diff --git a/content/html/content/src/nsGenericHTMLElement.cpp b/content/html/content/src/nsGenericHTMLElement.cpp
index 71bb5cf4b2bc..ca0a04334cf1 100644
--- a/content/html/content/src/nsGenericHTMLElement.cpp
+++ b/content/html/content/src/nsGenericHTMLElement.cpp
@@ -4439,6 +4439,9 @@ nsGenericHTMLElement::InternalGetExistingAttrNameFromQName(const nsAString& aStr
   }
 
   NS_ConvertUTF16toUTF8 utf8name(aStr);
+  if (mNodeInfo->NamespaceEquals(kNameSpaceID_None)) {
+    ToLowerCase(utf8name);
+  }
 
   PRInt32 i, count = 0;
   mAttributes->GetAttributeCount(count);