Only 'form associated' elements have a potential form owner.

Source/core/html/HTMLLabelElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2010 Apple Inc. All rights reserved.
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 67 matching lines @@
     }
 
     if (Element* element = treeScope().getElementById(controlId)) {
         if (supportsLabels(element))
             return toLabelableElement(element);
     }
 
     return 0;
 }
 
-HTMLFormElement* HTMLLabelElement::form() const
+HTMLFormElement* HTMLLabelElement::virtualForm() const
 {
     return FormAssociatedElement::findAssociatedForm(this, 0);
 }
 
 void HTMLLabelElement::setActive(bool down)
 {
     if (down == active())
         return;
 
     // Update our status first.
@@ skipping 85 matching lines @@
 
 void HTMLLabelElement::accessKeyAction(bool sendMouseEvents)
 {
     if (HTMLElement* element = control())
         element->accessKeyAction(sendMouseEvents);
     else
         HTMLElement::accessKeyAction(sendMouseEvents);
 }
 
 } // namespace