I recently looked into using Velocity on a mobile application. I need simple animations but they need to be performant.

I was dismayed to find that Velocity and almost every other animation library out there still uses DOM-based tweening rather than CSS transitions.

Even some of the faster libs out there that leverage CSS transforms are still pushing frames manually and sucking up CPU cycles running code-based animations.

Why are people so convinced that code-based animations are somehow faster or better than pure CSS transitions?

I certainly understand and appreciate the flexibility of code-based animations (easy chaining, full control, older browser support etc.), but for simple things like panels sliding in and out of view, nothing beats transform3d and transitions on platforms when these things are available (which is most mobile devices, these days).