gecko-dev/testing/web-platform/tests/fullscreen/api/resources/attempt-fullscreen.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>IFrame Fullscreen API success reporter</title>
<body>
<script>
reportFullscreenSuccessful = (success) => {
  return () => {
    parent.postMessage({"report": {
      "api": "fullscreen",
      "result": success,
      "frame": window.name
    }}, "*");
  };
};

window.addEventListener('message', e => {
  if (e.data.action == "report") {
    document.onfullscreenchange = reportFullscreenSuccessful(true);
    document.onfullscreenerror = reportFullscreenSuccessful(false);
    document.body.requestFullscreen();
  }
});
</script>