation.pause();
+  assert_equals(animation.playState, 'paused');
+  assert_true(animation.pending);
+}, 'reports \'paused\' for a pause-pending animation');
+
+test(t => {
+  const animation = createDiv(t).animate({}, 0);
+  assert_equals(animation.playState, 'finished');
+  assert_true(animation.pending);
+}, 'reports \'finished\' for a finished-pending animation');
+
+test(t => {
+  const animation = createDiv(t).animate({}, 100 * MS_PER_SEC);
+  // Set up the pending playback rate
+  animation.updatePlaybackRate(-1);
+  // Call play again so that we seek to the end while remaining play-pending
+  animation.play();
+  // For a pending animation, the play state should always report what the
+  // play state _will_ be once we finish pending.
+  assert_equals(animation.playState, 'running');
+  assert_true(animation.pending);
+}, 'reports the play state based on the pending playback rate');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/playing-an-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/playing-an-animation.html
new file mode 100644
index 0000000000..01e036ae57
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/playing-an-animation.html
@@ -0,0 +1,177 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Playing an animation</title>
+<link rel="help"
+      href="https://drafts.csswg.org/web-animations/#playing-an-animation-section">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+test(t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.currentTime = 1 * MS_PER_SEC;
+  assert_time_equals_literal(animation.currentTime, 1 * MS_PER_SEC);
+  animation.play();
+  assert_time_equals_literal(animation.currentTime, 1 * MS_PER_SEC);
+}, 'Playing a running animation leaves the current time unchanged');
+
+test(t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.finish();
+  assert_time_equals_literal(animation.currentTime, 100 * MS_PER_SEC);
+  animation.play();
+  assert_time_equals_literal(animation.currentTime, 0);
+}, 'Playing a finished animation seeks back to the start');
+
+test(t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.playbackRate = -1;
+  animation.currentTime = 0;
+  assert_time_equals_literal(animation.currentTime, 0);
+  animation.play();
+  assert_time_equals_literal(animation.currentTime, 100 * MS_PER_SEC);
+}, 'Playing a finished and reversed animation seeks to end');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.finish();
+
+  // Initiate a pause then abort it
+  animation.pause();
+  animation.play();
+
+  // Wait to return to running state
+  await animation.ready;
+
+  assert_true(animation.currentTime < 100 * 1000,
+              'After aborting a pause when finished, the current time should'
+              + ' jump back to the start of the animation');
+}, 'Playing a pause-pending but previously finished animation seeks back to'
+   + ' to the start');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.finish();
+  await animation.ready;
+
+  animation.play();
+  assert_equals(animation.startTime, null, 'start time is unresolved');
+}, 'Playing a finished animation clears the start time');
+
+test(t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.cancel();
+  const promise = animation.ready;
+  animation.play();
+  assert_not_equals(animation.ready, promise);
+}, 'The ready promise should be replaced if the animation is not already'
+   + ' pending');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  const promise = animation.ready;
+  const promiseResult = await promise;
+  assert_equals(promiseResult, animation);
+  assert_equals(animation.ready, promise);
+}, 'A pending ready promise should be resolved and not replaced when the'
+   + ' animation enters the running state');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.currentTime = 50 * MS_PER_SEC;
+  await animation.ready;
+
+  animation.pause();
+  await animation.ready;
+
+  const holdTime = animation.currentTime;
+
+  animation.play();
+  await animation.ready;
+
+  assert_less_than_equal(
+    animation.startTime,
+    animation.timeline.currentTime - holdTime + TIME_PRECISION
+  );
+}, 'Resuming an animation from paused calculates start time from hold time');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await animation.ready;
+
+  // Go to pause-pending state
+  animation.pause();
+  assert_true(animation.pending, 'Animation is pending');
+  const pauseReadyPromise = animation.ready;
+
+  // Now play again immediately (abort the pause)
+  animation.play();
+  assert_true(animation.pending, 'Animation is still pending');
+  assert_equals(animation.ready, pauseReadyPromise,
+                'The pause Promise is re-used when playing while waiting'
+                + ' to pause');
+
+  // Sanity check: Animation proceeds to running state
+  await animation.ready;
+  assert_true(!animation.pending && animation.playState === 'running',
+              'Animation is running after aborting a pause');
+}, 'If a pause operation is interrupted, the ready promise is reused');
+
+promise_test(async t => {
+  // Seek animation beyond target end
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.currentTime = -100 * MS_PER_SEC;
+  await animation.ready;
+
+  // Set pending playback rate to the opposite direction
+  animation.updatePlaybackRate(-1);
+  assert_true(animation.pending);
+  assert_equals(animation.playbackRate, 1);
+
+  // When we play, we should seek to the target end, NOT to zero (which
+  // is where we would seek to if we used the playbackRate of 1.
+  animation.play();
+  assert_time_equals_literal(animation.currentTime, 100 * MS_PER_SEC);
+}, 'A pending playback rate is used when determining auto-rewind behavior');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.cancel();
+  assert_equals(animation.startTime, null,
+                'Start time should be unresolved');
+
+  const playTime = animation.timeline.currentTime;
+  animation.play();
+  assert_true(animation.pending, 'Animation should be play-pending');
+
+  await animation.ready;
+
+  assert_false(animation.pending, 'animation should no longer be pending');
+  assert_time_greater_than_equal(animation.startTime, playTime,
+                      'The start time of the playing animation should be set');
+}, 'Playing a canceled animation sets the start time');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.playbackRate = -1;
+  animation.cancel();
+  assert_equals(animation.startTime, null,
+                'Start time should be unresolved');
+
+  const playTime = animation.timeline.currentTime;
+  animation.play();
+  assert_true(animation.pending, 'Animation should be play-pending');
+
+  await animation.ready;
+
+  assert_false(animation.pending, 'Animation should no longer be pending');
+  assert_time_greater_than_equal(animation.startTime, playTime + 100 * MS_PER_SEC,
+                      'The start time of the playing animation should be set');
+}, 'Playing a canceled animation backwards sets the start time');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/reverse-running-animation-ref.html b/testing/web-platform/tests/web-animations/timing-model/animations/reverse-running-animation-ref.html
new file mode 100644
index 0000000000..7bf5b03c1e
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/reverse-running-animation-ref.html
@@ -0,0 +1,17 @@
+<!DOCTYPE html>
+<title>Reference for reverse running animation</title>
+<style>
+  #box {
+    background: green;
+    height: 40px;
+    width: 40px;
+  }
+</style>
+<body>
+  <div id="box"></div>
+  <p id="notes">
+    This test animates the box color from green to red and reverses the play
+    direction shortly after the midpoint. If the box remains red, the test
+    failed.
+  </p>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/reverse-running-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/reverse-running-animation.html
new file mode 100644
index 0000000000..c9eb2a068e
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/reverse-running-animation.html
@@ -0,0 +1,50 @@
+
+<!DOCTYPE html>
+<html class="reftest-wait">
+<meta charset="UTF-8">
+<title>reverse running animation</title>
+<link rel="match" href="reverse-running-animation-ref.html">
+<script src="/common/reftest-wait.js"></script>
+<script src="../../testcommon.js"></script>
+<style>
+  #box {
+    background: green;
+    height: 40px;
+    width: 40px;
+  }
+</style>
+<body>
+  <div id="box"></div>
+  <p id="notes">
+    This test animates the box color from green to red and reverses the play
+    direction shortly after the midpoint. If the box remains red, the test
+    failed.
+  </p>
+</body>
+<script>
+  onload = async function() {
+    const box = document.getElementById('box');
+    const duration = 10000;
+    const anim =
+        box.animate({ bacground: [ 'green', 'red' ] },
+                    { duration: duration, easing: 'steps(2, jump-none)' });
+    anim.currentTime = duration / 2;
+    anim.ready.then(() => {
+      const startTime = anim.timeline.currentTime;
+      waitForAnimationFrames(2).then(() => {
+        anim.reverse();
+        anim.ready.then(() => {
+          const reversalTime = anim.timeline.currentTime;
+          const forwardPlayingTime = reversalTime - startTime;
+          const checkIfDone = () => {
+            if (anim.timeline.currentTime - reversalTime > forwardPlayingTime)
+              takeScreenshot();
+            else
+              requestAnimationFrame(checkIfDone);
+          };
+          requestAnimationFrame(checkIfDone);
+        });
+      });
+    });
+  };
+</script>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/reversing-an-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/reversing-an-animation.html
new file mode 100644
index 0000000000..8d869d72aa
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/reversing-an-animation.html
@@ -0,0 +1,266 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Reversing an animation</title>
+<link rel="help"
+      href="https://drafts.csswg.org/web-animations/#reversing-an-animation-section">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, { duration: 100 * MS_PER_SEC,
+                                      iterations: Infinity });
+
+  await animation.ready;
+  // Wait a frame because if currentTime is still 0 when we call
+  // reverse(), it will throw (per spec).
+  await waitForAnimationFrames(1);
+
+  assert_greater_than_equal(animation.currentTime, 0,
+    'currentTime expected to be greater than 0, one frame after starting');
+  animation.currentTime = 50 * MS_PER_SEC;
+  const previousPlaybackRate = animation.playbackRate;
+  animation.reverse();
+  assert_equals(animation.playbackRate, previousPlaybackRate,
+                'Playback rate should not have changed');
+  await animation.ready;
+
+  assert_equals(animation.playbackRate, -previousPlaybackRate,
+                'Playback rate should be inverted');
+}, 'Reversing an animation inverts the playback rate');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, { duration: 100 * MS_PER_SEC,
+                                      iterations: Infinity });
+  animation.currentTime = 50 * MS_PER_SEC;
+  animation.pause();
+
+  await animation.ready;
+
+  animation.reverse();
+  await animation.ready;
+
+  assert_equals(animation.playState, 'running',
+    'Animation.playState should be "running" after reverse()');
+}, 'Reversing an animation plays a pausing animation');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, 100 * MS_PER_SEC);
+  animation.currentTime = 50 * MS_PER_SEC;
+  animation.reverse();
+
+  assert_equals(animation.currentTime, 50 * MS_PER_SEC,
+    'The current time should not change it is in the middle of ' +
+    'the animation duration');
+}, 'Reversing an animation maintains the same current time');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, { duration: 200 * MS_PER_SEC,
+                                      delay: -100 * MS_PER_SEC });
+  assert_true(animation.pending,
+              'The animation is pending before we call reverse');
+
+  animation.reverse();
+
+  assert_true(animation.pending,
+              'The animation is still pending after calling reverse');
+}, 'Reversing an animation does not cause it to leave the pending state');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, { duration: 200 * MS_PER_SEC,
+                                      delay: -100 * MS_PER_SEC });
+  let readyResolved = false;
+  animation.ready.then(() => { readyResolved = true; });
+
+  animation.reverse();
+
+  await Promise.resolve();
+  assert_false(readyResolved,
+               'ready promise should not have been resolved yet');
+}, 'Reversing an animation does not cause it to resolve the ready promise');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, 100 * MS_PER_SEC);
+  animation.currentTime = 200 * MS_PER_SEC;
+  animation.reverse();
+
+  assert_equals(animation.currentTime, 100 * MS_PER_SEC,
+    'reverse() should start playing from the animation effect end ' +
+    'if the playbackRate > 0 and the currentTime > effect end');
+}, 'Reversing an animation when playbackRate > 0 and currentTime > ' +
+   'effect end should make it play from the end');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, 100 * MS_PER_SEC);
+
+  animation.currentTime = -200 * MS_PER_SEC;
+  animation.reverse();
+
+  assert_equals(animation.currentTime, 100 * MS_PER_SEC,
+    'reverse() should start playing from the animation effect end ' +
+    'if the playbackRate > 0 and the currentTime < 0');
+}, 'Reversing an animation when playbackRate > 0 and currentTime < 0 ' +
+   'should make it play from the end');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, 100 * MS_PER_SEC);
+  animation.playbackRate = -1;
+  animation.currentTime = -200 * MS_PER_SEC;
+  animation.reverse();
+
+  assert_equals(animation.currentTime, 0,
+    'reverse() should start playing from the start of animation time ' +
+    'if the playbackRate < 0 and the currentTime < 0');
+}, 'Reversing an animation when playbackRate < 0 and currentTime < 0 ' +
+   'should make it play from the start');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, 100 * MS_PER_SEC);
+  animation.playbackRate = -1;
+  animation.currentTime = 200 * MS_PER_SEC;
+  animation.reverse();
+
+  assert_equals(animation.currentTime, 0,
+    'reverse() should start playing from the start of animation time ' +
+    'if the playbackRate < 0 and the currentTime > effect end');
+}, 'Reversing an animation when playbackRate < 0 and currentTime > effect ' +
+   'end should make it play from the start');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, { duration: 100 * MS_PER_SEC,
+                                      iterations: Infinity });
+  animation.currentTime = -200 * MS_PER_SEC;
+
+  assert_throws_dom('InvalidStateError',
+    () => { animation.reverse(); },
+    'reverse() should throw InvalidStateError ' +
+    'if the playbackRate > 0 and the currentTime < 0 ' +
+    'and the target effect is positive infinity');
+}, 'Reversing an animation when playbackRate > 0 and currentTime < 0 ' +
+   'and the target effect end is positive infinity should throw an exception');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate({}, { duration: 100 * MS_PER_SEC,
+                                               iterations: Infinity });
+  animation.currentTime = -200 * MS_PER_SEC;
+
+  try { animation.reverse(); } catch(e) { }
+
+  assert_equals(animation.playbackRate, 1, 'playbackRate is unchanged');
+
+  await animation.ready;
+  assert_equals(animation.playbackRate, 1, 'playbackRate remains unchanged');
+}, 'When reversing throws an exception, the playback rate remains unchanged');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, { duration: 100 * MS_PER_SEC,
+                                      iterations: Infinity });
+  animation.currentTime = -200 * MS_PER_SEC;
+  animation.playbackRate = 0;
+
+  try {
+    animation.reverse();
+  } catch (e) {
+    assert_unreached(`Unexpected exception when calling reverse(): ${e}`);
+  }
+}, 'Reversing animation when playbackRate = 0 and currentTime < 0 ' +
+   'and the target effect end is positive infinity should NOT throw an ' +
+   'exception');
+
+test(t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, { duration: 100 * MS_PER_SEC,
+                                      iterations: Infinity });
+  animation.playbackRate = -1;
+  animation.currentTime = -200 * MS_PER_SEC;
+  animation.reverse();
+
+  assert_equals(animation.currentTime, 0,
+    'reverse() should start playing from the start of animation time ' +
+    'if the playbackRate < 0 and the currentTime < 0 ' +
+    'and the target effect is positive infinity');
+}, 'Reversing an animation when playbackRate < 0 and currentTime < 0 ' +
+   'and the target effect end is positive infinity should make it play ' +
+   'from the start');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animation = div.animate({}, 100 * MS_PER_SEC);
+  animation.playbackRate = 0;
+  animation.currentTime = 50 * MS_PER_SEC;
+  animation.reverse();
+
+  await animation.ready;
+  assert_equals(animation.playbackRate, 0,
+    'reverse() should preserve playbackRate if the playbackRate == 0');
+  assert_equals(animation.currentTime, 50 * MS_PER_SEC,
+    'reverse() should not affect the currentTime if the playbackRate == 0');
+}, 'Reversing when when playbackRate == 0 should preserve the current ' +
+   'time and playback rate');
+
+test(t => {
+  const div = createDiv(t);
+  const animation =
+    new Animation(new KeyframeEffect(div, null, 100 * MS_PER_SEC));
+  assert_equals(animation.currentTime, null);
+
+  animation.reverse();
+
+  assert_equals(animation.currentTime, 100 * MS_PER_SEC,
+    'animation.currentTime should be at its effect end');
+}, 'Reversing an idle animation from starts playing the animation');
+
+test(t => {
+  const div = createDiv(t);
+  const animation =
+    new Animation(new KeyframeEffect(div, null, 100 * MS_PER_SEC), null);
+
+  assert_throws_dom('InvalidStateError', () => { animation.reverse(); });
+}, 'Reversing an animation without an active timeline throws an ' +
+   'InvalidStateError');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await animation.ready;
+
+  animation.updatePlaybackRate(2);
+  animation.reverse();
+
+  await animation.ready;
+  assert_equals(animation.playbackRate, -2);
+}, 'Reversing should use the negative pending playback rate');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, {
+    duration: 100 * MS_PER_SEC,
+    iterations: Infinity,
+  });
+  animation.currentTime = -200 * MS_PER_SEC;
+  await animation.ready;
+
+  animation.updatePlaybackRate(2);
+  assert_throws_dom('InvalidStateError', () => { animation.reverse(); });
+  assert_equals(animation.playbackRate, 1);
+
+  await animation.ready;
+  assert_equals(animation.playbackRate, 2);
+}, 'When reversing fails, it should restore any previous pending playback'
+   + ' rate');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/seamlessly-updating-the-playback-rate-of-an-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/seamlessly-updating-the-playback-rate-of-an-animation.html
new file mode 100644
index 0000000000..dffbeabd59
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/seamlessly-updating-the-playback-rate-of-an-animation.html
@@ -0,0 +1,171 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>Seamlessly updating the playback rate of an animation</title>
+<link rel="help"
+  href="https://drafts.csswg.org/web-animations-1/#seamlessly-updating-the-playback-rate-of-an-animation">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await animation.ready;
+
+  animation.currentTime = 50 * MS_PER_SEC;
+
+  animation.updatePlaybackRate(0.5);
+  await animation.ready;
+  // Since the animation is in motion (and we want to test it while it is in
+  // motion!) we can't assert that the current time == 50s but we can check
+  // that the current time is NOT re-calculated by simply substituting in the
+  // new playback rate (i.e. without adjusting the start time). If that were
+  // the case the currentTime would jump to 25s. So we just test the currentTime
+  // hasn't gone backwards.
+  assert_greater_than_equal(animation.currentTime, 50 * MS_PER_SEC,
+    'Reducing the playback rate should not change the current time ' +
+    'of a playing animation');
+
+  animation.updatePlaybackRate(2);
+  await animation.ready;
+  // Likewise, we test here that the current time does not jump to 100s as it
+  // would if we naively applied a playbackRate of 2 without adjusting the
+  // startTime.
+  assert_less_than(animation.currentTime, 100 * MS_PER_SEC,
+    'Increasing the playback rate should not change the current time ' +
+    'of a playing animation');
+}, 'Updating the playback rate maintains the current time');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await animation.ready;
+
+  assert_false(animation.pending);
+  animation.updatePlaybackRate(2);
+  assert_true(animation.pending);
+}, 'Updating the playback rate while running makes the animation pending');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.currentTime = 50 * MS_PER_SEC;
+  assert_true(animation.pending);
+
+  animation.updatePlaybackRate(0.5);
+
+  // Check that the hold time is updated as expected
+  assert_time_equals_literal(animation.currentTime, 50 * MS_PER_SEC);
+
+  await animation.ready;
+
+  // As above, check that the currentTime is not calculated by simply
+  // substituting in the updated playbackRate without updating the startTime.
+  assert_greater_than_equal(animation.currentTime, 50 * MS_PER_SEC,
+    'Reducing the playback rate should not change the current time ' +
+    'of a play-pending animation');
+}, 'Updating the playback rate on a play-pending animation maintains'
+   + ' the current time');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.currentTime = 50 * MS_PER_SEC;
+  await animation.ready;
+
+  animation.pause();
+  animation.updatePlaybackRate(0.5);
+
+  assert_greater_than_equal(animation.currentTime, 50 * MS_PER_SEC);
+}, 'Updating the playback rate on a pause-pending animation maintains'
+   + ' the current time');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+
+  animation.updatePlaybackRate(2);
+  animation.updatePlaybackRate(3);
+  animation.updatePlaybackRate(4);
+
+  assert_equals(animation.playbackRate, 1);
+  await animation.ready;
+
+  assert_equals(animation.playbackRate, 4);
+}, 'If a pending playback rate is set multiple times, the latest wins');
+
+test(t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.cancel();
+
+  animation.updatePlaybackRate(2);
+  assert_equals(animation.playbackRate, 2);
+  assert_false(animation.pending);
+}, 'In the idle state, the playback rate is applied immediately');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.pause();
+  await animation.ready;
+
+  animation.updatePlaybackRate(2);
+  assert_equals(animation.playbackRate, 2);
+  assert_false(animation.pending);
+}, 'In the paused state, the playback rate is applied immediately');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.finish();
+  assert_time_equals_literal(animation.currentTime, 100 * MS_PER_SEC);
+  assert_false(animation.pending);
+
+  animation.updatePlaybackRate(2);
+  assert_equals(animation.playbackRate, 2);
+  assert_time_equals_literal(animation.currentTime, 100 * MS_PER_SEC);
+  assert_false(animation.pending);
+}, 'Updating the playback rate on a finished animation maintains'
+   + ' the current time');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.finish();
+  assert_time_equals_literal(animation.currentTime, 100 * MS_PER_SEC);
+  assert_false(animation.pending);
+
+  animation.updatePlaybackRate(0);
+  assert_equals(animation.playbackRate, 0);
+  assert_time_equals_literal(animation.currentTime, 100 * MS_PER_SEC);
+  assert_false(animation.pending);
+}, 'Updating the playback rate to zero on a finished animation maintains'
+   + ' the current time');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await animation.ready;
+
+  // Get the animation in a state where it has an unresolved current time,
+  // a resolved start time (so it is not 'idle') and but no pending play task.
+  animation.timeline = null;
+  animation.startTime = 0;
+  assert_equals(animation.currentTime, null);
+  assert_equals(animation.playState, 'running');
+
+  // Make the effect end infinite.
+  animation.effect.updateTiming({ endDelay: 1e38 });
+
+  // Now we want to check that when we go to set a negative playback rate we
+  // don't end up throwing an InvalidStateError (which would happen if we ended
+  // up applying the auto-rewind behavior).
+  animation.updatePlaybackRate(-1);
+
+  // Furthermore, we should apply the playback rate immediately since the
+  // current time is unresolved.
+  assert_equals(animation.playbackRate, -1,
+    'We apply the pending playback rate immediately if the current time is ' +
+    'unresolved');
+  assert_false(animation.pending);
+}, 'Updating the negative playback rate with the unresolved current time and'
+   + ' a positive infinite associated effect end should not throw an'
+   + ' exception');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-current-time-of-an-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-current-time-of-an-animation.html
new file mode 100644
index 0000000000..809877345f
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-current-time-of-an-animation.html
@@ -0,0 +1,167 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>Setting the current time of an animation</title>
+<link rel="help"
+  href="https://drafts.csswg.org/web-animations-1/#setting-the-current-time-of-an-animation">
+<script src='/resources/testharness.js'></script>
+<script src='/resources/testharnessreport.js'></script>
+<script src='../../testcommon.js'></script>
+<body>
+<div id='log'></div>
+<script>
+'use strict';
+
+test(t => {
+  const anim = new Animation();
+  assert_equals(anim.playState, 'idle');
+  assert_equals(anim.currentTime, null);
+
+  // This should not throw because the currentTime is already null.
+  anim.currentTime = null;
+}, 'Setting the current time of a pending animation to unresolved does not'
+   + ' throw a TypeError');
+
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await anim.ready;
+
+  assert_greater_than_equal(anim.currentTime, 0);
+  assert_throws_js(TypeError, () => {
+    anim.currentTime = null;
+  });
+}, 'Setting the current time of a playing animation to unresolved throws a'
+   + ' TypeError');
+
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await anim.ready;
+  anim.pause();
+
+  assert_greater_than_equal(anim.currentTime, 0);
+  assert_throws_js(TypeError, () => {
+    anim.currentTime = null;
+  });
+}, 'Setting the current time of a paused animation to unresolved throws a'
+   + ' TypeError');
+
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+
+  assert_throws_js(TypeError, () => {
+    animation.currentTime = CSSNumericValue.parse("30%");
+  });
+  assert_throws_js(TypeError, () => {
+    animation.currentTime = CSSNumericValue.parse("30deg");
+  });
+
+  animation.currentTime = 2000;
+  assert_equals(animation.currentTime, 2000, "Set current time using double");
+
+  animation.currentTime = CSSNumericValue.parse("3000");
+  assert_equals(animation.currentTime, 3000, "Set current time using " +
+    "CSSNumericValue number value");
+
+  animation.currentTime = CSSNumericValue.parse("4000ms");
+  assert_equals(animation.currentTime, 4000, "Set current time using " +
+    "CSSNumericValue milliseconds value");
+
+  animation.currentTime = CSSNumericValue.parse("50s");
+  assert_equals(animation.currentTime, 50000, "Set current time using " +
+    "CSSNumericValue seconds value");
+}, 'Validate different value types that can be used to set current time');
+
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await anim.ready;
+  anim.pause();
+
+  // We should be pause-pending now
+  assert_true(anim.pending);
+  assert_equals(anim.playState, 'paused');
+
+  // Apply a pending playback rate
+  anim.updatePlaybackRate(2);
+  assert_equals(anim.playbackRate, 1);
+
+  // Setting the current time should apply the pending playback rate
+  anim.currentTime = 50 * MS_PER_SEC;
+  assert_equals(anim.playbackRate, 2);
+  assert_false(anim.pending);
+
+  // Sanity check that the current time is preserved
+  assert_time_equals_literal(anim.currentTime, 50 * MS_PER_SEC);
+}, 'Setting the current time of a pausing animation applies a pending playback'
+   + ' rate');
+
+
+// The following tests verify that currentTime can be set outside of the normal
+// bounds of an animation.
+
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await anim.ready;
+
+  anim.currentTime = 200 * MS_PER_SEC;
+  assert_equals(anim.playState, 'finished');
+  assert_time_equals_literal(anim.currentTime, 200 * MS_PER_SEC);
+}, 'Setting the current time after the end with a positive playback rate');
+
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await anim.ready;
+
+  anim.currentTime = -100 * MS_PER_SEC;
+  assert_equals(anim.playState, 'running');
+  assert_time_equals_literal(anim.currentTime, -100 * MS_PER_SEC);
+
+  await waitForAnimationFrames(2);
+  assert_greater_than(anim.currentTime, -100 * MS_PER_SEC);
+}, 'Setting a negative current time with a positive playback rate');
+
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.updatePlaybackRate(-1);
+  await anim.ready;
+
+  anim.currentTime = 200 * MS_PER_SEC;
+  assert_equals(anim.playState, 'running');
+  assert_time_equals_literal(anim.currentTime, 200 * MS_PER_SEC);
+
+  await waitForAnimationFrames(2);
+  assert_less_than(anim.currentTime, 200 * MS_PER_SEC);
+}, 'Setting the current time after the end with a negative playback rate');
+
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.updatePlaybackRate(-1);
+  await anim.ready;
+
+  anim.currentTime = -100 * MS_PER_SEC;
+  assert_equals(anim.playState, 'finished');
+  assert_time_equals_literal(anim.currentTime, -100 * MS_PER_SEC);
+}, 'Setting a negative current time with a negative playback rate');
+
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.updatePlaybackRate(0);
+  await anim.ready;
+
+  // An animation with a playback rate of zero is never in the finished state
+  // even if currentTime is outside the normal range of [0, effect end].
+  anim.currentTime = 200 * MS_PER_SEC;
+  assert_equals(anim.playState, 'running');
+  assert_time_equals_literal(anim.currentTime, 200 * MS_PER_SEC);
+  await waitForAnimationFrames(2);
+  assert_time_equals_literal(anim.currentTime, 200 * MS_PER_SEC);
+
+  anim.currentTime = -200 * MS_PER_SEC;
+  assert_equals(anim.playState, 'running');
+  assert_time_equals_literal(anim.currentTime, -200 * MS_PER_SEC);
+  await waitForAnimationFrames(2);
+  assert_time_equals_literal(anim.currentTime, -200 * MS_PER_SEC);
+
+}, 'Setting the current time on an animation with a zero playback rate');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-playback-rate-of-an-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-playback-rate-of-an-animation.html
new file mode 100644
index 0000000000..a1f9e4f3ac
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-playback-rate-of-an-animation.html
@@ -0,0 +1,110 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Setting the playback rate of an animation</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#setting-the-playback-rate-of-an-animation">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.playbackRate = 2;
+  await animation.ready;
+
+  const previousAnimationCurrentTime = animation.currentTime;
+  const previousTimelineCurrentTime = animation.timeline.currentTime;
+
+  await waitForAnimationFrames(1);
+
+  const animationCurrentTimeDifference =
+    animation.currentTime - previousAnimationCurrentTime;
+  const timelineCurrentTimeDifference =
+    animation.timeline.currentTime - previousTimelineCurrentTime;
+
+  assert_times_equal(
+    animationCurrentTimeDifference,
+    timelineCurrentTimeDifference * animation.playbackRate,
+    'The current time should increase two times faster than timeline'
+  );
+}, 'The playback rate affects the rate of progress of the current time');
+
+test(t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.currentTime = 50 * MS_PER_SEC;
+  animation.playbackRate = 2;
+  assert_time_equals_literal(animation.currentTime, 50 * MS_PER_SEC);
+}, 'Setting the playback rate while play-pending preserves the current time');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.currentTime = 50 * MS_PER_SEC;
+  await animation.ready;
+  animation.playbackRate = 2;
+  assert_greater_than_equal(animation.currentTime, 50 * MS_PER_SEC);
+  assert_less_than(animation.currentTime, 100 * MS_PER_SEC);
+}, 'Setting the playback rate while playing preserves the current time');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.currentTime = 50 * MS_PER_SEC;
+  animation.updatePlaybackRate(2);
+  animation.playbackRate = 1;
+  await animation.ready;
+  assert_equals(animation.playbackRate, 1);
+}, 'Setting the playback rate should clear any pending playback rate');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.currentTime = 50 * MS_PER_SEC;
+  animation.pause();
+  await animation.ready;
+  animation.playbackRate = 2;
+  // Ensure that the animation remains paused and current time is preserved.
+  assert_equals(animation.playState, 'paused');
+  assert_time_equals_literal(animation.currentTime, 50 * MS_PER_SEC);
+}, 'Setting the playback rate while paused preserves the current time and '
+    + 'state');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.currentTime = 150 * MS_PER_SEC;
+  await animation.ready;
+  animation.playbackRate = 2;
+  // Ensure that current time is preserved and does not snap to the effect end
+  // time.
+  assert_equals(animation.playState, 'finished');
+  assert_time_equals_literal(animation.currentTime, 150 * MS_PER_SEC);
+}, 'Setting the playback rate while finished preserves the current time');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.currentTime = 150 * MS_PER_SEC;
+  await animation.ready;
+  assert_equals(animation.playState, 'finished');
+  animation.playbackRate = -1;
+  // Ensure that current time does not snap to the effect end time and that the
+  // animation resumes playing.
+  assert_equals(animation.playState, 'running');
+  assert_time_equals_literal(animation.currentTime, 150 * MS_PER_SEC);
+  await waitForAnimationFrames(2);
+  assert_less_than(animation.currentTime, 150 * MS_PER_SEC);
+}, 'Reversing the playback rate while finished restarts the animation');
+
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await animation.ready;
+  animation.currentTime = 50 * MS_PER_SEC;
+  animation.playbackRate = 0;
+  // Ensure that current time does not drift.
+  assert_equals(animation.playState, 'running');
+  await waitForAnimationFrames(2);
+  assert_time_equals_literal(animation.currentTime, 50 * MS_PER_SEC);
+}, 'Setting a zero playback rate while running preserves the current time');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-start-time-of-an-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-start-time-of-an-animation.html
new file mode 100644
index 0000000000..fee3f1e0de
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-start-time-of-an-animation.html
@@ -0,0 +1,329 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Setting the start time of an animation</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#setting-the-start-time-of-an-animation">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<script src="../../resources/timing-override.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  null);
+
+  assert_throws_js(TypeError, () => {
+    animation.startTime = CSSNumericValue.parse("30%");
+  });
+  assert_throws_js(TypeError, () => {
+    animation.startTime = CSSNumericValue.parse("30deg");
+  });
+
+  animation.startTime = 2000;
+  assert_equals(animation.startTime, 2000, "Set start time using double");
+
+  animation.startTime = CSSNumericValue.parse("3000");
+  assert_equals(animation.startTime, 3000, "Set start time using " +
+    "CSSNumericValue number value");
+
+  animation.startTime = CSSNumericValue.parse("4000ms");
+  assert_equals(animation.startTime, 4000, "Set start time using " +
+    "CSSNumericValue milliseconds value");
+
+  animation.startTime = CSSNumericValue.parse("50s");
+  assert_equals(animation.startTime, 50000, "Set start time using " +
+    "CSSNumericValue seconds value");
+}, 'Validate different value types that can be used to set start time');
+
+test(t => {
+  // It should only be possible to set *either* the start time or the current
+  // time for an animation that does not have an active timeline.
+
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  null);
+
+  assert_equals(animation.currentTime, null, 'Intial current time');
+  assert_equals(animation.startTime, null, 'Intial start time');
+
+  animation.currentTime = 1000;
+  assert_equals(animation.currentTime, 1000,
+                'Setting the current time succeeds');
+  assert_equals(animation.startTime, null,
+                'Start time remains null after setting current time');
+
+  animation.startTime = 1000;
+  assert_equals(animation.startTime, 1000,
+                'Setting the start time succeeds');
+  assert_equals(animation.currentTime, null,
+                'Setting the start time clears the current time');
+
+  animation.startTime = null;
+  assert_equals(animation.startTime, null,
+                'Setting the start time to an unresolved time succeeds');
+  assert_equals(animation.currentTime, null, 'The current time is unaffected');
+
+}, 'Setting the start time of an animation without an active timeline');
+
+test(t => {
+  // Setting an unresolved start time on an animation without an active
+  // timeline should not clear the current time.
+
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  null);
+
+  assert_equals(animation.currentTime, null, 'Intial current time');
+  assert_equals(animation.startTime, null, 'Intial start time');
+
+  animation.currentTime = 1000;
+  assert_equals(animation.currentTime, 1000,
+                'Setting the current time succeeds');
+  assert_equals(animation.startTime, null,
+                'Start time remains null after setting current time');
+
+  animation.startTime = null;
+  assert_equals(animation.startTime, null, 'Start time remains unresolved');
+  assert_equals(animation.currentTime, 1000, 'Current time is unaffected');
+
+}, 'Setting an unresolved start time an animation without an active timeline'
+   + ' does not clear the current time');
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+
+  // So long as a hold time is set, querying the current time will return
+  // the hold time.
+
+  // Since the start time is unresolved at this point, setting the current time
+  // will set the hold time
+  animation.currentTime = 1000;
+  assert_equals(animation.currentTime, 1000,
+                'The current time is calculated from the hold time');
+
+  // If we set the start time, however, we should clear the hold time.
+  animation.startTime = document.timeline.currentTime - 2000;
+  assert_time_equals_literal(animation.currentTime, 2000,
+                             'The current time is calculated from the start'
+                             + ' time, not the hold time');
+
+  // Sanity check
+  assert_equals(animation.playState, 'running',
+                'Animation reports it is running after setting a resolved'
+                + ' start time');
+}, 'Setting the start time clears the hold time');
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+
+  // Set up a running animation (i.e. both start time and current time
+  // are resolved).
+  animation.startTime = document.timeline.currentTime - 1000;
+  assert_equals(animation.playState, 'running');
+  assert_time_equals_literal(animation.currentTime, 1000,
+                             'Current time is resolved for a running animation');
+
+  // Clear start time
+  animation.startTime = null;
+  assert_time_equals_literal(animation.currentTime, 1000,
+                             'Hold time is set after start time is made'
+                             + ' unresolved');
+  assert_equals(animation.playState, 'paused',
+                'Animation reports it is paused after setting an unresolved'
+                + ' start time');
+}, 'Setting an unresolved start time sets the hold time');
+
+promise_test(async t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+
+  let readyPromiseCallbackCalled = false;
+  animation.ready.then(() => { readyPromiseCallbackCalled = true; } );
+
+  // Put the animation in the play-pending state
+  animation.play();
+
+  // Sanity check
+  assert_true(animation.pending && animation.playState === 'running',
+              'Animation is in play-pending state');
+
+  // Setting the start time should resolve the 'ready' promise, i.e.
+  // it should schedule a microtask to run the promise callbacks.
+  animation.startTime = document.timeline.currentTime;
+  assert_false(readyPromiseCallbackCalled,
+               'Ready promise callback is not called synchronously');
+
+  // If we schedule another microtask then it should run immediately after
+  // the ready promise resolution microtask.
+  await Promise.resolve();
+  assert_true(readyPromiseCallbackCalled,
+              'Ready promise callback called after setting startTime');
+}, 'Setting the start time resolves a pending ready promise');
+
+promise_test(async t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+
+  let readyPromiseCallbackCalled = false;
+  animation.ready.then(() => { readyPromiseCallbackCalled = true; } );
+
+  // Put the animation in the pause-pending state
+  animation.startTime = document.timeline.currentTime;
+  animation.pause();
+
+  // Sanity check
+  assert_true(animation.pending && animation.playState === 'paused',
+              'Animation is in pause-pending state');
+
+  // Setting the start time should resolve the 'ready' promise although
+  // the resolution callbacks when be run in a separate microtask.
+  animation.startTime = null;
+  assert_false(readyPromiseCallbackCalled,
+               'Ready promise callback is not called synchronously');
+
+  await Promise.resolve();
+  assert_true(readyPromiseCallbackCalled,
+              'Ready promise callback called after setting startTime');
+}, 'Setting the start time resolves a pending pause task');
+
+promise_test(async t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+
+  // Put the animation in the play-pending state
+  animation.play();
+
+  // Sanity check
+  assert_true(animation.pending, 'Animation is pending');
+  assert_equals(animation.playState, 'running',
+                'Animation is play-pending');
+  assert_equals(animation.startTime, null, 'Start time is null');
+
+  // Even though the startTime is already null, setting it to the same value
+  // should still cancel the pending task.
+  animation.startTime = null;
+  assert_false(animation.pending, 'Animation is no longer pending');
+  assert_equals(animation.playState, 'paused', 'Animation is paused');
+}, 'Setting an unresolved start time on a play-pending animation makes it'
+   + ' paused');
+
+promise_test(async t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+
+  // Set start time such that the current time is past the end time
+  animation.startTime = document.timeline.currentTime
+                        - 110 * MS_PER_SEC;
+  assert_equals(animation.playState, 'finished',
+                'Seeked to finished state using the startTime');
+
+  // If the 'did seek' flag is true, the current time should be greater than
+  // the effect end.
+  assert_greater_than(animation.currentTime,
+                      animation.effect.getComputedTiming().endTime,
+                      'Setting the start time updated the finished state with'
+                      + ' the \'did seek\' flag set to true');
+
+  // Furthermore, that time should persist if we have correctly updated
+  // the hold time
+  const finishedCurrentTime = animation.currentTime;
+  await waitForAnimationFrames(1);
+  assert_equals(animation.currentTime, finishedCurrentTime,
+                'Current time does not change after seeking past the effect'
+                + ' end time by setting the current time');
+}, 'Setting the start time updates the finished state');
+
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+
+  // We should be play-pending now
+  assert_true(anim.pending);
+  assert_equals(anim.playState, 'running');
+
+  // Apply a pending playback rate
+  anim.updatePlaybackRate(2);
+  assert_equals(anim.playbackRate, 1);
+  assert_true(anim.pending);
+
+  // Setting the start time should apply the pending playback rate
+  anim.startTime = anim.timeline.currentTime - 25 * MS_PER_SEC;
+  assert_equals(anim.playbackRate, 2);
+  assert_false(anim.pending);
+
+  // Sanity check that the start time is preserved and current time is
+  // calculated using the new playback rate
+  assert_times_equal(anim.startTime,
+                     anim.timeline.currentTime - 25 * MS_PER_SEC);
+  assert_time_equals_literal(anim.currentTime, 50 * MS_PER_SEC);
+}, 'Setting the start time of a play-pending animation applies a pending playback rate');
+
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await anim.ready;
+
+  // We should be running now
+  assert_false(anim.pending);
+  assert_equals(anim.playState, 'running');
+
+  // Apply a pending playback rate
+  anim.updatePlaybackRate(2);
+  assert_equals(anim.playbackRate, 1);
+  assert_true(anim.pending);
+
+  // Setting the start time should apply the pending playback rate
+  anim.startTime = anim.timeline.currentTime - 25 * MS_PER_SEC;
+  assert_equals(anim.playbackRate, 2);
+  assert_false(anim.pending);
+
+  // Sanity check that the start time is preserved and current time is
+  // calculated using the new playback rate
+  assert_times_equal(anim.startTime,
+                     anim.timeline.currentTime - 25 * MS_PER_SEC);
+  assert_time_equals_literal(parseInt(anim.currentTime.toPrecision(5), 10), 50 * MS_PER_SEC);
+}, 'Setting the start time of a playing animation applies a pending playback rate');
+
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await anim.ready;
+  assert_equals(anim.playState, 'running');
+
+  // Setting the start time updates the finished state. The hold time is not
+  // constrained by the effect end time.
+  anim.startTime = -200 * MS_PER_SEC;
+  assert_equals(anim.playState, 'finished');
+
+  assert_times_equal(anim.currentTime,
+                     document.timeline.currentTime + 200 * MS_PER_SEC);
+}, 'Setting the start time on a running animation updates the play state');
+
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await anim.ready;
+
+  // Setting the start time updates the finished state. The hold time is not
+  // constrained by the normal range of the animation time.
+  anim.currentTime = 100 * MS_PER_SEC;
+  assert_equals(anim.playState, 'finished');
+  anim.playbackRate = -1;
+  assert_equals(anim.playState, 'running');
+  anim.startTime = -200 * MS_PER_SEC;
+  assert_equals(anim.playState, 'finished');
+  assert_times_equal(anim.currentTime,
+                     -document.timeline.currentTime - 200 * MS_PER_SEC);
+}, 'Setting the start time on a reverse running animation updates the play '
+   + 'state');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-target-effect-of-an-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-target-effect-of-an-animation.html
new file mode 100644
index 0000000000..60ea1850fc
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-target-effect-of-an-animation.html
@@ -0,0 +1,129 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Setting the target effect of an animation</title>
+<link rel='help' href='https://drafts.csswg.org/web-animations/#setting-the-target-effect'>
+<script src='/resources/testharness.js'></script>
+<script src='/resources/testharnessreport.js'></script>
+<script src='../../testcommon.js'></script>
+<body>
+<div id='log'></div>
+<script>
+'use strict';
+
+promise_test(t => {
+  const anim = createDiv(t).animate({ marginLeft: [ '0px', '100px' ] },
+                                    100 * MS_PER_SEC);
+  assert_true(anim.pending);
+
+  const originalReadyPromise = anim.ready.catch(err => {
+    assert_unreached('Original ready promise should not be rejected');
+  });
+
+  anim.effect = null;
+  assert_equals(anim.playState, 'finished');
+  assert_true(anim.pending);
+
+  return originalReadyPromise;
+}, 'If new effect is null and old effect is not null the animation becomes'
+   + ' finish-pending');
+
+promise_test(async t => {
+  const anim = new Animation();
+  anim.pause();
+  assert_true(anim.pending);
+
+  anim.effect = new KeyframeEffect(createDiv(t),
+                                   { marginLeft: [ '0px', '100px' ] },
+                                   100 * MS_PER_SEC);
+  assert_true(anim.pending);
+  await anim.ready;
+
+  assert_false(anim.pending);
+  assert_equals(anim.playState, 'paused');
+}, 'If animation has a pending pause task, reschedule that task to run ' +
+   'as soon as animation is ready.');
+
+promise_test(async t => {
+  const anim = new Animation();
+  anim.play();
+  assert_true(anim.pending);
+
+  anim.effect = new KeyframeEffect(createDiv(t),
+                                   { marginLeft: [ '0px', '100px' ] },
+                                   100 * MS_PER_SEC);
+  assert_true(anim.pending);
+  await anim.ready;
+
+  assert_false(anim.pending);
+  assert_equals(anim.playState, 'running');
+}, 'If animation has a pending play task, reschedule that task to run ' +
+   'as soon as animation is ready to play new effect.');
+
+promise_test(async t => {
+  const anim = createDiv(t).animate({ marginLeft: [ '0px', '100px' ] },
+                                    100 * MS_PER_SEC);
+  assert_equals(anim.playState, 'running');
+  assert_true(anim.pending);
+
+  const originalEffect = anim.effect;
+  const originalReadyPromise = anim.ready;
+
+  anim.effect = null;
+  assert_equals(anim.playState, 'finished');
+  assert_true(anim.pending);
+
+  anim.effect = originalEffect;
+  assert_equals(anim.playState, 'running');
+  assert_true(anim.pending);
+
+  await originalReadyPromise;
+
+  assert_equals(anim.playState, 'running');
+  assert_false(anim.pending);
+}, 'The pending play task should be rescheduled even after temporarily setting'
+   + ' the effect to null');
+
+promise_test(async t => {
+  const animA = createDiv(t).animate({ marginLeft: [ '0px', '100px' ] },
+                                     100 * MS_PER_SEC);
+  const animB = new Animation();
+
+  await animA.ready;
+
+  animB.effect = animA.effect;
+  assert_equals(animA.effect, null);
+  assert_equals(animA.playState, 'finished');
+}, 'When setting the effect of an animation to the effect of an existing ' +
+   'animation, the existing animation\'s target effect should be set to null.');
+
+test(t => {
+  const animA = createDiv(t).animate({ marginLeft: [ '0px', '100px' ] },
+                                     100 * MS_PER_SEC);
+  const animB = new Animation();
+  const effect = animA.effect;
+  animA.currentTime = 50 * MS_PER_SEC;
+  animB.currentTime = 20 * MS_PER_SEC;
+  assert_equals(effect.getComputedTiming().progress, 0.5,
+                'Original timing comes from first animation');
+  animB.effect = effect;
+  assert_equals(effect.getComputedTiming().progress, 0.2,
+                'After setting the effect on a different animation, ' +
+                'it uses the new animation\'s timing');
+}, 'After setting the target effect of animation to the target effect of an ' +
+   'existing animation, the target effect\'s timing is updated to reflect ' +
+   'the current time of the new animation.');
+
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.updatePlaybackRate(2);
+  assert_equals(anim.playbackRate, 1);
+
+  anim.effect = null;
+  await anim.ready;
+
+  assert_equals(anim.playbackRate, 2);
+}, 'Setting the target effect to null causes a pending playback rate to be'
+   + ' applied');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-timeline-of-an-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-timeline-of-an-animation.html
new file mode 100644
index 0000000000..d4f802152c
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/setting-the-timeline-of-an-animation.html
@@ -0,0 +1,255 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Setting the timeline of an animation</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#setting-the-timeline">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+// ---------------------------------------------------------------------
+//
+// Tests from no timeline to timeline
+//
+// ---------------------------------------------------------------------
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  null);
+  animation.currentTime = 50 * MS_PER_SEC;
+  assert_equals(animation.playState, 'paused');
+
+  animation.timeline = document.timeline;
+
+  assert_equals(animation.playState, 'paused');
+  assert_time_equals_literal(animation.currentTime, 50 * MS_PER_SEC);
+}, 'After setting timeline on paused animation it is still paused');
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  null);
+  animation.currentTime = 200 * MS_PER_SEC;
+  assert_equals(animation.playState, 'paused');
+
+  animation.timeline = document.timeline;
+
+  assert_equals(animation.playState, 'paused');
+  assert_time_equals_literal(animation.currentTime, 200 * MS_PER_SEC);
+}, 'After setting timeline on animation paused outside active interval'
+   + ' it is still paused');
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  null);
+  assert_equals(animation.playState, 'idle');
+
+  animation.timeline = document.timeline;
+
+  assert_equals(animation.playState, 'idle');
+}, 'After setting timeline on an idle animation without a start time'
+   + ' it is still idle');
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  null);
+  animation.startTime = document.timeline.currentTime;
+  assert_equals(animation.playState, 'running');
+
+  animation.timeline = document.timeline;
+
+  assert_equals(animation.playState, 'running');
+}, 'After transitioning from a null timeline on an animation with a start time'
+   + ' it is still running');
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  null);
+  animation.startTime = document.timeline.currentTime - 200 * MS_PER_SEC;
+  assert_equals(animation.playState, 'running');
+
+  animation.timeline = document.timeline;
+
+  assert_equals(animation.playState, 'finished');
+}, 'After transitioning from a null timeline on an animation with a ' +
+   'sufficiently ancient start time it is finished');
+
+promise_test(async t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  null);
+  animation.play();
+  assert_true(animation.pending && animation.playState === 'running',
+              'Animation is initially play-pending');
+
+  animation.timeline = document.timeline;
+
+  assert_true(animation.pending && animation.playState === 'running',
+              'Animation is still play-pending after setting timeline');
+
+  await animation.ready;
+  assert_true(!animation.pending && animation.playState === 'running',
+              'Animation plays after it finishes pending');
+}, 'After setting timeline on a play-pending animation it begins playing'
+   + ' after pending');
+
+promise_test(async t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  null);
+  animation.startTime = document.timeline.currentTime;
+  animation.pause();
+  animation.timeline = null;
+  assert_true(animation.pending && animation.playState === 'paused',
+              'Animation is initially pause-pending');
+
+  animation.timeline = document.timeline;
+
+  assert_true(animation.pending && animation.playState === 'paused',
+              'Animation is still pause-pending after setting timeline');
+
+  await animation.ready;
+  assert_true(!animation.pending && animation.playState === 'paused',
+              'Animation pauses after it finishes pending');
+}, 'After setting timeline on a pause-pending animation it becomes paused'
+   + ' after pending');
+
+// ---------------------------------------------------------------------
+//
+// Tests from timeline to no timeline
+//
+// ---------------------------------------------------------------------
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+  animation.currentTime = 50 * MS_PER_SEC;
+  assert_false(animation.pending);
+  assert_equals(animation.playState, 'paused');
+
+  animation.timeline = null;
+
+  assert_false(animation.pending);
+  assert_equals(animation.playState, 'paused');
+  assert_time_equals_literal(animation.currentTime, 50 * MS_PER_SEC);
+}, 'After clearing timeline on paused animation it is still paused');
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+  const initialStartTime = document.timeline.currentTime - 200 * MS_PER_SEC;
+  animation.startTime = initialStartTime;
+  assert_equals(animation.playState, 'finished');
+
+  animation.timeline = null;
+
+  assert_equals(animation.playState, 'running');
+  assert_times_equal(animation.startTime, initialStartTime);
+}, 'After clearing timeline on finished animation it is running');
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+  const initialStartTime = document.timeline.currentTime - 50 * MS_PER_SEC;
+  animation.startTime = initialStartTime;
+  assert_equals(animation.playState, 'running');
+
+  animation.timeline = null;
+
+  assert_equals(animation.playState, 'running');
+  assert_times_equal(animation.startTime, initialStartTime);
+}, 'After clearing timeline on running animation it is still running');
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+  assert_equals(animation.playState, 'idle');
+
+  animation.timeline = null;
+
+  assert_equals(animation.playState, 'idle');
+  assert_equals(animation.startTime, null);
+}, 'After clearing timeline on idle animation it is still idle');
+
+test(t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  assert_true(animation.pending && animation.playState === 'running');
+
+  animation.timeline = null;
+
+  assert_true(animation.pending && animation.playState === 'running');
+}, 'After clearing timeline on play-pending animation it is still pending');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  assert_true(animation.pending && animation.playState === 'running');
+
+  animation.timeline = null;
+  animation.timeline = document.timeline;
+
+  assert_true(animation.pending && animation.playState === 'running');
+  await animation.ready;
+  assert_true(!animation.pending && animation.playState === 'running');
+}, 'After clearing and re-setting timeline on play-pending animation it'
+   + ' begins to play');
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+  animation.startTime = document.timeline.currentTime;
+  animation.pause();
+  assert_true(animation.pending && animation.playState === 'paused');
+
+  animation.timeline = null;
+
+  assert_true(animation.pending && animation.playState === 'paused');
+}, 'After clearing timeline on a pause-pending animation it is still pending');
+
+promise_test(async t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+  animation.startTime = document.timeline.currentTime;
+  animation.pause();
+  assert_true(animation.pending && animation.playState === 'paused');
+
+  animation.timeline = null;
+  animation.timeline = document.timeline;
+
+  assert_true(animation.pending && animation.playState === 'paused');
+  await animation.ready;
+  assert_true(!animation.pending && animation.playState === 'paused');
+}, 'After clearing and re-setting timeline on a pause-pending animation it'
+   + ' completes pausing');
+
+promise_test(async t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+  const initialStartTime = document.timeline.currentTime - 50 * MS_PER_SEC;
+  animation.startTime = initialStartTime;
+  animation.pause();
+  animation.play();
+
+  animation.timeline = null;
+  animation.timeline = document.timeline;
+
+  await animation.ready;
+  assert_times_equal(animation.startTime, initialStartTime);
+}, 'After clearing and re-setting timeline on an animation in the middle of'
+   + ' an aborted pause, it continues playing using the same start time');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/sync-start-times-ref.html b/testing/web-platform/tests/web-animations/timing-model/animations/sync-start-times-ref.html
new file mode 100644
index 0000000000..fc843a132f
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/sync-start-times-ref.html
@@ -0,0 +1,20 @@
+
+<!DOCTYPE html>
+<meta charset="UTF-8">
+<title>Reference for sync start times</title>
+<style>
+  #notes {
+    position: absolute;
+    left: 0px;
+    top: 100px;
+  }
+</style>
+
+<body>
+  <p id="notes">
+    This test creates a pair of animations, starts the first animation and then
+    syncs the second animation to align with the first. The test passes if the
+    box associated with the first animation is completely occluded by the
+    second.
+  </p>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/sync-start-times.html b/testing/web-platform/tests/web-animations/timing-model/animations/sync-start-times.html
new file mode 100644
index 0000000000..e9ef6762ea
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/sync-start-times.html
@@ -0,0 +1,72 @@
+
+<!DOCTYPE html>
+<html class="reftest-wait">
+<meta charset="UTF-8">
+<title>sync start times</title>
+<link rel="match" href="sync-start-times-ref.html">
+<script src="/common/reftest-wait.js"></script>
+<style>
+  #box-1, #box-2 {
+    border: 1px solid white;
+    height: 40px;
+    left: 40px;
+    position: absolute;
+    top: 40px;
+    width: 40px;
+    /* To ensure Chrome to render the two boxes (one actively
+       animating and the other not) with the same subpixel offset
+       when there is subpixel translation during animation. */
+    will-change: transform;
+  }
+  #box-1 {
+    background: blue;
+    z-index: 1;
+  }
+  #box-2 {
+    background: white;
+    z-index: 2;
+  }
+  #notes {
+    position: absolute;
+    left: 0px;
+    top: 100px;
+  }
+</style>
+
+<body>
+  <div id="box-1"></div>
+  <div id="box-2"></div>
+  <p id="notes">
+    This test creates a pair of animations, starts the first animation and then
+    syncs the second animation to align with the first. The test passes if the
+    box associated with the first animation is completely occluded by the
+    second.
+  </p>
+</body>
+<script>
+  onload = function() {
+    function createAnimation(elementId) {
+      const elem = document.getElementById(elementId);
+      const keyframes = [
+        { transform: 'translateX(0px)' },
+        { transform: 'translateX(200px)' }
+      ];
+      const anim = elem.animate(keyframes, { duration: 1000 });
+      anim.pause();
+      return anim;
+    };
+
+    const anim1 = createAnimation('box-1');
+    const anim2 = createAnimation('box-2');
+
+    anim1.currentTime = 500;
+    anim1.play();
+
+    anim1.ready.then(() => {
+      anim2.startTime = anim1.startTime;
+      requestAnimationFrame(() => {
+        takeScreenshot();
+      });
+    });
+  };
+</script>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/the-current-time-of-an-animation.html b/testing/web-platform/tests/web-animations/timing-model/animations/the-current-time-of-an-animation.html
new file mode 100644
index 0000000000..77a6b716d2
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/the-current-time-of-an-animation.html
@@ -0,0 +1,75 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>The current time of an animation</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#the-current-time-of-an-animation">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<script src="../../resources/timing-override.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+
+  animation.play();
+  assert_equals(animation.currentTime, 0,
+    'Current time returns the hold time set when entering the play-pending ' +
+    'state');
+}, 'The current time returns the hold time when set');
+
+promise_test(async t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  null);
+
+  await animation.ready;
+  assert_equals(animation.currentTime, null);
+}, 'The current time is unresolved when there is no associated timeline ' +
+   '(and no hold time is set)');
+
+// FIXME: Test that the current time is unresolved when we have an inactive
+// timeline if we find a way of creating an inactive timeline!
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+
+  animation.startTime = null;
+  assert_equals(animation.currentTime, null);
+}, 'The current time is unresolved when the start time is unresolved ' +
+   '(and no hold time is set)');
+
+test(t => {
+  const animation =
+    new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
+                  document.timeline);
+
+  animation.playbackRate = 2;
+  animation.startTime = document.timeline.currentTime - 25 * MS_PER_SEC;
+
+  const timelineTime = document.timeline.currentTime;
+  const startTime = animation.startTime;
+  const playbackRate = animation.playbackRate;
+  assert_times_equal(animation.currentTime,
+                     (timelineTime - startTime) * playbackRate,
+                     'Animation has a unresolved start time');
+}, 'The current time is calculated from the timeline time, start time and ' +
+   'playback rate');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  animation.playbackRate = 0;
+
+  await animation.ready;
+  await waitForAnimationFrames(1);
+  assert_time_equals_literal(animation.currentTime, 0);
+}, 'The current time does not progress if playback rate is 0');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-fast-ref.html b/testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-fast-ref.html
new file mode 100644
index 0000000000..e996815da8
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-fast-ref.html
@@ -0,0 +1,17 @@
+<!DOCTYPE html>
+<title>Reference for update playback rate zero</title>
+<style>
+  #box {
+    background: green;
+    height: 40px;
+    width: 40px;
+  }
+</style>
+<body>
+  <div id="box"></div>
+  <p id="notes">
+    This test creates a running animation and changes its playback rate
+    part way through. If the box remains red when the screenshot is captured
+    the test fails.
+  </p>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-fast.html b/testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-fast.html
new file mode 100644
index 0000000000..c3df1c1bf0
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-fast.html
@@ -0,0 +1,52 @@
+
+<!DOCTYPE html>
+<html class="reftest-wait">
+<meta charset="UTF-8">
+<title>Update playback rate zero</title>
+<link rel="match" href="update-playback-rate-fast-ref.html">
+<script src="/common/reftest-wait.js"></script>
+<script src="../../testcommon.js"></script>
+<style>
+  #box {
+    background: green;
+    height: 40px;
+    width: 40px;
+  }
+</style>
+<body>
+  <div id="box"></div>
+  <p id="notes">
+    This test creates a running animation and changes its playback rate
+    part way through. If the box remains red when the screenshot is captured
+    the test fails.
+  </p>
+</body>
+<script>
+  onload = async function() {
+    const box = document.getElementById('box');
+    const duration = 2000;
+    const anim =
+        box.animate({ bacground: [ 'red', 'green' ] },
+                    { duration: duration, easing: 'steps(2, jump-none)' });
+    anim.ready.then(() => {
+      const startTime = anim.timeline.currentTime;
+      waitForAnimationFrames(2).then(() => {
+        anim.updatePlaybackRate(2);
+        anim.ready.then(() => {
+          const updateTime = anim.timeline.currentTime;
+          const baseProgress = (updateTime - startTime) / duration;
+          const checkIfDone = () => {
+            const progress =
+               2 * (anim.timeline.currentTime - updateTime) / duration +
+               baseProgress;
+            if (progress > 0.5)
+              takeScreenshot();
+            else
+              requestAnimationFrame(checkIfDone);
+          };
+          requestAnimationFrame(checkIfDone);
+        });
+      });
+    });
+  };
+</script>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-zero-ref.html b/testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-zero-ref.html
new file mode 100644
index 0000000000..399fd5ce7d
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-zero-ref.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<title>Reference for update playback rate zero</title>
+<style>
+  #box {
+    background: green;
+    height: 40px;
+    width: 40px;
+  }
+</style>
+<body>
+  <div id="box"></div>
+  <p id="notes">
+    This test creates a running animation and halts its playback rate
+    part way through. If the box transitions to red the test fails.
+  </p>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-zero.html b/testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-zero.html
new file mode 100644
index 0000000000..db1544ee92
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/update-playback-rate-zero.html
@@ -0,0 +1,46 @@
+
+<!DOCTYPE html>
+<html class="reftest-wait">
+<meta charset="UTF-8">
+<title>Update playback rate zero</title>
+<link rel="match" href="update-playback-rate-zero-ref.html">
+<script src="/common/reftest-wait.js"></script>
+<script src="../../testcommon.js"></script>
+<style>
+  #box {
+    background: green;
+    height: 40px;
+    width: 40px;
+  }
+</style>
+<body>
+  <div id="box"></div>
+  <p id="notes">
+    This test creates a running animation and halts its playback rate
+    part way through. If the box transitions to red the test fails.
+  </p>
+</body>
+<script>
+  onload = async function() {
+    const box = document.getElementById('box');
+    const duration = 2000;
+    const anim =
+        box.animate({ bacground: [ 'green', 'red' ] },
+                    { duration: duration, easing: 'steps(2, jump-none)' });
+    anim.ready.then(() => {
+      const startTime = anim.timeline.currentTime;
+      waitForAnimationFrames(2).then(() => {
+        anim.updatePlaybackRate(0);
+        anim.ready.then(() => {
+          const checkIfDone = () => {
+            if (anim.timeline.currentTime - startTime > duration / 2)
+              takeScreenshot();
+            else
+              requestAnimationFrame(checkIfDone);
+          };
+          requestAnimationFrame(checkIfDone);
+        });
+      });
+    });
+  };
+</script>
diff --git a/testing/web-platform/tests/web-animations/timing-model/animations/updating-the-finished-state.html b/testing/web-platform/tests/web-animations/timing-model/animations/updating-the-finished-state.html
new file mode 100644
index 0000000000..4d3cc7950b
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/animations/updating-the-finished-state.html
@@ -0,0 +1,457 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Updating the finished state</title>
+<meta name="timeout" content="long">
+<link rel="help" href="https://drafts.csswg.org/web-animations/#updating-the-finished-state">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<body>
+<div id="log"></div>
+<script>
+'use strict';
+
+// --------------------------------------------------------------------
+//
+// TESTS FOR UPDATING THE HOLD TIME
+//
+// --------------------------------------------------------------------
+
+// CASE 1: playback rate > 0 and current time >= target effect end
+// (Also the start time is resolved and there is pending task)
+
+// Did seek = false
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+
+  // Here and in the following tests we wait until ready resolves as
+  // otherwise we don't have a resolved start time. We test the case
+  // where the start time is unresolved in a subsequent test.
+  await anim.ready;
+
+  // Seek to 1ms before the target end and then wait 1ms
+  anim.currentTime = 100 * MS_PER_SEC - 1;
+  await waitForAnimationFramesWithDelay(1);
+
+  assert_equals(anim.currentTime, 100 * MS_PER_SEC,
+                'Hold time is set to target end clamping current time');
+}, 'Updating the finished state when playing past end');
+
+// Did seek = true
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+
+  await anim.ready;
+
+  anim.currentTime = 200 * MS_PER_SEC;
+  await waitForNextFrame();
+
+  assert_equals(anim.currentTime, 200 * MS_PER_SEC,
+                'Hold time is set so current time should NOT change');
+}, 'Updating the finished state when seeking past end');
+
+// Test current time == target end
+//
+// We can't really write a test for current time == target end with
+// did seek = false since that would imply setting up an animation where
+// the next animation frame time happens to exactly align with the target end.
+//
+// Fortunately, we don't need to test that case since even if the implementation
+// fails to set the hold time on such a tick, it should be mostly unobservable
+// (on the subsequent tick the hold time will be set to the same value anyway).
+
+// Did seek = true
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  await anim.ready;
+
+  anim.currentTime = 100 * MS_PER_SEC;
+  await waitForNextFrame();
+
+  assert_equals(anim.currentTime, 100 * MS_PER_SEC,
+                'Hold time is set so current time should NOT change');
+}, 'Updating the finished state when seeking exactly to end');
+
+
+// CASE 2: playback rate < 0 and current time <= 0
+// (Also the start time is resolved and there is pending task)
+
+// Did seek = false
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.playbackRate = -1;
+  anim.play(); // Make sure animation is not initially finished
+
+  await anim.ready;
+
+  // Seek to 1ms before 0 and then wait 1ms
+  anim.currentTime = 1;
+  await waitForAnimationFramesWithDelay(1);
+
+  assert_equals(anim.currentTime, 0 * MS_PER_SEC,
+                'Hold time is set to zero clamping current time');
+}, 'Updating the finished state when playing in reverse past zero');
+
+// Did seek = true
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.playbackRate = -1;
+  anim.play();
+
+  await anim.ready;
+
+  anim.currentTime = -100 * MS_PER_SEC;
+  await waitForNextFrame();
+
+  assert_equals(anim.currentTime, -100 * MS_PER_SEC,
+                'Hold time is set so current time should NOT change');
+}, 'Updating the finished state when seeking a reversed animation past zero');
+
+// As before, it's difficult to test current time == 0 for did seek = false but
+// it doesn't really matter.
+
+// Did seek = true
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.playbackRate = -1;
+  anim.play();
+  await anim.ready;
+
+  anim.currentTime = 0;
+  await waitForNextFrame();
+
+  assert_equals(anim.currentTime, 0 * MS_PER_SEC,
+                'Hold time is set so current time should NOT change');
+}, 'Updating the finished state when seeking a reversed animation exactly'
+   + ' to zero');
+
+// CASE 3: playback rate > 0 and current time < target end OR
+//         playback rate < 0 and current time > 0
+// (Also the start time is resolved and there is pending task)
+
+// Did seek = false; playback rate > 0
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+
+  // We want to test that the hold time is cleared so first we need to
+  // put the animation in a state where the hold time is set.
+  anim.finish();
+  await anim.ready;
+
+  assert_equals(anim.currentTime, 100 * MS_PER_SEC,
+                'Hold time is initially set');
+  // Then extend the duration so that the hold time is cleared and on
+  // the next tick the current time will increase.
+  anim.effect.updateTiming({
+    duration: anim.effect.getComputedTiming().duration * 2,
+  });
+  await waitForNextFrame();
+
+  assert_greater_than(anim.currentTime, 100 * MS_PER_SEC,
+                      'Hold time is not set so current time should increase');
+}, 'Updating the finished state when playing before end');
+
+// Did seek = true; playback rate > 0
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.finish();
+  await anim.ready;
+
+  anim.currentTime = 50 * MS_PER_SEC;
+  // When did seek = true, updating the finished state: (i) updates
+  // the animation's start time and (ii) clears the hold time.
+  // We can test both by checking that the currentTime is initially
+  // updated and then increases.
+  assert_equals(anim.currentTime, 50 * MS_PER_SEC, 'Start time is updated');
+  await waitForNextFrame();
+
+  assert_greater_than(anim.currentTime, 50 * MS_PER_SEC,
+                      'Hold time is not set so current time should increase');
+}, 'Updating the finished state when seeking before end');
+
+// Did seek = false; playback rate < 0
+//
+// Unfortunately it is not possible to test this case. We need to have
+// a hold time set, a resolved start time, and then perform some
+// operation that updates the finished state with did seek set to true.
+//
+// However, the only situation where this could arrive is when we
+// replace the timeline and that procedure is likely to change. For all
+// other cases we either have an unresolved start time (e.g. when
+// paused), we don't have a set hold time (e.g. regular playback), or
+// the current time is zero (and anything that gets us out of that state
+// will set did seek = true).
+
+// Did seek = true; playback rate < 0
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.playbackRate = -1;
+  await anim.ready;
+
+  anim.currentTime = 50 * MS_PER_SEC;
+  assert_equals(anim.currentTime, 50 * MS_PER_SEC, 'Start time is updated');
+  await waitForNextFrame();
+
+  assert_less_than(anim.currentTime, 50 * MS_PER_SEC,
+                    'Hold time is not set so current time should decrease');
+}, 'Updating the finished state when seeking a reversed animation before end');
+
+// CASE 4: playback rate == 0
+
+// current time < 0
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.playbackRate = 0;
+  await anim.ready;
+
+  anim.currentTime = -100 * MS_PER_SEC;
+  await waitForNextFrame();
+
+  assert_equals(anim.currentTime, -100 * MS_PER_SEC,
+                'Hold time should not be cleared so current time should'
+                + ' NOT change');
+}, 'Updating the finished state when playback rate is zero and the'
+   + ' current time is less than zero');
+
+// current time < target end
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.playbackRate = 0;
+  await anim.ready;
+
+  anim.currentTime = 50 * MS_PER_SEC;
+  await waitForNextFrame();
+
+  assert_equals(anim.currentTime, 50 * MS_PER_SEC,
+                'Hold time should not be cleared so current time should'
+                + ' NOT change');
+}, 'Updating the finished state when playback rate is zero and the'
+   + ' current time is less than end');
+
+// current time > target end
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.playbackRate = 0;
+  await anim.ready;
+
+  anim.currentTime = 200 * MS_PER_SEC;
+  await waitForNextFrame();
+
+  assert_equals(anim.currentTime, 200 * MS_PER_SEC,
+                'Hold time should not be cleared so current time should'
+                + ' NOT change');
+}, 'Updating the finished state when playback rate is zero and the'
+   + ' current time is greater than end');
+
+// CASE 5: current time unresolved
+
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.cancel();
+  // Trigger a change that will cause the "update the finished state"
+  // procedure to run.
+  anim.effect.updateTiming({ duration: 200 * MS_PER_SEC });
+  assert_equals(anim.currentTime, null,
+                'The animation hold time / start time should not be updated');
+  // The "update the finished state" procedure is supposed to run after any
+  // change to timing, but just in case an implementation defers that, let's
+  // wait a frame and check that the hold time / start time has still not been
+  // updated.
+  await waitForAnimationFrames(1);
+
+  assert_equals(anim.currentTime, null,
+                'The animation hold time / start time should not be updated');
+}, 'Updating the finished state when current time is unresolved');
+
+// CASE 6: has a pending task
+
+test(t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.cancel();
+  anim.currentTime = 75 * MS_PER_SEC;
+  anim.play();
+  // We now have a pending task and a resolved current time.
+  //
+  // In the next step we will adjust the timing so that the current time
+  // is greater than the target end. At this point the "update the finished
+  // state" procedure should run and if we fail to check for a pending task
+  // we will set the hold time to the target end, i.e. 50ms.
+  anim.effect.updateTiming({ duration: 50 * MS_PER_SEC });
+  assert_equals(anim.currentTime, 75 * MS_PER_SEC,
+                'Hold time should not be updated');
+}, 'Updating the finished state when there is a pending task');
+
+// CASE 7: start time unresolved
+
+// Did seek = false
+promise_test(async t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.cancel();
+  // Make it so that only the start time is unresolved (to avoid overlapping
+  // with the test case where current time is unresolved)
+  anim.currentTime = 150 * MS_PER_SEC;
+  // Trigger a change that will cause the "update the finished state"
+  // procedure to run (did seek = false).
+  anim.effect.updateTiming({ duration: 200 * MS_PER_SEC });
+  await waitForAnimationFrames(1);
+
+  assert_equals(anim.currentTime, 150 * MS_PER_SEC,
+                'The animation hold time should not be updated');
+  assert_equals(anim.startTime, null,
+                'The animation start time should not be updated');
+}, 'Updating the finished state when start time is unresolved and'
+   + ' did seek = false');
+
+// Did seek = true
+test(t => {
+  const anim = createDiv(t).animate(null, 100 * MS_PER_SEC);
+  anim.cancel();
+  anim.currentTime = 150 * MS_PER_SEC;
+  // Trigger a change that will cause the "update the finished state"
+  // procedure to run.
+  anim.currentTime = 50 * MS_PER_SEC;
+  assert_equals(anim.currentTime, 50 * MS_PER_SEC,
+                'The animation hold time should not be updated');
+  assert_equals(anim.startTime, null,
+                'The animation start time should not be updated');
+}, 'Updating the finished state when start time is unresolved and'
+   + ' did seek = true');
+
+// --------------------------------------------------------------------
+//
+// TESTS FOR RUNNING FINISH NOTIFICATION STEPS
+//
+// --------------------------------------------------------------------
+
+function waitForFinishEventAndPromise(animation) {
+  const eventPromise = new Promise(resolve => {
+    animation.onfinish = resolve;
+  });
+  return Promise.all([eventPromise, animation.finished]);
+}
+
+promise_test(t => {
+  const animation = createDiv(t).animate(null, 1);
+  animation.onfinish =
+    t.unreached_func('Seeking to finish should not fire finish event');
+  animation.finished.then(
+    t.unreached_func('Seeking to finish should not resolve finished promise'));
+  animation.currentTime = 1;
+  animation.currentTime = 0;
+  animation.pause();
+  return waitForAnimationFrames(3);
+}, 'Finish notification steps don\'t run when the animation seeks to finish'
+   + ' and then seeks back again');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 1);
+  await animation.ready;
+
+  return waitForFinishEventAndPromise(animation);
+}, 'Finish notification steps run when the animation completes normally');
+
+promise_test(async t => {
+  const effect = new KeyframeEffect(null, null, 1);
+  const animation = new Animation(effect, document.timeline);
+  animation.play();
+  await animation.ready;
+
+  return waitForFinishEventAndPromise(animation);
+}, 'Finish notification steps run when an animation without a target'
+   + ' effect completes normally');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 1);
+  await animation.ready;
+
+  animation.currentTime = 10;
+  return waitForFinishEventAndPromise(animation);
+}, 'Finish notification steps run when the animation seeks past finish');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 1);
+  await animation.ready;
+
+  // Register for notifications now since once we seek away from being
+  // finished the 'finished' promise will be replaced.
+  const finishNotificationSteps = waitForFinishEventAndPromise(animation);
+  animation.finish();
+  animation.currentTime = 0;
+  animation.pause();
+  return finishNotificationSteps;
+}, 'Finish notification steps run when the animation completes with .finish(),'
+   + ' even if we then seek away');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 1);
+  const initialFinishedPromise = animation.finished;
+  await animation.finished;
+
+  animation.currentTime = 0;
+  assert_not_equals(initialFinishedPromise, animation.finished);
+}, 'Animation finished promise is replaced after seeking back to start');
+
+promise_test(async t => {
+  const animation = createDiv(t).animate(null, 1);
+  const initialFinishedPromise = animation.finished;
+  await animation.finished;
+
+  animation.play();
+  assert_not_equals(initialFinishedPromise, animation.finished);
+}, 'Animation finished promise is replaced after replaying from start');
+
+async_test(t => {
+  const animation = createDiv(t).animate(null, 1);
+  animation.onfinish = event => {
+    animation.currentTime = 0;
+    animation.onfinish = event => {
+      t.done();
+    };
+  };
+}, 'Animation finish event is fired again after seeking back to start');
+
+async_test(t => {
+  const animation = createDiv(t).animate(null, 1);
+  animation.onfinish = event => {
+    animation.play();
+    animation.onfinish = event => {
+      t.done();
+    };
+  };
+}, 'Animation finish event is fired again after replaying from start');
+
+async_test(t => {
+  const anim = createDiv(t).animate(null,
+                                    { duration: 100000, endDelay: 50000 });
+  anim.onfinish = t.step_func(event => {
+    assert_unreached('finish event should not be fired');
+  });
+
+  anim.ready.then(() => {
+    anim.currentTime = 100000;
+    return waitForAnimationFrames(2);
+  }).then(t.step_func(() => {
+    t.done();
+  }));
+}, 'finish event is not fired at the end of the active interval when the'
+   + ' endDelay has not expired');
+
+async_test(t => {
+  const anim = createDiv(t).animate(null,
+                                    { duration: 100000, endDelay: 30000 });
+  anim.ready.then(() => {
+    anim.currentTime = 110000; // during endDelay
+    anim.onfinish = t.step_func(event => {
+      assert_unreached('onfinish event should not be fired during endDelay');
+    });
+    return waitForAnimationFrames(2);
+  }).then(t.step_func(() => {
+    anim.onfinish = t.step_func(event => {
+      t.done();
+    });
+    anim.currentTime = 130000; // after endTime
+  }));
+}, 'finish event is fired after the endDelay has expired');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/time-transformations/transformed-progress.html b/testing/web-platform/tests/web-animations/timing-model/time-transformations/transformed-progress.html
new file mode 100644
index 0000000000..960e333c09
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/time-transformations/transformed-progress.html
@@ -0,0 +1,391 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Transformed progress</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#calculating-the-transformed-progress">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<script src="../../resources/easing-tests.js"></script>
+<body>
+<div id="log"></div>
+<div id="target"></div>
+<script>
+'use strict';
+
+for (const params of gEasingTests) {
+  test(t => {
+    const target = createDiv(t);
+    const anim   = target.animate(null, { duration: 1000,
+                                          fill: 'forwards',
+                                          easing: params.easing });
+
+    for (const sampleTime of [0, 250, 500, 750, 1000]) {
+      anim.currentTime = sampleTime;
+      const portion = sampleTime / anim.effect.getComputedTiming().duration;
+      const expectedProgress = params.easingFunction(portion);
+      assert_approx_equals(anim.effect.getComputedTiming().progress,
+                           expectedProgress,
+                           0.01,
+                           'The progress should be approximately ' +
+                           `${expectedProgress} at ${sampleTime}ms`);
+    }
+  }, `Transformed progress for ${params.desc}`);
+}
+
+// Additional tests for various boundary conditions of step timing functions.
+
+const gStepTimingFunctionTests = [
+  {
+    description: 'Test bounds point of step-start easing',
+    effect:     {
+                  delay: 1000,
+                  duration: 1000,
+                  fill: 'both',
+                  easing: 'steps(2, start)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 0 },
+                  { currentTime: 999,  progress: 0 },
+                  { currentTime: 1000, progress: 0.5 },
+                  { currentTime: 1499, progress: 0.5 },
+                  { currentTime: 1500, progress: 1 },
+                  { currentTime: 2000, progress: 1 }
+                ]
+  },
+  {
+    description: 'Test bounds point of step-start easing with reverse direction',
+    effect:     {
+                  delay: 1000,
+                  duration: 1000,
+                  fill: 'both',
+                  direction: 'reverse',
+                  easing: 'steps(2, start)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 1 },
+                  { currentTime: 1001, progress: 1 },
+                  { currentTime: 1500, progress: 1 },
+                  { currentTime: 1501, progress: 0.5 },
+                  { currentTime: 2000, progress: 0 },
+                  { currentTime: 2500, progress: 0 }
+                ]
+  },
+  {
+    description: 'Test bounds point of step-start easing ' +
+                 'with iterationStart not at a transition point',
+    effect:     {
+                  delay: 1000,
+                  duration: 1000,
+                  fill: 'both',
+                  iterationStart: 0.25,
+                  easing: 'steps(2, start)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 0.5 },
+                  { currentTime: 999,  progress: 0.5 },
+                  { currentTime: 1000, progress: 0.5 },
+                  { currentTime: 1249, progress: 0.5 },
+                  { currentTime: 1250, progress: 1 },
+                  { currentTime: 1749, progress: 1 },
+                  { currentTime: 1750, progress: 0.5 },
+                  { currentTime: 2000, progress: 0.5 },
+                  { currentTime: 2500, progress: 0.5 },
+                ]
+  },
+  {
+    description: 'Test bounds point of step-start easing ' +
+                 'with iterationStart and delay',
+    effect:     {
+                  delay: 1000,
+                  duration: 1000,
+                  fill: 'both',
+                  iterationStart: 0.5,
+                  easing: 'steps(2, start)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 0.5 },
+                  { currentTime: 999,  progress: 0.5 },
+                  { currentTime: 1000, progress: 1 },
+                  { currentTime: 1499, progress: 1 },
+                  { currentTime: 1500, progress: 0.5 },
+                  { currentTime: 2000, progress: 1 }
+                ]
+  },
+  {
+    description: 'Test bounds point of step-start easing ' +
+                 'with iterationStart and reverse direction',
+    effect:     {
+                  delay: 1000,
+                  duration: 1000,
+                  fill: 'both',
+                  iterationStart: 0.5,
+                  direction: 'reverse',
+                  easing: 'steps(2, start)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 1 },
+                  { currentTime: 1000, progress: 1 },
+                  { currentTime: 1001, progress: 0.5 },
+                  { currentTime: 1499, progress: 0.5 },
+                  { currentTime: 1500, progress: 1 },
+                  { currentTime: 1999, progress: 1 },
+                  { currentTime: 2000, progress: 0.5 },
+                  { currentTime: 2500, progress: 0.5 }
+                ]
+  },
+  {
+    description: 'Test bounds point of step(4, start) easing ' +
+                 'with iterationStart 0.75 and delay',
+    effect:     {
+                  duration: 1000,
+                  fill: 'both',
+                  delay: 1000,
+                  iterationStart: 0.75,
+                  easing: 'steps(4, start)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 0.75 },
+                  { currentTime: 999,  progress: 0.75 },
+                  { currentTime: 1000, progress: 1 },
+                  { currentTime: 2000, progress: 1 },
+                  { currentTime: 2500, progress: 1 }
+                ]
+  },
+  {
+    description: 'Test bounds point of step-start easing ' +
+                 'with alternate direction',
+    effect:     {
+                  duration: 1000,
+                  fill: 'both',
+                  delay: 1000,
+                  iterations: 2,
+                  iterationStart: 1.5,
+                  direction: 'alternate',
+                  easing: 'steps(2, start)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 1 },
+                  { currentTime: 1000, progress: 1 },
+                  { currentTime: 1001, progress: 0.5 },
+                  { currentTime: 2999, progress: 1 },
+                  { currentTime: 3000, progress: 0.5 },
+                  { currentTime: 3500, progress: 0.5 }
+                ]
+  },
+  {
+    description: 'Test bounds point of step-start easing ' +
+                 'with alternate-reverse direction',
+    effect:     {
+                  duration: 1000,
+                  fill: 'both',
+                  delay: 1000,
+                  iterations: 2,
+                  iterationStart: 0.5,
+                  direction: 'alternate-reverse',
+                  easing: 'steps(2, start)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 1 },
+                  { currentTime: 1000, progress: 1 },
+                  { currentTime: 1001, progress: 0.5 },
+                  { currentTime: 2999, progress: 1 },
+                  { currentTime: 3000, progress: 0.5 },
+                  { currentTime: 3500, progress: 0.5 }
+                ]
+  },
+  {
+    description: 'Test bounds point of step-end easing',
+    effect:     {
+                  delay: 1000,
+                  duration: 1000,
+                  fill: 'both',
+                  easing: 'steps(2, end)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 0 },
+                  { currentTime: 999,  progress: 0 },
+                  { currentTime: 1000, progress: 0 },
+                  { currentTime: 1499, progress: 0 },
+                  { currentTime: 1500, progress: 0.5 },
+                  { currentTime: 2000, progress: 1 }
+                ]
+  },
+  {
+    description: 'Test bounds point of step-end easing ' +
+                 'with iterationStart and delay',
+    effect:     {
+                  duration: 1000,
+                  fill: 'both',
+                  delay: 1000,
+                  iterationStart: 0.5,
+                  easing: 'steps(2, end)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 0 },
+                  { currentTime: 999,  progress: 0 },
+                  { currentTime: 1000, progress: 0.5 },
+                  { currentTime: 1499, progress: 0.5 },
+                  { currentTime: 1500, progress: 0 },
+                  { currentTime: 1999, progress: 0 },
+                  { currentTime: 2000, progress: 0.5 },
+                  { currentTime: 2500, progress: 0.5 }
+                ]
+  },
+  {
+    description: 'Test bounds point of step-end easing ' +
+                 'with iterationStart not at a transition point',
+    effect:     {
+                  delay: 1000,
+                  duration: 1000,
+                  fill: 'both',
+                  iterationStart: 0.75,
+                  easing: 'steps(2, end)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 0.5 },
+                  { currentTime: 999,  progress: 0.5 },
+                  { currentTime: 1000, progress: 0.5 },
+                  { currentTime: 1249, progress: 0.5 },
+                  { currentTime: 1250, progress: 0 },
+                  { currentTime: 1749, progress: 0 },
+                  { currentTime: 1750, progress: 0.5 },
+                  { currentTime: 2000, progress: 0.5 },
+                  { currentTime: 2500, progress: 0.5 },
+                ]
+  },
+  {
+    description: 'Test bounds point of steps(jump-both) easing',
+    effect:     {
+                  delay: 1000,
+                  duration: 1000,
+                  fill: 'both',
+                  easing: 'steps(2, jump-both)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 0 },
+                  { currentTime: 999,  progress: 0 },
+                  { currentTime: 1000, progress: 1/3 },
+                  { currentTime: 1499, progress: 1/3 },
+                  { currentTime: 1500, progress: 2/3 },
+                  { currentTime: 2000, progress: 1 }
+                ]
+  },
+  {
+    description: 'Test bounds point of steps(jump-both) easing ' +
+                 'with iterationStart and delay',
+    effect:     {
+                  duration: 1000,
+                  fill: 'both',
+                  delay: 1000,
+                  iterationStart: 0.5,
+                  easing: 'steps(2, jump-both)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 1/3 },
+                  { currentTime: 999,  progress: 1/3 },
+                  { currentTime: 1000, progress: 2/3 },
+                  { currentTime: 1499, progress: 2/3 },
+                  { currentTime: 1500, progress: 1/3 },
+                  { currentTime: 1999, progress: 1/3 },
+                  { currentTime: 2000, progress: 2/3 },
+                  { currentTime: 2500, progress: 2/3 }
+                ]
+  },
+  {
+    description: 'Test bounds point of steps(jump-both) easing ' +
+                 'with iterationStart not at a transition point',
+    effect:     {
+                  delay: 1000,
+                  duration: 1000,
+                  fill: 'both',
+                  iterationStart: 0.75,
+                  easing: 'steps(2, jump-both)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 2/3 },
+                  { currentTime: 999,  progress: 2/3 },
+                  { currentTime: 1000, progress: 2/3 },
+                  { currentTime: 1249, progress: 2/3 },
+                  { currentTime: 1250, progress: 1/3 },
+                  { currentTime: 1749, progress: 1/3 },
+                  { currentTime: 1750, progress: 2/3 },
+                  { currentTime: 2000, progress: 2/3 },
+                  { currentTime: 2500, progress: 2/3 }
+                ]
+  },
+  {
+    description: 'Test bounds point of steps(jump-none) easing',
+    effect:     {
+                  delay: 1000,
+                  duration: 1000,
+                  fill: 'both',
+                  easing: 'steps(2, jump-none)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 0 },
+                  { currentTime: 1000, progress: 0 },
+                  { currentTime: 1499, progress: 0 },
+                  { currentTime: 1500, progress: 1 },
+                  { currentTime: 2000, progress: 1 }
+                ]
+  },
+  {
+    description: 'Test bounds point of steps(jump-none) easing ' +
+                 'with iterationStart and delay',
+    effect:     {
+                  duration: 1000,
+                  fill: 'both',
+                  delay: 1000,
+                  iterationStart: 0.5,
+                  easing: 'steps(2, jump-none)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 0 },
+                  { currentTime: 999,  progress: 0 },
+                  { currentTime: 1000, progress: 1 },
+                  { currentTime: 1499, progress: 1 },
+                  { currentTime: 1500, progress: 0 },
+                  { currentTime: 1999, progress: 0 },
+                  { currentTime: 2000, progress: 1 },
+                  { currentTime: 2500, progress: 1 }
+                ]
+  },
+  {
+    description: 'Test bounds point of steps(jump-none) easing ' +
+                 'with iterationStart not at a transition point',
+    effect:     {
+                  delay: 1000,
+                  duration: 1000,
+                  fill: 'both',
+                  iterationStart: 0.75,
+                  easing: 'steps(2, jump-none)'
+                },
+    conditions: [
+                  { currentTime: 0,    progress: 1 },
+                  { currentTime: 999,  progress: 1 },
+                  { currentTime: 1000, progress: 1 },
+                  { currentTime: 1249, progress: 1 },
+                  { currentTime: 1250, progress: 0 },
+                  { currentTime: 1749, progress: 0 },
+                  { currentTime: 1750, progress: 1 },
+                  { currentTime: 2000, progress: 1 },
+                  { currentTime: 2500, progress: 1 }
+                ]
+  },
+];
+
+for (const options of gStepTimingFunctionTests) {
+  test(t => {
+    const target = createDiv(t);
+    const animation = target.animate(null, options.effect);
+    for (const condition of options.conditions) {
+      animation.currentTime = condition.currentTime;
+      assert_equals(animation.effect.getComputedTiming().progress,
+                    condition.progress,
+                    `Progress at ${animation.currentTime}ms`);
+    }
+  }, options.description);
+}
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/web-animations/timing-model/timelines/document-timelines.html b/testing/web-platform/tests/web-animations/timing-model/timelines/document-timelines.html
new file mode 100644
index 0000000000..c71d73331c
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/timelines/document-timelines.html
@@ -0,0 +1,50 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>Document timelines</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#document-timelines">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<div id="log"></div>
+<script>
+'use strict';
+
+async_test(t => {
+  assert_greater_than_equal(document.timeline.currentTime, 0,
+    'The current time is initially is positive or zero');
+
+  // document.timeline.currentTime should be set even before document
+  // load fires. We expect this code to be run before document load and hence
+  // the above assertion is sufficient.
+  // If the following assertion fails, this test needs to be redesigned.
+  assert_not_equals(document.readyState, 'complete',
+    'Test is running prior to document load');
+
+  // Test that the document timeline's current time is measured from
+  // navigationStart.
+  //
+  // We can't just compare document.timeline.currentTime to
+  // window.performance.now() because currentTime is only updated on a sample
+  // so we use requestAnimationFrame instead.
+  window.requestAnimationFrame(rafTime => {
+    t.step(() => {
+      assert_equals(document.timeline.currentTime, rafTime,
+                    'The current time matches requestAnimationFrame time');
+    });
+    t.done();
+  });
+}, 'Document timelines report current time relative to navigationStart');
+
+async_test(t => {
+  window.requestAnimationFrame(rafTime => {
+    t.step(() => {
+      const iframe = document.createElement('iframe');
+      document.body.appendChild(iframe);
+      assert_greater_than_equal(iframe.contentDocument.timeline.currentTime, 0,
+        'The current time of a new iframe is initially is positive or zero');
+    });
+    t.done();
+  });
+}, 'Child frames do not report negative initial times');
+
+</script>
diff --git a/testing/web-platform/tests/web-animations/timing-model/timelines/resources/target-frame.html b/testing/web-platform/tests/web-animations/timing-model/timelines/resources/target-frame.html
new file mode 100644
index 0000000000..18ee4fd8a2
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/timelines/resources/target-frame.html
@@ -0,0 +1,76 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <meta charset="utf-8">
+  <style type="text/css">
+      #target {
+        background: green;
+        height:  50px;
+        width:  50px;
+      }
+  </style>
+</head>
+<body>
+  <div id="target"></div>
+</body>
+<script src="../../../testcommon.js"></script>
+<script type="text/javascript">
+  function sendResult(message) {
+    top.postMessage(message, '*');
+  }
+
+  function waitForAnimationReady(anim) {
+    // Resolution of the ready promise, though UA dependent, is expected to
+    // happen within a few frames. Throttling rendering of the frame owning
+    // the animation's timeline may delay resolution of the ready promise,
+    // resulting in a test failure.
+    let frameTimeout = 10;
+    let resolved = false;
+    return new Promise((resolve, reject) => {
+      anim.ready.then(() => {
+        resolved = true;
+        resolve('PASS');
+      });
+      const tick = () => {
+        requestAnimationFrame(() => {
+          if (!resolved) {
+            if (--frameTimeout == 0)
+              resolve('FAIL: Animation is still pending');
+            else
+              tick();
+          }
+        });
+      };
+      tick();
+    });
+  }
+
+  function verifyAnimationIsUpdating() {
+    return new Promise(resolve => {
+      waitForAnimationFrames(3).then(() => {
+        const opacity = getComputedStyle(target).opacity;
+        const result =
+            (opacity == 1) ? 'FAIL: opacity remained unchanged' : 'PASS';
+        resolve(result);
+      });
+    });
+  }
+
+  async function runTest() {
+    const anim = document.getAnimations()[0];
+    if (!anim) {
+      setResult('FAIL: Failed to create animation');
+      return;
+    }
+    waitForAnimationReady(anim).then(result => {
+      if (result != 'PASS') {
+        sendResult(result);
+        return;
+      }
+      verifyAnimationIsUpdating().then(result => {
+        sendResult(result);
+      });
+    });
+  }
+</script>
+</html>
diff --git a/testing/web-platform/tests/web-animations/timing-model/timelines/resources/timeline-frame.html b/testing/web-platform/tests/web-animations/timing-model/timelines/resources/timeline-frame.html
new file mode 100644
index 0000000000..9c8cdabc9d
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/timelines/resources/timeline-frame.html
@@ -0,0 +1,20 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <meta charset="utf-8">
+</head>
+<script type="text/javascript">
+  const targetWindow = window.top.a;
+  const element = targetWindow.document.getElementById('target');
+  const keyframes = { opacity: [1, 0.2] };
+  const options = {
+    duration: 1000,
+    // Use this document's timeline rather then the timeline of the
+    // element's document.
+    timeline: document.timeline,
+    fill: 'forwards'
+  };
+  element.animate(keyframes, options);
+  targetWindow.runTest();
+</script>
+</html>
diff --git a/testing/web-platform/tests/web-animations/timing-model/timelines/sibling-iframe-timeline.html b/testing/web-platform/tests/web-animations/timing-model/timelines/sibling-iframe-timeline.html
new file mode 100644
index 0000000000..8a611c8579
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/timelines/sibling-iframe-timeline.html
@@ -0,0 +1,58 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>Animate using sibling iframe's timeline</title>
+</head>
+<body></body>
+<script src="/common/get-host-info.sub.js"></script>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script type="text/javascript">
+  'use strict';
+
+  function crossSiteUrl(filename) {
+    const url =
+        get_host_info().HTTP_REMOTE_ORIGIN +
+            '/web-animations/timing-model/timelines/resources/' +
+            filename;
+    return url;
+  }
+
+  function loadFrame(name, path, hidden) {
+    return new Promise(resolve => {
+      const frame = document.createElement('iframe');
+      if (hidden)
+        frame.style = 'visibility: hidden;';
+      frame.name = name;
+      document.body.appendChild(frame);
+      frame.onload = () => {
+        resolve();
+      }
+      frame.src = crossSiteUrl(path);
+    });
+  }
+
+ function waitForTestResults() {
+    return new Promise(resolve => {
+      const listener = (evt) => {
+        window.removeEventListener('message', listener);
+        resolve(evt.data);
+      };
+      window.addEventListener('message', listener);
+    });
+  }
+
+  promise_test(async t => {
+    const promise = waitForTestResults().then((data) => {
+      assert_equals(data, 'PASS');
+    });
+    // Animate an element in frame A.
+    await loadFrame('a', 'target-frame.html', false);
+    // Animation's timeline is in hidden frame B.
+    await loadFrame('b', 'timeline-frame.html', true);
+
+   return promise;
+  }, 'animation tied to another frame\'s timeline runs properly');
+</script>
+</html>
diff --git a/testing/web-platform/tests/web-animations/timing-model/timelines/timelines.html b/testing/web-platform/tests/web-animations/timing-model/timelines/timelines.html
new file mode 100644
index 0000000000..d570eed5c2
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/timelines/timelines.html
@@ -0,0 +1,112 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>Timelines</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#timelines">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<style>
+@keyframes opacity-animation {
+  from { opacity: 1; }
+  to { opacity: 0; }
+}
+</style>
+<div id="log"></div>
+<script>
+'use strict';
+
+promise_test(t => {
+  const valueAtStart = document.timeline.currentTime;
+  const timeAtStart = window.performance.now();
+  while (window.performance.now() - timeAtStart < 50) {
+    // Wait 50ms
+  }
+  assert_equals(document.timeline.currentTime, valueAtStart,
+    'Timeline time does not change within an animation frame');
+  return waitForAnimationFrames(1).then(() => {
+    assert_greater_than(document.timeline.currentTime, valueAtStart,
+      'Timeline time increases between animation frames');
+  });
+}, 'Timeline time increases once per animation frame');
+
+async_test(t => {
+  const iframe = document.createElement('iframe');
+  iframe.width = 10;
+  iframe.height = 10;
+
+  iframe.addEventListener('load', t.step_func(() => {
+    const iframeTimeline = iframe.contentDocument.timeline;
+    const valueAtStart   = iframeTimeline.currentTime;
+    const timeAtStart    = window.performance.now();
+    while (iframe.contentWindow.performance.now() - timeAtStart < 50) {
+      // Wait 50ms
+    }
+    assert_equals(iframeTimeline.currentTime, valueAtStart,
+      'Timeline time within an iframe does not change within an '
+      + ' animation frame');
+
+    iframe.contentWindow.requestAnimationFrame(t.step_func_done(() => {
+      assert_greater_than(iframeTimeline.currentTime, valueAtStart,
+        'Timeline time within an iframe increases between animation frames');
+      iframe.remove();
+    }));
+  }));
+
+  document.body.appendChild(iframe);
+}, 'Timeline time increases once per animation frame in an iframe');
+
+async_test(t => {
+  const startTime = document.timeline.currentTime;
+  let firstRafTime;
+
+  requestAnimationFrame(() => {
+    t.step(() => {
+      assert_greater_than_equal(document.timeline.currentTime, startTime,
+                                'Timeline time should have progressed');
+      firstRafTime = document.timeline.currentTime;
+    });
+  });
+
+  requestAnimationFrame(() => {
+    t.step(() => {
+      assert_equals(document.timeline.currentTime, firstRafTime,
+                    'Timeline time should be the same');
+    });
+    t.done();
+  });
+}, 'Timeline time should be the same for all RAF callbacks in an animation'
+   + ' frame');
+
+async_test(t => {
+  const div = createDiv(t);
+  const animation = div.animate(null, 100 * MS_PER_SEC);
+
+  animation.ready.then(t.step_func(() => {
+    const readyTimelineTime = document.timeline.currentTime;
+    requestAnimationFrame(t.step_func_done(() => {
+      assert_equals(readyTimelineTime, document.timeline.currentTime,
+                    'There should be a microtask checkpoint');
+    }));
+  }));
+}, 'Performs a microtask checkpoint after updating timelins');
+
+async_test(t => {
+  const div = createDiv(t);
+  let readyPromiseRan = false;
+  let finishedPromiseRan = false;
+  div.style.animation = 'opacity-animation 1ms';
+  let anim = div.getAnimations()[0];
+  anim.ready.then(t.step_func(() => {
+    readyPromiseRan = true;
+  }));
+  div.addEventListener('animationstart', t.step_func(() => {
+    assert_true(readyPromiseRan, 'It should run ready promise before animationstart event');
+  }));
+  anim.finished.then(t.step_func(() => {
+    finishedPromiseRan = true;
+  }));
+  div.addEventListener('animationend', t.step_func_done(() => {
+    assert_true(finishedPromiseRan, 'It should run finished promise before animationend event');
+  }));
+}, 'Runs finished promise before animation events');
+</script>
diff --git a/testing/web-platform/tests/web-animations/timing-model/timelines/update-and-send-events-replacement.html b/testing/web-platform/tests/web-animations/timing-model/timelines/update-and-send-events-replacement.html
new file mode 100644
index 0000000000..d6ed734831
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/timelines/update-and-send-events-replacement.html
@@ -0,0 +1,1017 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>Update animations and send events (replacement)</title>
+<link rel="help" href="https://drafts.csswg.org/web-animations/#update-animations-and-send-events">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<style>
+@keyframes opacity-animation {
+  to { opacity: 1 }
+}
+</style>
+<div id="log"></div>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation when another covers the same properties');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  await animB.finished;
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation after another animation finishes');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate(
+    { opacity: 1, width: '100px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+
+  const animB = div.animate(
+    { width: '200px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  await animB.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  const animC = div.animate(
+    { opacity: 0.5 },
+    { duration: 1, fill: 'forwards' }
+  );
+  await animC.finished;
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+  assert_equals(animC.replaceState, 'active');
+}, 'Removes an animation after multiple other animations finish');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate(
+    { opacity: 1 },
+    { duration: 100 * MS_PER_SEC, fill: 'forwards' }
+  );
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  await animB.finished;
+
+  assert_equals(animB.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  // Seek animA to just before it finishes since we want to test the behavior
+  // when the animation finishes by the ticking of the timeline, not by seeking
+  // (that is covered in a separate test).
+
+  animA.currentTime = 99.99 * MS_PER_SEC;
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation after it finishes');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate(
+    { opacity: 1 },
+    { duration: 100 * MS_PER_SEC, fill: 'forwards' }
+  );
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  animB.finish();
+
+  // Replacement should not happen until the next time the "update animations
+  // and send events" procedure runs.
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation after seeking another animation');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate(
+    { opacity: 1 },
+    { duration: 100 * MS_PER_SEC, fill: 'forwards' }
+  );
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  await animB.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  animA.finish();
+
+  // Replacement should not happen until the next time the "update animations
+  // and send events" procedure runs.
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation after seeking it');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate({ opacity: 1 }, 1);
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  animB.effect.updateTiming({ fill: 'forwards' });
+
+  // Replacement should not happen until the next time the "update animations
+  // and send events" procedure runs.
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation after updating the fill mode of another animation');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, 1);
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  animA.effect.updateTiming({ fill: 'forwards' });
+
+  // Replacement should not happen until the next time the "update animations
+  // and send events" procedure runs.
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation after updating its fill mode');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate({ opacity: 1 }, 1);
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  animB.effect = new KeyframeEffect(
+    div,
+    { opacity: 1 },
+    {
+      duration: 1,
+      fill: 'forwards',
+    }
+  );
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, "Removes an animation after updating another animation's effect to one with different timing");
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, 1);
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  await animB.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  animA.effect = new KeyframeEffect(
+    div,
+    { opacity: 1 },
+    {
+      duration: 1,
+      fill: 'forwards',
+    }
+  );
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation after updating its effect to one with different timing');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate(
+    { opacity: 1 },
+    { duration: 100 * MS_PER_SEC, fill: 'forwards' }
+  );
+
+  await animA.finished;
+
+  // Set up a timeline that makes animB finished
+  animB.timeline = new DocumentTimeline({
+    originTime:
+      document.timeline.currentTime - 100 * MS_PER_SEC - animB.startTime,
+  });
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, "Removes an animation after updating another animation's timeline");
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate(
+    { opacity: 1 },
+    { duration: 100 * MS_PER_SEC, fill: 'forwards' }
+  );
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+
+  await animB.finished;
+
+  // Set up a timeline that makes animA finished
+  animA.timeline = new DocumentTimeline({
+    originTime:
+      document.timeline.currentTime - 100 * MS_PER_SEC - animA.startTime,
+  });
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation after updating its timeline');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate(
+    { width: '100px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  animB.effect.setKeyframes({ width: '100px', opacity: 1 });
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, "Removes an animation after updating another animation's effect's properties");
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate(
+    { opacity: 1, width: '100px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  const animB = div.animate(
+    { width: '200px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  animA.effect.setKeyframes({ width: '100px' });
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, "Removes an animation after updating its effect's properties");
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate(
+    { width: '100px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  animB.effect = new KeyframeEffect(
+    div,
+    { width: '100px', opacity: 1 },
+    { duration: 1, fill: 'forwards' }
+  );
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, "Removes an animation after updating another animation's effect to one with different properties");
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate(
+    { opacity: 1, width: '100px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  const animB = div.animate(
+    { width: '200px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  animA.effect = new KeyframeEffect(
+    div,
+    { width: '100px' },
+    {
+      duration: 1,
+      fill: 'forwards',
+    }
+  );
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation after updating its effect to one with different properties');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate(
+    { marginLeft: '10px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  const animB = div.animate(
+    { margin: '20px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation when another animation uses a shorthand');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate(
+    { margin: '10px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  const animB = div.animate(
+    {
+      marginLeft: '10px',
+      marginTop: '20px',
+      marginRight: '30px',
+      marginBottom: '40px',
+    },
+    { duration: 1, fill: 'forwards' }
+  );
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation that uses a shorthand');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate(
+    { marginLeft: '10px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  const animB = div.animate(
+    { marginInlineStart: '20px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation by another animation using logical properties');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate(
+    { marginInlineStart: '10px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  const animB = div.animate(
+    { marginLeft: '20px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation using logical properties');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate(
+    { marginTop: '10px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  const animB = div.animate(
+    { marginInlineStart: '20px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  div.style.writingMode = 'vertical-rl';
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation by another animation using logical properties after updating the context');
+
+promise_test(async t => {
+  const divA = createDiv(t);
+  const divB = createDiv(t);
+
+  const animA = divA.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = divB.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  animB.effect.target = divA;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, "Removes an animation after updating another animation's effect's target");
+
+promise_test(async t => {
+  const divA = createDiv(t);
+  const divB = createDiv(t);
+
+  const animA = divA.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = divB.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  animA.effect.target = divB;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, "Removes an animation after updating its effect's target");
+
+promise_test(async t => {
+  const divA = createDiv(t);
+  const divB = createDiv(t);
+
+  const animA = divA.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = divB.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  animB.effect = new KeyframeEffect(
+    divA,
+    { opacity: 1 },
+    {
+      duration: 1,
+      fill: 'forwards',
+    }
+  );
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, "Removes an animation after updating another animation's effect to one with a different target");
+
+promise_test(async t => {
+  const divA = createDiv(t);
+  const divB = createDiv(t);
+
+  const animA = divA.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = divB.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  animA.effect = new KeyframeEffect(
+    divB,
+    { opacity: 1 },
+    {
+      duration: 1,
+      fill: 'forwards',
+    }
+  );
+
+  assert_equals(animA.replaceState, 'active');
+  assert_equals(animB.replaceState, 'active');
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Removes an animation after updating its effect to one with a different target');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  div.style.animation = 'opacity-animation 1ms forwards';
+  const cssAnimation = div.getAnimations()[0];
+
+  const scriptAnimation = div.animate(
+    { opacity: 1 },
+    {
+      duration: 1,
+      fill: 'forwards',
+    }
+  );
+  await scriptAnimation.finished;
+
+  assert_equals(cssAnimation.replaceState, 'active');
+  assert_equals(scriptAnimation.replaceState, 'active');
+}, 'Does NOT remove a CSS animation tied to markup');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  div.style.animation = 'opacity-animation 1ms forwards';
+  const cssAnimation = div.getAnimations()[0];
+
+  // Break tie to markup
+  div.style.animationName = 'none';
+  assert_equals(cssAnimation.playState, 'idle');
+
+  // Restart animation
+  cssAnimation.play();
+
+  const scriptAnimation = div.animate(
+    { opacity: 1 },
+    {
+      duration: 1,
+      fill: 'forwards',
+    }
+  );
+  await scriptAnimation.finished;
+
+  assert_equals(cssAnimation.replaceState, 'removed');
+  assert_equals(scriptAnimation.replaceState, 'active');
+}, 'Removes a CSS animation no longer tied to markup');
+
+promise_test(async t => {
+  // Setup transition
+  const div = createDiv(t);
+  div.style.opacity = '0';
+  div.style.transition = 'opacity 1ms';
+  getComputedStyle(div).opacity;
+  div.style.opacity = '1';
+  const cssTransition = div.getAnimations()[0];
+  cssTransition.effect.updateTiming({ fill: 'forwards' });
+
+  const scriptAnimation = div.animate(
+    { opacity: 1 },
+    {
+      duration: 1,
+      fill: 'forwards',
+    }
+  );
+  await scriptAnimation.finished;
+
+  assert_equals(cssTransition.replaceState, 'active');
+  assert_equals(scriptAnimation.replaceState, 'active');
+}, 'Does NOT remove a CSS transition tied to markup');
+
+promise_test(async t => {
+  // Setup transition
+  const div = createDiv(t);
+  div.style.opacity = '0';
+  div.style.transition = 'opacity 1ms';
+  getComputedStyle(div).opacity;
+  div.style.opacity = '1';
+  const cssTransition = div.getAnimations()[0];
+  cssTransition.effect.updateTiming({ fill: 'forwards' });
+
+  // Break tie to markup
+  div.style.transitionProperty = 'none';
+  assert_equals(cssTransition.playState, 'idle');
+
+  // Restart transition
+  cssTransition.play();
+
+  const scriptAnimation = div.animate(
+    { opacity: 1 },
+    {
+      duration: 1,
+      fill: 'forwards',
+    }
+  );
+  await scriptAnimation.finished;
+
+  assert_equals(cssTransition.replaceState, 'removed');
+  assert_equals(scriptAnimation.replaceState, 'active');
+}, 'Removes a CSS transition no longer tied to markup');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const eventWatcher = new EventWatcher(t, animA, 'remove');
+
+  const event = await eventWatcher.wait_for('remove');
+
+  assert_times_equal(event.timelineTime, document.timeline.currentTime);
+  assert_times_equal(event.currentTime, 1);
+}, 'Dispatches an event when removing');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const eventWatcher = new EventWatcher(t, animA, 'remove');
+
+  await eventWatcher.wait_for('remove');
+
+  // Check we don't get another event
+  animA.addEventListener(
+    'remove',
+    t.step_func(() => {
+      assert_unreached('remove event should not be fired a second time');
+    })
+  );
+
+  // Restart animation
+  animA.play();
+
+  await waitForNextFrame();
+
+  // Finish animation
+  animA.finish();
+
+  await waitForNextFrame();
+}, 'Does NOT dispatch a remove event twice');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate(
+    { opacity: 1 },
+    { duration: 100 * MS_PER_SEC, fill: 'forwards' }
+  );
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+
+  animB.finish();
+  animB.currentTime = 0;
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'active');
+}, "Does NOT remove an animation after making a redundant change to another animation's current time");
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate(
+    { opacity: 1 },
+    { duration: 100 * MS_PER_SEC, fill: 'forwards' }
+  );
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  await animB.finished;
+
+  assert_equals(animA.replaceState, 'active');
+
+  animA.finish();
+  animA.currentTime = 0;
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'active');
+}, 'Does NOT remove an animation after making a redundant change to its current time');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate(
+    { opacity: 1 },
+    { duration: 100 * MS_PER_SEC, fill: 'forwards' }
+  );
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+
+  // Set up a timeline that makes animB finished but then restore it
+  animB.timeline = new DocumentTimeline({
+    originTime:
+      document.timeline.currentTime - 100 * MS_PER_SEC - animB.startTime,
+  });
+  animB.timeline = document.timeline;
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'active');
+}, "Does NOT remove an animation after making a redundant change to another animation's timeline");
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate(
+    { opacity: 1 },
+    { duration: 100 * MS_PER_SEC, fill: 'forwards' }
+  );
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  await animB.finished;
+
+  assert_equals(animA.replaceState, 'active');
+
+  // Set up a timeline that makes animA finished but then restore it
+  animA.timeline = new DocumentTimeline({
+    originTime:
+      document.timeline.currentTime - 100 * MS_PER_SEC - animA.startTime,
+  });
+  animA.timeline = document.timeline;
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'active');
+}, 'Does NOT remove an animation after making a redundant change to its timeline');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate(
+    { marginLeft: '100px' },
+    {
+      duration: 1,
+      fill: 'forwards',
+    }
+  );
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+
+  // Redundant change
+  animB.effect.setKeyframes({ marginLeft: '100px', opacity: 1 });
+  animB.effect.setKeyframes({ marginLeft: '100px' });
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'active');
+}, "Does NOT remove an animation after making a redundant change to another animation's effect's properties");
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animA = div.animate(
+    { marginLeft: '100px' },
+    {
+      duration: 1,
+      fill: 'forwards',
+    }
+  );
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'active');
+
+  // Redundant change
+  animA.effect.setKeyframes({ opacity: 1 });
+  animA.effect.setKeyframes({ marginLeft: '100px' });
+
+  await waitForNextFrame();
+
+  assert_equals(animA.replaceState, 'active');
+}, "Does NOT remove an animation after making a redundant change to its effect's properties");
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  animB.timeline = new DocumentTimeline();
+
+  await animA.finished;
+
+  // If, for example, we only update the timeline for animA before checking
+  // replacement state, then animB will not be finished and animA will not be
+  // replaced.
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Updates ALL timelines before checking for replacement');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+
+  const events = [];
+  const logEvent = (targetName, eventType) => {
+    events.push(`${targetName}:${eventType}`);
+  };
+
+  animA.addEventListener('finish', () => logEvent('animA', 'finish'));
+  animA.addEventListener('remove', () => logEvent('animA', 'remove'));
+  animB.addEventListener('finish', () => logEvent('animB', 'finish'));
+  animB.addEventListener('remove', () => logEvent('animB', 'remove'));
+
+  await animA.finished;
+
+  // Allow all events to be dispatched
+
+  await waitForNextFrame();
+
+  assert_array_equals(events, [
+    'animA:finish',
+    'animB:finish',
+    'animA:remove',
+  ]);
+}, 'Dispatches remove events after finish events');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+
+  const eventWatcher = new EventWatcher(t, animA, 'remove');
+
+  await animA.finished;
+
+  let rAFReceived = false;
+  requestAnimationFrame(() => (rAFReceived = true));
+
+  await eventWatcher.wait_for('remove');
+
+  assert_false(
+    rAFReceived,
+    'remove event should be fired before requestAnimationFrame'
+  );
+}, 'Fires remove event before requestAnimationFrame');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate(
+    { width: '100px' },
+    { duration: 1, fill: 'forwards' }
+  );
+  const animC = div.animate(
+    { opacity: 0.5, width: '200px' },
+    { duration: 1, fill: 'forwards' }
+  );
+
+  // In the event handler for animA (which should be fired before that of animB)
+  // we make a change to animC so that it no longer covers animB.
+  //
+  // If the remove event for animB is not already queued by this point, it will
+  // fail to fire.
+  animA.addEventListener('remove', () => {
+    animC.effect.setKeyframes({
+      opacity: 0.5,
+    });
+  });
+
+  const eventWatcher = new EventWatcher(t, animB, 'remove');
+  await eventWatcher.wait_for('remove');
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'removed');
+  assert_equals(animC.replaceState, 'active');
+}, 'Queues all remove events before running them');
+
+promise_test(async t => {
+  const outerIframe = document.createElement('iframe');
+  outerIframe.width = 10;
+  outerIframe.height = 10;
+  await insertFrameAndAwaitLoad(t, outerIframe, document);
+
+  const innerIframe = document.createElement('iframe');
+  innerIframe.width = 10;
+  innerIframe.height = 10;
+  await insertFrameAndAwaitLoad(t, innerIframe, outerIframe.contentDocument);
+
+  const div = createDiv(t, innerIframe.contentDocument);
+
+  const animA = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+  const animB = div.animate({ opacity: 1 }, { duration: 1, fill: 'forwards' });
+
+  // Sanity check: The timeline for these animations should be the default
+  // document timeline for div.
+  assert_equals(animA.timeline, innerIframe.contentDocument.timeline);
+  assert_equals(animB.timeline, innerIframe.contentDocument.timeline);
+
+  await animA.finished;
+
+  assert_equals(animA.replaceState, 'removed');
+  assert_equals(animB.replaceState, 'active');
+}, 'Performs removal in deeply nested iframes');
+
+</script>
diff --git a/testing/web-platform/tests/web-animations/timing-model/timelines/update-and-send-events.html b/testing/web-platform/tests/web-animations/timing-model/timelines/update-and-send-events.html
new file mode 100644
index 0000000000..255e013f27
--- /dev/null
+++ b/testing/web-platform/tests/web-animations/timing-model/timelines/update-and-send-events.html
@@ -0,0 +1,257 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>Update animations and send events</title>
+<meta name="timeout" content="long">
+<link rel="help" href="https://drafts.csswg.org/web-animations/#update-animations-and-send-events">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../../testcommon.js"></script>
+<div id="log"></div>
+<script>
+'use strict';
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animation = div.animate(null, 100 * MS_PER_SEC);
+
+  // The ready promise should be resolved as part of micro-task checkpoint
+  // after updating the current time of all timeslines in the procedure to
+  // "update animations and send events".
+  await animation.ready;
+
+  let rAFReceived = false;
+  requestAnimationFrame(() => rAFReceived = true);
+
+  const eventWatcher = new EventWatcher(t, animation, 'cancel');
+  animation.cancel();
+
+  await eventWatcher.wait_for('cancel');
+
+  assert_false(rAFReceived,
+    'cancel event should be fired before requestAnimationFrame');
+}, 'Fires cancel event before requestAnimationFrame');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const animation = div.animate(null, 100 * MS_PER_SEC);
+
+  // Like the above test, the ready promise should be resolved micro-task
+  // checkpoint after updating the current time of all timeslines in the
+  // procedure to "update animations and send events".
+  await animation.ready;
+
+  let rAFReceived = false;
+  requestAnimationFrame(() => rAFReceived = true);
+
+  const eventWatcher = new EventWatcher(t, animation, 'finish');
+  animation.finish();
+
+  await eventWatcher.wait_for('finish');
+
+  assert_false(rAFReceived,
+    'finish event should be fired before requestAnimationFrame');
+}, 'Fires finish event before requestAnimationFrame');
+
+function animationType(anim) {
+  if (anim instanceof CSSAnimation) {
+    return 'CSSAnimation';
+  } else if (anim instanceof CSSTransition) {
+    return 'CSSTransition';
+  } else {
+    return 'ScriptAnimation';
+  }
+}
+
+promise_test(async t => {
+  createStyle(t, { '@keyframes anim': '' });
+  const div = createDiv(t);
+
+  getComputedStyle(div).marginLeft;
+  div.style = 'animation: anim 100s; ' +
+              'transition: margin-left 100s; ' +
+              'margin-left: 100px;';
+  div.animate(null, 100 * MS_PER_SEC);
+  const animations = div.getAnimations();
+
+  let receivedEvents = [];
+  animations.forEach(anim => {
+    anim.onfinish = event => {
+      receivedEvents.push({
+        type: animationType(anim) + ':' + event.type,
+        timeStamp: event.timeStamp
+      });
+    };
+  });
+
+  await Promise.all(animations.map(anim => anim.ready));
+
+  // Setting current time to the time just before the effect end.
+  animations.forEach(anim => anim.currentTime = 100 * MS_PER_SEC - 1);
+
+  await waitForNextFrame();
+
+  assert_array_equals(receivedEvents.map(event => event.type),
+    [ 'CSSTransition:finish', 'CSSAnimation:finish',
+      'ScriptAnimation:finish' ],
+    'finish events for various animation type should be sorted by composite ' +
+    'order');
+}, 'Sorts finish events by composite order');
+
+promise_test(async t => {
+  createStyle(t, { '@keyframes anim': '' });
+  const div = createDiv(t);
+
+  let receivedEvents = [];
+  function receiveEvent(type, timeStamp) {
+    receivedEvents.push({ type, timeStamp });
+  }
+
+  div.onanimationcancel = event => receiveEvent(event.type, event.timeStamp);
+  div.ontransitioncancel = event => receiveEvent(event.type, event.timeStamp);
+
+  getComputedStyle(div).marginLeft;
+  div.style = 'animation: anim 100s; ' +
+              'transition: margin-left 100s; ' +
+              'margin-left: 100px;';
+  div.animate(null, 100 * MS_PER_SEC);
+  const animations = div.getAnimations();
+
+  animations.forEach(anim => {
+    anim.oncancel = event => {
+      receiveEvent(animationType(anim) + ':' + event.type, event.timeStamp);
+    };
+  });
+
+  await Promise.all(animations.map(anim => anim.ready));
+
+  const timeInAnimationReady = document.timeline.currentTime;
+
+  // Call cancel() in reverse composite order.  I.e. canceling for script
+  // animation happen first, then for CSS animation and CSS transition.
+  // 'cancel' events for these animations should be sorted by composite
+  // order.
+  animations.reverse().forEach(anim => anim.cancel());
+
+  // requestAnimationFrame callback which is actually the _same_ frame since we
+  // are currently operating in the `ready` callbac of the animations which
+  // happens as part of the "Update animations and send events" procedure
+  // _before_ we run animation frame callbacks.
+  await waitForAnimationFrames(1);
+
+  assert_times_equal(timeInAnimationReady, document.timeline.currentTime,
+    'A rAF callback should happen in the same frame');
+
+  assert_array_equals(receivedEvents.map(event => event.type),
+    // This ordering needs more clarification in the spec, but the intention is
+    // that the cancel playback event fires before the equivalent CSS cancel
+    // event in each case.
+    [ 'CSSTransition:cancel', 'CSSAnimation:cancel', 'ScriptAnimation:cancel',
+      'transitioncancel', 'animationcancel' ],
+    'cancel events should be sorted by composite order');
+}, 'Sorts cancel events by composite order');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  getComputedStyle(div).marginLeft;
+  div.style = 'transition: margin-left 100s; margin-left: 100px;';
+  const anim = div.getAnimations()[0];
+
+  let receivedEvents = [];
+  anim.oncancel = event => receivedEvents.push(event);
+
+  const eventWatcher = new EventWatcher(t, div, 'transitionstart');
+  await eventWatcher.wait_for('transitionstart');
+
+  const timeInEventCallback = document.timeline.currentTime;
+
+  // Calling cancel() queues a cancel event
+  anim.cancel();
+
+  await waitForAnimationFrames(1);
+  assert_times_equal(timeInEventCallback, document.timeline.currentTime,
+    'A rAF callback should happen in the same frame');
+
+  assert_array_equals(receivedEvents, [],
+    'The queued cancel event shouldn\'t be dispatched in the same frame');
+
+  await waitForAnimationFrames(1);
+  assert_array_equals(receivedEvents.map(event => event.type), ['cancel'],
+    'The cancel event should be dispatched in a later frame');
+}, 'Queues a cancel event in transitionstart event callback');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  getComputedStyle(div).marginLeft;
+  div.style = 'transition: margin-left 100s; margin-left: 100px;';
+  const anim = div.getAnimations()[0];
+
+  let receivedEvents = [];
+  anim.oncancel = event => receivedEvents.push(event);
+  div.ontransitioncancel = event => receivedEvents.push(event);
+
+  await anim.ready;
+
+  anim.cancel();
+
+  await waitForAnimationFrames(1);
+
+  assert_array_equals(receivedEvents.map(event => event.type),
+    [ 'cancel', 'transitioncancel' ],
+    'Playback and CSS events for the same transition should be sorted by ' +
+    'schedule event time and composite order');
+}, 'Sorts events for the same transition');
+
+promise_test(async t => {
+  const div = createDiv(t);
+  const anim = div.animate(null, 100 * MS_PER_SEC);
+
+  let receivedEvents = [];
+  anim.oncancel = event => receivedEvents.push(event);
+  anim.onfinish = event => receivedEvents.push(event);
+
+  await anim.ready;
+
+  anim.finish();
+  anim.cancel();
+
+  await waitForAnimationFrames(1);
+
+  assert_array_equals(receivedEvents.map(event => event.type),
+    [ 'finish', 'cancel' ],
+    'Calling finish() synchronously queues a finish event when updating the ' +
+    'finish state so it should appear before the cancel event');
+}, 'Playback events with the same timeline retain the order in which they are' +
+   'queued');
+
+promise_test(async t => {
+  const div = createDiv(t);
+
+  // Create two animations with separate timelines
+
+  const timelineA = document.timeline;
+  const animA = div.animate(null, 100 * MS_PER_SEC);
+
+  const timelineB = new DocumentTimeline();
+  const animB = new Animation(
+    new KeyframeEffect(div, null, 100 * MS_PER_SEC),
+    timelineB
+  );
+  animB.play();
+
+  animA.currentTime = 99.9 * MS_PER_SEC;
+  animB.currentTime = 99.9 * MS_PER_SEC;
+
+  // When the next tick happens both animations should be updated, and we will
+  // notice that they are now finished. As a result their finished promise
+  // callbacks should be queued. All of that should happen before we run the
+  // next microtask checkpoint and actually run the promise callbacks and
+  // hence the calls to cancel should not stop the existing callbacks from
+  // being run.
+
+  animA.finished.then(() => { animB.cancel() });
+  animB.finished.then(() => { animA.cancel() });
+
+  await Promise.all([animA.finished, animB.finished]);
+}, 'All timelines are updated before running microtasks');
+
+</script>
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