WebKit, the browser engine used by Apple's Safari and Google's Chrome, is apparently the first browser engine to fully pass Acid3. WebKit has long scored 100/100, but now it appears to have passed the final requirement of smooth performance on animations.
According to the Surfin' Safari Blog:
“Today we would like to announce that WebKit is the first browser engine to fully pass Acid3. A while back, we posted that we scored 100/100 and matched the reference rendering. Now, thanks to recent speedups in JavaScript, DOM and rendering, we have passed the third condition, smooth animation on reference hardware.”
You can learn more here.