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

Source/core/editing/FrameSelection.cpp

 /*
  * Copyright (C) 2004, 2008, 2009, 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1721 matching lines @@
 
 // Scans logically forward from "start", including any child frames.
 static HTMLFormElement* scanForForm(Node* start)
 {
     if (!start)
         return 0;
     Element* element = start->isElementNode() ? toElement(start) : ElementTraversal::next(*start);
     for (; element; element = ElementTraversal::next(*element)) {
         if (element->hasTagName(formTag))
             return toHTMLFormElement(element);
-        if (element->isHTMLElement() && toHTMLElement(element)->isFormControlElement())
-            return toHTMLFormControlElement(element)->form();
+        if (element->isHTMLElement()) {
+            HTMLFormElement* owner = toHTMLElement(element)->formOwner();

[arv (Not doing code reviews), 2013/12/10 20:37:20]

You could do:

if (HTMLFormElement* owner = toHTMLElement(element)->formOwner())
  return owner;

I'm not sure which style is generally preferred.

+            if (owner)
+                return owner;
+        }
         if (element->hasTagName(frameTag) || element->hasTagName(iframeTag)) {
             Node* childDocument = toHTMLFrameElementBase(element)->contentDocument();
             if (HTMLFormElement* frameResult = scanForForm(childDocument))
                 return frameResult;
         }
     }
     return 0;
 }
 
 // We look for either the form containing the current focus, or for one immediately after it
 HTMLFormElement* FrameSelection::currentForm() const
 {
     // Start looking either at the active (first responder) node, or where the selection is.
     Node* start = m_frame->document()->focusedElement();
     if (!start)
         start = this->start().deprecatedNode();
 
     // Try walking up the node tree to find a form element.
     Node* node;
     for (node = start; node; node = node->parentNode()) {
         if (node->hasTagName(formTag))
             return toHTMLFormElement(node);
-        if (node->isHTMLElement() && toHTMLElement(node)->isFormControlElement())
-            return toHTMLFormControlElement(node)->form();
+        if (node->isHTMLElement()) {
+            HTMLFormElement* owner = toHTMLElement(node)->formOwner();
+            if (owner)
+                return owner;
+        }
     }
 
     // Try walking forward in the node tree to find a form element.
     return scanForForm(start);
 }
 
 void FrameSelection::revealSelection(const ScrollAlignment& alignment, RevealExtentOption revealExtentOption)
 {
     LayoutRect rect;
 
@@ skipping 77 matching lines @@
     sel.showTreeForThis();
 }
 
 void showTree(const WebCore::FrameSelection* sel)
 {
     if (sel)
         sel->showTreeForThis();
 }
 
 #endif