<html>
  <head>
    <script>
      function load() {
        // Let the test page know that it can try to navigate.
        top.postMessage("onload", "*");
        // We're starting an infinite loop, but we need to time out after a
        // while, or the loop will keep running until shutdown.
        let t0 = performance.now();
        while (performance.now() - t0 < 5000) {
            document.getElementById("output").innerText = Math.random();
        }
      }
    </script>
  </head>
  <body onload="load()">
    <p id="output"></p>
  </body>
</html>
