file_document_write.html

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<!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>