Use Shadow DOM to display fallback content for images

Source/core/html/HTMLImageElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google 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 16 matching lines @@
 #include "core/html/HTMLElement.h"
 #include "core/html/HTMLImageLoader.h"
 #include "core/html/canvas/CanvasImageSource.h"
 #include "platform/graphics/GraphicsTypes.h"
 #include "wtf/WeakPtr.h"
 
 namespace blink {
 
 class HTMLFormElement;
 class ImageCandidate;
+class ShadowRoot;
 
 class HTMLImageElement final : public HTMLElement, public CanvasImageSource {
     DEFINE_WRAPPERTYPEINFO();
 public:
     class ViewportChangeListener;
 
     static PassRefPtrWillBeRawPtr<HTMLImageElement> create(Document&);
     static PassRefPtrWillBeRawPtr<HTMLImageElement> create(Document&, HTMLFormElement*, bool createdByParser);
     static PassRefPtrWillBeRawPtr<HTMLImageElement> createForJSConstructor(Document&, int width, int height);
 
     virtual ~HTMLImageElement();
     virtual void trace(Visitor*) override;
 
     int width(bool ignorePendingStylesheets = false);
     int height(bool ignorePendingStylesheets = false);
 
     int naturalWidth() const;
     int naturalHeight() const;
     const String& currentSrc() const;
 
     bool isServerMap() const;
 
-    const AtomicString& altText() const;
+    virtual String altText() const override final;
 
     ImageResource* cachedImage() const { return imageLoader().image(); }
     void setImageResource(ImageResource* i) { imageLoader().setImage(i); };
 
     void setLoadingImageDocument() { imageLoader().setLoadingImageDocument(); }
 
     void setHeight(int);
 
     KURL src() const;
     void setSrc(const String&);
 
     void setWidth(int);
 
     int x() const;
     int y() const;
 
     bool complete() const;
 
     bool hasPendingActivity() const { return imageLoader().hasPendingActivity(); }
 
     virtual bool canContainRangeEndPoint() const override { return false; }
 
     void addClient(ImageLoaderClient* client) { imageLoader().addClient(client); }
     void removeClient(ImageLoaderClient* client) { imageLoader().removeClient(client); }
 
     virtual const AtomicString imageSourceURL() const override;
 
     virtual HTMLFormElement* formOwner() const override;
     void formRemovedFromTree(const Node& formRoot);
+    virtual void ensureFallbackContent();
+    virtual void ensurePrimaryContent();
 
     // CanvasImageSourceImplementations
     virtual PassRefPtr<Image> getSourceImageForCanvas(SourceImageMode, SourceImageStatus*) const override;
     virtual bool wouldTaintOrigin(SecurityOrigin*) const override;
     virtual FloatSize sourceSize() const override;
     virtual FloatSize defaultDestinationSize() const override;
     virtual const KURL& sourceURL() const override;
 
     // public so that HTMLPictureElement can call this as well.
     void selectSourceURL(ImageLoader::UpdateFromElementBehavior);
+    void reattachFallbackContent();
+    void setUseFallbackContent();
+    void setIsFallbackImage() { m_isFallbackImage = true; }
+
 protected:
     explicit HTMLImageElement(Document&, HTMLFormElement* = 0, bool createdByParser = false);
 
     virtual void didMoveToNewDocument(Document& oldDocument) override;
+    virtual bool useFallbackContent() const { return m_useFallbackContent; }
 
+    virtual void didAddUserAgentShadowRoot(ShadowRoot&) override;
+    virtual PassRefPtr<RenderStyle> customStyleForRenderer() override;
 private:
     virtual bool areAuthorShadowsAllowed() const override { return false; }
 
     virtual void parseAttribute(const QualifiedName&, const AtomicString&) override;
     virtual bool isPresentationAttribute(const QualifiedName&) const override;
     virtual void collectStyleForPresentationAttribute(const QualifiedName&, const AtomicString&, MutableStylePropertySet*) override;
 
     virtual void attach(const AttachContext& = AttachContext()) override;
     virtual RenderObject* createRenderer(RenderStyle*) override;
 
@@ skipping 25 matching lines @@
     Member<HTMLFormElement> m_form;
 #else
     WeakPtr<HTMLFormElement> m_form;
 #endif
     AtomicString m_bestFitImageURL;
     float m_imageDevicePixelRatio;
     unsigned m_formWasSetByParser : 1;
     unsigned m_elementCreatedByParser : 1;
     // Intrinsic sizing is viewport dependant if the 'w' descriptor was used for the picked resource.
     unsigned m_intrinsicSizingViewportDependant : 1;
+    unsigned m_useFallbackContent : 1;
+    unsigned m_isFallbackImage : 1;
 };
 
 } // namespace blink
 
 #endif // HTMLImageElement_h