src/css/CSSSerialize.h (CSSSerializeRGB): Fix a bug; note rgba is not valid in CSS 2.1
(CSSStyleDeclarationImp::isRelativelyPositioned): New
[CSSOM View] document.elementFromPoint() should return the top-most element

(StackingContext::boxFromPoint): New
(ViewCSSImp::boxFromPoint, Box::boxFromPoint): Refine