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chromium/third_party/blink/web_tests/http/tests/inspector-protocol/resources/stale-while-revalidate.html 

<body>
<script>
function loadScript() {
  const script = document.createElement("script");
  script.src = "stale-script.php";
  document.body.appendChild(script);
}
loadScript();
setTimeout(loadScript, 10);
</script>
</body>