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chromium/third_party/blink/web_tests/external/wpt/html/browsers/windows/nested-browsing-contexts/resources/frameElement-sibling-accessed.html 

<!DOCTYPE html>
<meta charset="utf-8">
<title>This page will set its document.domain on request so that its sibling can access it</title>

<h1>I did get loaded</h1>

<script>
"use strict";

window.onmessage = e => {
  const { newDocumentDomain } = e.data;
  document.domain = newDocumentDomain;

  parent.postMessage("done", "*");
};
</script>