Non-activated default buttons prevent implicit form submission.

Source/core/html/HTMLButtonElement.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, 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 42 matching lines @@
     virtual void parseAttribute(const QualifiedName&, const AtomicString&) OVERRIDE;
     virtual bool isPresentationAttribute(const QualifiedName&) const OVERRIDE;
     virtual void defaultEventHandler(Event*);
 
     virtual bool appendFormData(FormDataList&, bool);
 
     virtual bool isEnumeratable() const { return true; }
     virtual bool supportLabels() const OVERRIDE { return true; }
     virtual bool isInteractiveContent() const OVERRIDE;
 
-    virtual bool isSuccessfulSubmitButton() const;
+    virtual bool canBeSuccessfulSubmitButton() const OVERRIDE;
     virtual bool isActivatedSubmit() const;
     virtual void setActivatedSubmit(bool flag);
 
     virtual void accessKeyAction(bool sendMouseEvents);
     virtual bool isURLAttribute(const Attribute&) const OVERRIDE;
 
     virtual bool canStartSelection() const { return false; }
 
     virtual bool isOptionalFormControl() const { return true; }
     virtual bool recalcWillValidate() const;
 
     Type m_type;
     bool m_isActivatedSubmit;
 };
 
 } // namespace
 
 #endif