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+<!DOCTYPE html>
+<html>
+<head>
+<title>If prototype is already an interface prototype object, Document.registerElement() throws a NotSupportedError</title>
+<meta name="author" title="Sergey G. Grekhov" href="mailto:sgrekhov@unipro.ru">
+<meta name="assert" content="If PROTOTYPE is already an interface prototype object for any interface object, throw a NotSupportedError and stop">
+<link rel="help" href="http://www.w3.org/TR/custom-elements/#instantiating-custom-elements">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../testcommon.js"></script>
+</head>
+<body>
+<div id="log"></div>
+<script>
+test(function() {
+    var doc = newHTMLDocument();
+    HTML5_ELEMENTS.forEach(function(tagName) {
+        var obj = doc.createElement(tagName);
+        var name = 'x-a-' + tagName;
+        assert_throws('NotSupportedError', function() {
+            doc.registerElement(name, {prototype: obj.constructor.prototype});
+        }, 'Exception should be thrown in case of attempt to register element ' +
+            'if prototype is already an interface prototype object (' + name + ')');
+    });
+}, 'Test Document.registerElement() throws NotSupportedError ' +
+    'if prototype is already an interface prototype object');
+
+
+test(function() {
+    var doc = newHTMLDocument();
+    var proto = Object.create(HTMLElement.prototype);
+    doc.registerElement('x-b', {
+        prototype: proto
+    });
+    assert_throws('NotSupportedError', function() {
+        doc.registerElement('x-b', {
+            prototype: proto
+        });
+    }, 'Exception should be thrown if registring custom element type with already used prototype');
+}, 'Test Document.registerElement() throws NotSupportedError ' +
+    'if prototype is already used for another custom element type');
+</script>
+</body>
+</html>