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chromium/third_party/blink/web_tests/external/wpt/longtask-timing/shared-renderer/resources/frame-with-longtask.html 

<!DOCTYPE HTML>
<meta charset=utf-8>
<meta name="viewport" content="width=device-width">

<title>Long Task Frame</title>
<body>
<h1>Long Task plus PostMessage</h1>

<script>
  const begin = window.performance.now();
  while (window.performance.now() < begin + 60);
  window.opener.postMessage('Finished.', '*');
</script>
</body>