On the other hand, when you do a CSS animation (e.g. using the transform or webkit-transform css properties), you leave it to the GPU to simply calculate where the element should be as often as it can. As soon as it calculates the element’s new position and places it there, the GPU starts the calculation again. This means the animation is always as smooth as the GPU can possibly make it, no matter what. If you animate too many elements for the GPU to handle it may still have trouble, but instead of ugly stuttering, it will be a more graceful drop in framerate.
That’s why CSS animation is a must for mobile targeted websites. The good news is that all mobile browsers (the ones we care about anyway) support CSS3’s animation techniques, so there’s no need for capability detection and fallbacks.