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+<!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>