Use Shadow DOM to display fallback content for images

Source/core/html/HTMLElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004-2007, 2009, 2014 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 81 matching lines @@
     static const AtomicString& eventNameForAttributeName(const QualifiedName& attrName);
 
     virtual bool matchesReadOnlyPseudoClass() const override;
     virtual bool matchesReadWritePseudoClass() const override;
 
     static const AtomicString& eventParameterName();
 
     HTMLMenuElement* contextMenu() const;
     void setContextMenu(HTMLMenuElement*);
 
+    virtual void ensureFallbackContent() { }
+    virtual void ensurePrimaryContent() { }
+    virtual String altText() const { return String(); }
+
 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*);
 
     virtual void parseAttribute(const QualifiedName&, const AtomicString&) override;
@@ skipping 61 matching lines @@
     template<typename T> inline bool is##thisType(const PassRefPtr<T>& node) { return is##thisType(node.get()); } \
     template<typename T> inline bool is##thisType(const RefPtr<T>& node) { return is##thisType(node.get()); } \
     template <> inline bool isElementOfType<const thisType>(const HTMLElement& element) { return is##thisType(element); } \
     DEFINE_ELEMENT_TYPE_CASTS_WITH_FUNCTION(thisType)
 
 } // namespace blink
 
 #include "core/HTMLElementTypeHelpers.h"
 
 #endif // HTMLElement_h