One of the subtests in imported/w3c/web-platform-tests/web-animations/timing-model/timelines/update-and-send-events.html is skipped because it fails with a flaky output: 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'); We need to stop skipping this test