It looks really great (and it's quite fun to play) in mozilla-central nightly builds! (In earlier Firefox versions, with no SMIL support, the game logic still mostly works, but the enemies and items won't animate.)
Marek's been iterating on this game pretty rapidly, adding new features and fixing bugs every few days. His latest version will always be linked from his blog. Give it a try, and send him any feedback that you might have!