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diff --git a/dom/bindings/test/test_InstanceOf.html b/dom/bindings/test/test_InstanceOf.html
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+++ b/dom/bindings/test/test_InstanceOf.html
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+<!DOCTYPE HTML>
+<html>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=748983
+-->
+<head>
+  <meta charset="utf-8">
+  <title>Test for Bug 748983</title>
+  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
+</head>
+<body>
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=748983">Mozilla Bug 748983</a>
+<p id="display"></p>
+<div id="content" style="display: none">
+</div>
+<pre id="test">
+<script type="application/javascript">
+
+/** Test for Bug 748983 **/
+
+SimpleTest.waitForExplicitFinish();
+
+function runTest()
+{
+    ok(document instanceof EventTarget, "document is an event target")
+    ok(new XMLHttpRequest() instanceof XMLHttpRequest, "instanceof should work on XHR");
+    ok(HTMLElement.prototype instanceof Node, "instanceof needs to walk the prototype chain")
+
+    var otherWin = document.getElementById("testFrame").contentWindow;
+
+    ok(otherWin.HTMLElement.prototype instanceof otherWin.Node, "Same-origin instanceof of a interface prototype object should work, even if called cross-origin");
+    ok(!(otherWin.HTMLElement.prototype instanceof Node), "Cross-origin instanceof of a interface prototype object shouldn't work");
+
+    // We need to reset HTMLElement.prototype.__proto__ to the original value
+    // before using anything from the harness, otherwise the harness code breaks
+    // in weird ways.
+    HTMLElement.prototype.__proto__ = otherWin.Element.prototype;
+    var [ shouldSucceed, shouldFail ] = otherWin.runTest();
+    shouldSucceed = shouldSucceed && HTMLElement.prototype instanceof otherWin.Element;
+    shouldFail = shouldFail && HTMLElement.prototype instanceof Element;
+    HTMLElement.prototype.__proto__ = Element.prototype;
+
+    ok(shouldSucceed, "If an interface prototype object is on the protochain then instanceof with the interface object should succeed");
+    ok(!shouldFail, "If an interface prototype object is not on the protochain then instanceof with the interface object should succeed");
+
+    SimpleTest.finish();
+}
+
+</script>
+</pre>
+<iframe id="testFrame" src="file_InstanceOf.html" onload="runTest()"></iframe>
+</body>
+</html>