chromium/src/third_party/blink/web_tests/external/wpt/longtask-timing/shared-renderer/resources/frame-with-longtask.html - manifest_repos/chromium_src - Git at Google

 <!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>
	<!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>