[] (0) integrate upstream <picture> updates

git-svn-id: http://svn.whatwg.org/webapps@8680 340c8d12-0b0e-0410-8428-c7bf67bfef74
[] (0) integrate upstream <picture> updates

git-svn-id: http://svn.whatwg.org/webapps@8681 340c8d12-0b0e-0410-8428-c7bf67bfef74
Merge remote branch 'origin/master' into whatwg