`el.style[t] !== undefined` when the browser supports 2d transforms aswell as 3d transforms.

So *checks back on compatibility*, it's to cover FF3.5 - <10; IE9; Safari 3.1- <4; Opera 10.5 - <15 . Where 2d transforms were supported but not 3d. Without the further check, we would attempt to `translate3d(` on these, but be left with no movement.

I've been using it as a snippet for a while, and was lifted pretty much straight from the post at: http://tiffanybbrown.com/2012/09/04/testing-for-css-3d-transforms-support/