Be more strict about multiple documentElements

Source/core/dom/ContainerNode.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2013 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 134 matching lines @@
     if (newChild->isPseudoElement()) {
         exceptionState.throwDOMException(HierarchyRequestError, "The new child element is a pseudo-element.");
         return false;
     }
 
     if (containsConsideringHostElements(*newChild)) {
         exceptionState.throwDOMException(HierarchyRequestError, "The new child element contains the parent.");
         return false;
     }
 
-    if (oldChild && isDocumentNode()) {
-        if (!toDocument(this)->canReplaceChild(*newChild, *oldChild)) {
-            // FIXME: Adjust 'Document::canReplaceChild' to return some additional detail (or an error message).
-            exceptionState.throwDOMException(HierarchyRequestError, "Failed to replace child.");
-            return false;
-        }
-    } else if (!isChildTypeAllowed(*newChild)) {
+    if (isDocumentNode())
+        return toDocument(this)->canAcceptChild(*newChild, oldChild, exceptionState);
+
+    if (!isChildTypeAllowed(*newChild)) {
         exceptionState.throwDOMException(HierarchyRequestError, "Nodes of type '" + newChild->nodeName() + "' may not be inserted inside nodes of type '" + nodeName() + "'.");
         return false;
     }
 
     return true;
 }
 
 bool ContainerNode::checkAcceptChildGuaranteedNodeTypes(const Node& newChild, ExceptionState& exceptionState) const
 {
     ASSERT(isChildTypeAllowed(newChild));
@@ skipping 1262 matching lines @@
         return true;
 
     if (node->isElementNode() && toElement(node)->shadow())
         return true;
 
     return false;
 }
 #endif
 
 } // namespace blink