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<!DOCTYPE html>
<script>
const channel = new MessageChannel();
window.parent.postMessage(channel.port2, '*', [channel.port2]);
channel.port1.onmessage = e => { alert(e.data); channel.port1.postMessage("message event received") };
channel.port1.onmessageerror = () => channel.port1.postMessage("messageerror event received");
</script>