Backed out changeset cd9ea65ce9b9 (bug 1258205) for issues in wpt-2 tests

dom/base/Element.cpp

@@ -1177,12 +1177,14 @@ Element::SetAttribute(const nsAString& aName,
                       const nsAString& aValue,
                       ErrorResult& aError)
 {
-  aError = nsContentUtils::CheckQName(aName, false);
-  if (aError.Failed()) {
-    return;
-  }
   const nsAttrName* name = InternalGetExistingAttrNameFromQName(aName);
+
   if (!name) {
+    aError = nsContentUtils::CheckQName(aName, false);
+    if (aError.Failed()) {
+      return;
+    }
+
     nsCOMPtr<nsIAtom> nameAtom;
     if (IsHTMLElement() && IsInHTMLDocument()) {
       nsAutoString lower;