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Ian Hickson has blogged about the performance side of Acid 3:

The Acid3 test says "To pass the test, a browser must use its default settings, the animation has to be smooth, the score has to end on 100/100, and the final page has to look exactly, pixel for pixel, like this reference rendering". (Emphasis mine.)

There has been some question as to what "the animation has to be smooth" means.

The idea is to make sure that browsers focus on performance as well as standards. Performance isn't a standards-compliance issue, but it is something that affects all Web authors and users. If a browser passes all 100/100 subtests and gets the rendering pixel-for-pixel correct (including the favicon!), then it has passed the standards-compliance parts of the Acid3 test. The rest is just a competition for who can be the fastest.

To determine the "score" for performance in a browser that gets 100/100, click on the "A" of "Acid3" on the test after having run the test twice (so that the test uses the browser's cache). An alert should pop up, giving a total time elapsed, and reporting any tests that took longer than 33ms. Test 26 is the only one that should take any significant amount of time, as it contains a tight loop doing some common DOM and JS operations. The test has "passed", for the purposes of the "smoothness" criteria, if all the tests took less than 33ms (it'll give you a message saying "No JS errors and no timing issues." if this happens). Then the only issue is the total time — is it faster than all the other browsers?

This is easy to nitpick (Ian talks about the "what hardware?" question) but it is quite cool to see performance being part of the standards.