[e] (0) Try to clarify browsing context rules
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=20939
Affected topics: HTML, Security, Video and Audio

git-svn-id: http://svn.whatwg.org/webapps@7828 340c8d12-0b0e-0410-8428-c7bf67bfef74