Be more strict about multiple documentElements

Source/core/dom/ContainerNode.cpp

Index: Source/core/dom/ContainerNode.cpp
diff --git a/Source/core/dom/ContainerNode.cpp b/Source/core/dom/ContainerNode.cpp
index 84f313231a43233e5e35d4e1b4f7d396b4e287a9..78f305711ca43c989daf1cfc2d3c5103f13bab13 100644
--- a/Source/core/dom/ContainerNode.cpp
+++ b/Source/core/dom/ContainerNode.cpp
@@ -152,13 +152,10 @@ bool ContainerNode::checkAcceptChild(const Node* newChild, const Node* oldChild,
         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;
     }