<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
</head>
<body>
  <iframe id="iframe"></iframe>

  <script type="text/javascript">
    "use strict";
    addEventListener("load", () => {
      // Send a heap-minimize observer notification so our script cache is
      // cleared, and our content script isn't available for synchronous
      // insertion.
      window.dispatchEvent(new CustomEvent("MozHeapMinimize"));

      let iframe = document.getElementById("iframe");
      let doc = iframe.contentDocument;
      let win = iframe.contentWindow;
      doc.open("text/html");
      // We need to do two writes here. The first creates the document element,
      // which normally triggers parser blocking. The second triggers the
      // creation of the element we're about to query for, which would normally
      // happen asynchronously if the parser were blocked.
      doc.write("<div id=meh>");
      doc.write("<div id=beer></div>");

      let elem = doc.getElementById("beer");
      top.postMessage(elem instanceof win.HTMLDivElement ? "ok" : "fail",
                      "*");

      doc.close();
    }, {once: true});
  </script>
</body>
</html>
