Use Shadow DOM to display fallback content for images

Source/core/accessibility/AXRenderObject.cpp

 /*
 * Copyright (C) 2008 Apple Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1.  Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 271 matching lines @@
     if (m_renderer->isText())
         return StaticTextRole;
     if (cssBox && cssBox->isImage()) {
         if (isHTMLInputElement(node))
             return ariaHasPopup() ? PopUpButtonRole : ButtonRole;
         if (isSVGImage())
             return SVGRootRole;
         return ImageRole;
     }
 
+    // This is for the case where the fallback content kicks in.
+    // The normal case should be handled in the case above
+    if (node && node->hasTagName(HTMLNames::imgTag))
+        return ImageRole;
+
     // Note: if JavaScript is disabled, the renderer won't be a RenderHTMLCanvas.
     if (isHTMLCanvasElement(node) && m_renderer->isCanvas())
         return CanvasRole;
 
     if (cssBox && cssBox->isRenderView())
         return WebAreaRole;
 
     if (cssBox && cssBox->isTextField())
         return TextFieldRole;
 
@@ skipping 2042 matching lines @@
         if (label && label->renderer()) {
             LayoutRect labelRect = axObjectCache()->getOrCreate(label)->elementRect();
             result.unite(labelRect);
         }
     }
 
     return result;
 }
 
 } // namespace blink