--- a/dom/base/nsIContent.h
+++ b/dom/base/nsIContent.h
@@ -916,30 +916,33 @@ public:
 
   /**
    * If the content is a part of HTML editor, this returns editing
    * host content.  When the content is in designMode, this returns its body
    * element.  Also, when the content isn't editable, this returns null.
    */
   mozilla::dom::Element* GetEditingHost();
 
+  bool SupportsLangAttr() const {
+    return IsHTMLElement() || IsSVGElement() || IsXULElement();
+  }
+
   /**
    * Determining language. Look at the nearest ancestor element that has a lang
    * attribute in the XML namespace or is an HTML/SVG element and has a lang in
    * no namespace attribute.  Returns false if no language was specified.
    */
   bool GetLang(nsAString& aResult) const {
     for (const nsIContent* content = this; content; content = content->GetParent()) {
       if (content->GetAttrCount() > 0) {
         // xml:lang has precedence over lang on HTML elements (see
         // XHTML1 section C.7).
         bool hasAttr = content->GetAttr(kNameSpaceID_XML, nsGkAtoms::lang,
                                           aResult);
-        if (!hasAttr && (content->IsHTMLElement() || content->IsSVGElement() ||
-            content->IsXULElement())) {
+        if (!hasAttr && content->SupportsLangAttr()) {
           hasAttr = content->GetAttr(kNameSpaceID_None, nsGkAtoms::lang,
                                      aResult);
         }
         NS_ASSERTION(hasAttr || aResult.IsEmpty(),
                      "GetAttr that returns false should not make string non-empty");
         if (hasAttr) {
           return true;
         }