chromium/third_party/blink/web_tests/http/tests/security/resources/agent-equality-srcdoc.html

<!DOCTYPE html>
<html>
<head>
<title>srcdoc iframe test</title>
</head>
<body>
<iframe srcdoc="&lt;script&gt;window.parent.postMessage(internals.getAgentId(window), '*');&lt;/script&gt;">
</iframe>
<script>
// This test loads an iframe whose content is specified as its srcdoc
// attribute. It posts it's agent ID to this frame, and we report the agent
// IDs of this frame and the child iframe.
//
// Success condition: The agent IDs of this frame and the child frame are
// the same (the child frame is same-origin as the parent frame, unlike the
// data: URL's case. See:
// https://html.spec.whatwg.org/C#determining-the-origin )

function onMessage(evt) {
  let iframeAgentId = evt.data;
  let message = [
    'srcdoc iframe test',
    internals.getAgentId(window),
    iframeAgentId
  ];
  window.parent.postMessage(message, '*');
}

window.addEventListener('message', onMessage, {'once': true});
</script>
</body>
</html>