Here is a little proof of concept I whipped up: a side-scroller using nothing but HTML elements and Javascript. No canvas tag, no flash, just DOM manipulation. It's a testament to the hard work the Mozilla and WebKit teams have put into their renderers and Javascript implementations that something like this can run fairly smoothly. (I have a 2 GHz Core Duo; folks with less endowed processors might disagree with the "run smoothly" assertion.)