It seems standard practice for Web authors to measure performance in FPS (frames per second). There are two large problems with this.
First, FPS is meaningless above 60 or so, because you normally can't get more frames than that onto the screen. A well-designed software stack will actually prevent FPS from rising above 60 --- those extra frames are wasting power. More than 60 FPS means you have a bug (or you are doing very specialized full-screen gaming --- i.e. not a Web browser). I discussed this in more detail here.
Second, Web authors normally measure FPS as "how many times did my event handler run per second". But what they actually should be measuring is how many different frames the browser was able to paint to the screen per second, and that's a very different thing, since a setTimeout handler (for example) can run multiple times between browser repaints. It's understandable that Web authors would make such a mistake, since browsers haven't provided a way to measure the real FPS. However, in Firefox 4 we have added an API for this: mozPaintCount. Use it!
I think authors need to get into the habit of measuring FPS using mozPaintCount or something similar on other browsers and determining first whether their app is able to reach 50-60FPS. If it can, then further performance improvements can only be measured by increasing the complexity of the scene until FPS starts dropping off. Microsoft's FishIE Tank is the right approach. (I don't know how they try to measure FPS though; they're probably not using mozPaintCount, so it's probably not very accurate.)