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chromium/third_party/blink/web_tests/external/wpt/fetch/private-network-access/resources/iframed-no-preflight-received.html 

<!DOCTYPE html>
<meta charset="utf-8">
<title>Iframed</title>
<script>
  const uuid = new URL(window.location).searchParams.get("iframe-uuid");
  top.postMessage({ uuid, message: "no preflight received" }, "*");
</script>