Non-activated default buttons prevent implicit form submission.

Source/core/html/HTMLFormControlElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010 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 63 matching lines @@
 
     virtual const AtomicString& formControlType() const OVERRIDE = 0;
 
     virtual bool canTriggerImplicitSubmission() const { return false; }
 
     // Override in derived classes to get the encoded name=value pair for submitting.
     // Return true for a successful control (see HTML4-17.13.2).
     virtual bool appendFormData(FormDataList&, bool) { return false; }
     virtual String resultForDialogSubmit();
 
-    virtual bool isSuccessfulSubmitButton() const { return false; }
+    virtual bool canBeSuccessfulSubmitButton() const { return false; }
+    bool isSuccessfulSubmitButton() const;
     virtual bool isActivatedSubmit() const { return false; }
     virtual void setActivatedSubmit(bool) { }
 
     enum CheckValidityDispatchEvents { CheckValidityDispatchEventsAllowed, CheckValidityDispatchEventsNone };
 
     virtual bool willValidate() const;
     void updateVisibleValidationMessage();
     void hideVisibleValidationMessage();
     bool checkValidity(Vector<RefPtr<FormAssociatedElement> >* unhandledInvalidControls = 0, CheckValidityDispatchEvents = CheckValidityDispatchEventsAllowed);
     // This must be called when a validation constraint or control value is changed.
@@ skipping 110 matching lines @@
 
 inline const HTMLFormControlElement& toHTMLFormControlElement(const FormAssociatedElement& control)
 {
     ASSERT_WITH_SECURITY_IMPLICATION(control.isFormControlElement());
     return static_cast<const HTMLFormControlElement&>(control);
 }
 
 } // namespace
 
 #endif