chromium/third_party/blink/web_tests/external/wpt/fetch/fetch-later/resources/fetch-later.html

<!DOCTYPE html>
<head>
</head>
<body>
  <script>
    const PARAMS = new URL(location.href).searchParams;
    const TARGET_URL= decodeURIComponent(PARAMS.get('url')) || '';

    fetchLater(TARGET_URL, {activateAfter: 0});
    if (window.opener) {
      window.opener.postMessage("done", "*");
    }
  </script>
</body>