Codebase Browser
chromium
Go to App

chromium/third_party/blink/web_tests/external/wpt/html/infrastructure/safe-passing-of-structured-data/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>