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chromium/third_party/blink/web_tests/external/wpt/fetch/private-network-access/resources/iframer.html 

<!DOCTYPE html>
<meta charset="utf-8">
<title>Iframer</title>
<body></body>
<script>
  const child = document.createElement("iframe");
  child.src = new URL(window.location).searchParams.get("url");
  document.body.appendChild(child);
</script>