[giow] (3) Make sure :disabled matches fieldsets inside disabled fieldsets, since apparently that's a thing. Also, editorial cleanup around 'disabled'.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=26454
Affected topics: CSS, HTML

git-svn-id: http://svn.whatwg.org/webapps@8753 340c8d12-0b0e-0410-8428-c7bf67bfef74