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chromium
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chromium/chrome/test/data/webui/chromeos/ash_common/post_message_api/iframe.html 

<html>
<body>
<script type="module" src="chrome://webui-test/chromeos/ash_common/post_message_api/post_message_api_client_test.js"></script>
HELLO WORLD.
</body>
</html>