I just tried ratchet out on my Android phone - it works well under chrome, but there are some minor CSS problems with the directional buttons under the built in web browser. Performance seems ok, but not lightning quick.
On the ipad everything seems to work ok. I had a bit of trouble with sliding the toggles.
So on the face of it, building an actual HTML5 app should be feasible. Does anybody with an iphone or a windows phone want to comment on how it runs?