The opacity of an RGBa statement is inherited to it's parent element.
The link is available in https://web.archive.org/web/20100315031259/http://nicewebtype.com/notes/2009/07/12/rgba-text-shadow-in-safari-firefox/. In Comment, it is mentioned by Dave Hyatt that it might be bug in CG rather than Webkit. I don't have any reproducible test case but if someone can comment whether it is fixed or not. Thanks!
Pretty sure this was an OS bug that was fixed long ago.