Original code found here.
Changes I've made to original code: (a) eliminated two spurious redraws of entire background; (b) removed some CSS so you can see the framerate more easily when zooming, etc.; (c) removed empty div tag to make valid html5 (allowing these comments to remain visible); (d) changed background color of page to show transparency in canvas; (e) filled the background (once) at the beginning; (f) reduced the part of the ground that gets redrawn to just the bit under the ball (and even that is excessive); (h) removed the 60fps framerate cap.
I haven't eliminated any of the other horribleness on this page (yet) since I'm not sure how greatly it parallels equivalent stuff in the Flash comparison. E.g. using all kinds of array hocus pocus and regexp just to calculate the framerate seems to me to be ridiculous, but ... not my problem.