Sometime recently I wanted to make some sweet-ass parallax stuff. Fun times would be had, but what a lot of effort it was going to be.
There were several great solutions out there, and I eventually decided upon jparallax as being the most complete. However, that still didn't allow me the flexibility I wanted - I needed to be able to do more than just move elements, I wanted to stretch parts of my parallax to achieve faux-sideon perspective. And then I started thinking about other things you could do, and decided the best way to approach such an engine would be to allow modification to any css property and decide what effects were workable later.