mozilla-release/testing/web-platform/tests/animation-worklet/worklet-animation-local-time-before-start.https.html

<html class="reftest-wait">
<title>Animation Worklet local time set before start</title>
<link rel="help" href="https://drafts.css-houdini.org/css-animationworklet/">
<meta name="assert" content="The local time should be trimmed by the duration, e.g. this is equivalent to effect.localTime = 0">
<link rel="match" href="worklet-animation-local-time-before-start-ref.html">

<script src="/web-animations/testcommon.js"></script>
<script src="/common/reftest-wait.js"></script>
<script src="common.js"></script>

<style>
  #box {
    width: 100px;
    height: 100px;
    background-color: green;
  }
</style>

<div id="box"></div>

<script>
  registerConstantLocalTimeAnimator(-500).then(() => {
    const box = document.getElementById('box');
    const effect = new KeyframeEffect(box,
      [
      { transform: 'translateY(200px)' },
      { transform: 'translateY(0px)' }
      ], {
        duration: 1000,
        fill: 'backwards'
      }
    );

    const animation = new WorkletAnimation('constant_time', effect);
    animation.play();

    waitForAsyncAnimationFrames(1).then(_ => {
      takeScreenshot();
    });
  });
</script>