Use Shadow DOM to display fallback content for images

Source/core/html/HTMLImageElement.h

Index: Source/core/html/HTMLImageElement.h
diff --git a/Source/core/html/HTMLImageElement.h b/Source/core/html/HTMLImageElement.h
index 7b5d9a21699042b08fd862ba27b99ce2463e743d..518cfb44373ddbfa2fe93aca8c853b12c81329f7 100644
--- a/Source/core/html/HTMLImageElement.h
+++ b/Source/core/html/HTMLImageElement.h
@@ -57,7 +57,7 @@ public:
 
     bool isServerMap() const;
 
-    const AtomicString& altText() const;
+    virtual String altText() const OVERRIDE FINAL;
 
     CompositeOperator compositeOperator() const { return m_compositeOperator; }
 
@@ -89,6 +89,8 @@ public:
 
     virtual HTMLFormElement* formOwner() const OVERRIDE;
     void formRemovedFromTree(const Node& formRoot);
+    virtual void ensureFallbackContent() OVERRIDE FINAL;
+    virtual void ensurePrimaryContent() OVERRIDE FINAL;
 
     // CanvasImageSourceImplementations
     virtual PassRefPtr<Image> getSourceImageForCanvas(SourceImageMode, SourceImageStatus*) const;
@@ -99,11 +101,18 @@ public:
 
     // public so that HTMLPictureElement can call this as well.
     void selectSourceURL(ImageLoader::UpdateFromElementBehavior);
+    void reattachFallbackContent();
+    void setUseFallbackContent() { m_useFallbackContent = true; ensureUserAgentShadowRoot(); }
+    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; }
 
@@ -135,6 +144,7 @@ private:
     HTMLImageLoader& imageLoader() const { return *m_imageLoader; }
     void notifyViewportChanged();
     void createMediaQueryListIfDoesNotExist();
+    Text* altTextNode() { return m_altTextNode; }
 
     OwnPtrWillBeMember<HTMLImageLoader> m_imageLoader;
     RefPtrWillBeMember<ViewportChangeListener> m_listener;
@@ -143,6 +153,7 @@ private:
 #else
     WeakPtr<HTMLFormElement> m_form;
 #endif
+    Text* m_altTextNode;

[esprehn, 2014/09/09 19:52:41]

There's no reason for this, just do
userAgentShadowRoot()->getElementById("alt-text") whenever you need to get it,
then use setTextContent to update it.

     CompositeOperator m_compositeOperator;
     AtomicString m_bestFitImageURL;
     float m_imageDevicePixelRatio;
@@ -152,6 +163,8 @@ private:
     unsigned m_intrinsicSizingViewportDependant : 1;
     // Effective size is viewport dependant if the sizes attribute's effective size used v* length units.
     unsigned m_effectiveSizeViewportDependant : 1;
+    unsigned m_useFallbackContent : 1;
+    unsigned m_isFallbackImage : 1;
 };
 
 } // namespace blink