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chromium/third_party/blink/web_tests/external/wpt/wasm/serialization/module/resources/echo-iframe.html 

<!DOCTYPE html>
<meta charset="utf-8">
<title>A test page that echos back anything postMessaged to it to its parent</title>

<script>
"use strict";

window.onmessage = ({ data }) => {
  parent.postMessage(data, "*");
};
</script>