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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;
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 HTMLDivElement ? "ok" : "fail",
"*");
doc.close();
}, {once: true});
</script>
</body>
</html>