[giow] (2) Define how focus sequential navigation works in slightly more detail; make navigating to an unfocusable control lead to the viewport being refocused.

Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=26907
Affected topics: HTML

git-svn-id: https://svn.whatwg.org/webapps@8845 340c8d12-0b0e-0410-8428-c7bf67bfef74

Author: Hixie