I really don't see the point. HTML5/JS don't really solve any problems that GTK/Qt don't (doubly so, since Qt can already handle that model). Cloud applications are one thing (easy access from anywhere is their one advantage), but making a cloud application a local application just makes zero sense to me (lets take the benefits of cloud, remove them, so we only have the handicaps!).
I can only see this /possibly/ working if you do the same thing as Chrome OS: target devices at the bottom end of the netbook range... but then again, Android is already stealing ChromeOS's niche- with the advantages of local applications!
(the Quake 2 demo was impressive... for a web browser. Remember, that's a -thirteen- year old game they got to work... and only using unreleased browsers...)