In just under a week, assuming all goes well, I'll be launching a version of Creatures & Castles on the Chrome Web Store.
Almost, but not quite... As far as canvas support goes, Google Chrome, IE9, Safari and Firefox 4 all seem to have the bases covered. I ran into no significant cross-platform issues. Performance was more than acceptable - especially on Windows - even though I was making fairly heavy use of alpha blending. It's a shame that Mac Chrome doesn't offer hardware acceleration on the Canvas.
There are tools such as Monkey that provide a more structured language (and can export to a number of target platforms including HTML5). I did not have time to test this tool, but the results look quite promising. I suspect it will be more useful as a prototyping tool than anything else. However given the pedigree behind it, I wouldn't be unduly surprised to see it used for more.