Unless it's a very simple game (very little animation, no physics etc) then a game that uses the DOM (and CSS) for game objects will perform horribly.

HTML5 canvas is definitely a possibility though. Check out http://impactjs.com/

Its a great game engine which will allow you to create all manner of high-performance games on iphone4+ hardware (i've never tested on <iphone4, so performance may be ok there too)