Allow to have a transparent UA dependent background.

third_party/WebKit/Source/core/layout/LayoutBox.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. 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
@@ skipping 1267 matching lines @@
     // of the paint phase. Potentially returning different results at different phases.
     geometry.calculate(*this, nullptr, GlobalPaintNormalPhase, style()->backgroundLayers(), backgroundRect);
     if (geometry.hasNonLocalGeometry())
         return false;
     paintedExtent = LayoutRect(geometry.destRect());
     return true;
 }
 
 bool LayoutBox::backgroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect) const
 {
+    // Instead of returning false unconditionnaly for the document, check if it
+    // is transparent.
     if (isDocumentElement() || backgroundStolenForBeingBody())
-        return false;
+        return !isTransparent();

[trchen, 2016/03/08 01:57:21]

This change is incorrect. The document element indeed never paints backgrounds,
and the body doesn't paint background if it is stolen as root background.

And I don't think it is related to the thing you want to achieve. If your
problem is things being considered opaque when they are not, why making more
things considered opaque helps?

 
     Color backgroundColor = resolveColor(CSSPropertyBackgroundColor);
     if (backgroundColor.hasAlpha())
         return false;
 
     // If the element has appearance, it might be painted by theme.
     // We cannot be sure if theme paints the background opaque.
     // In this case it is safe to not assume opaqueness.
     // FIXME: May be ask theme if it paints opaque.
     if (style()->hasAppearance())
@@ skipping 3378 matching lines @@
 }
 
 void LayoutBox::IntrinsicSizingInfo::transpose()
 {
     size = size.transposedSize();
     aspectRatio = aspectRatio.transposedSize();
     std::swap(hasWidth, hasHeight);
 }
 
 } // namespace blink