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chromium/third_party/blink/web_tests/external/wpt/web-animations/timing-model/timelines/resources/timeline-frame.html 

<!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>