LayoutTests/imported/w3c/web-platform-tests/webmessaging/broadcastchannel/resources/origin.html

<script>
const bc1 = new BroadcastChannel("ladila"),
      bc2 = new BroadcastChannel("ladila");
bc2.onmessage = e => {
  parent.postMessage(e.origin, "*");
}
bc1.postMessage("does not matter");
</script>