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

<!DOCTYPE html>
<meta charset="utf-8">
<title>Anchor</title>
<body></body>
<script>
  window.onmessage = (event) => {
    window.onmessage = (event) => parent.postMessage(event.data, "*");
    const { url } = event.data;
    const anchor = document.createElement('a');
    anchor.href = url;
    anchor.rel = 'opener';
    anchor.target = '_blank';
    document.body.appendChild(anchor);
    anchor.click();
  };
</script>