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

Source/core/html/HTMLElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2009 Apple Inc. All rights reserved.
  *
  * 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
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 60 matching lines @@
 
     void click();
 
     virtual void accessKeyAction(bool sendMouseEvents);
 
     bool ieForbidsInsertHTML() const;
 
     virtual bool rendererIsNeeded(const RenderStyle&);
     virtual RenderObject* createRenderer(RenderStyle*);
 
-    HTMLFormElement* form() const { return virtualForm(); }
+    HTMLFormElement* formOwner() const;
 
     HTMLFormElement* findFormAncestor() const;
 
     bool hasDirectionAuto() const;
     TextDirection directionalityIfhasDirAutoAttribute(bool& isAuto) const;
 
     virtual bool isHTMLUnknownElement() const { return false; }
 
     virtual bool isLabelable() const { return false; }
     // http://www.whatwg.org/specs/web-apps/current-work/multipage/elements.html#interactive-content
     virtual bool isInteractiveContent() const;
+    // http://www.whatwg.org/specs/web-apps/current-work/#form-associated-element
+    virtual bool isFormAssociatedElement() const { return false; }
     virtual void defaultEventHandler(Event*) OVERRIDE;
 
 protected:
     HTMLElement(const QualifiedName& tagName, Document&, ConstructionType);
 
     void addHTMLLengthToStyle(MutableStylePropertySet*, CSSPropertyID, const String& value);
     void addHTMLColorToStyle(MutableStylePropertySet*, CSSPropertyID, const String& color);
 
     void applyAlignmentAttributeToStyle(const AtomicString&, MutableStylePropertySet*);
     void applyBorderAttributeToStyle(const AtomicString&, MutableStylePropertySet*);
@@ skipping 34 matching lines @@
 inline HTMLElement::HTMLElement(const QualifiedName& tagName, Document& document, ConstructionType type = CreateHTMLElement)
     : Element(tagName, &document, type)
 {
     ASSERT(!tagName.localName().isNull());
     ScriptWrappable::init(this);
 }
 
 } // namespace WebCore
 
 #endif // HTMLElement_h