<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<script type="application/javascript">
"use strict";

addEventListener("message", event => {
  if ("ping" in event.data) {
    event.source.postMessage({ pong: event.data.ping }, event.origin);
  }
});
</script>
</head>
<body>
</body>
</html>
