Bug 1264323 - Set namespace when DOMParser.parseFromString(application/xhtml+xml). r=smaug

dom/base/nsDocument.cpp

@@ -13375,6 +13375,11 @@ nsIDocument::SetCachedEncoder(already_AddRefed<nsIDocumentEncoder> aEncoder)
 void
 nsIDocument::SetContentTypeInternal(const nsACString& aType)
 {
+  if (!IsHTMLOrXHTML() && mDefaultElementType == kNameSpaceID_None &&
+      aType.EqualsLiteral("application/xhtml+xml")) {
+    mDefaultElementType = kNameSpaceID_XHTML;
+  }
+
   mCachedEncoder = nullptr;
   mContentType = aType;
 }

testing/web-platform/meta/dom/nodes/Document-createElement-namespace.html.ini

@@ -6,6 +6,3 @@
   [Created element's namespace in created SVG document]
     expected: FAIL
 
-  [Created element's namespace in created XHTML document by DOMParser ('application/xhtml+xml')]
-    expected: FAIL
-