Strangely enough, the demo works using Webkit (Chrome) but not with Firefox : I cannot start the animation, as the Start button does not appear even if I resize the window properly.
It seems the minimum reported by window.innerWidth on 16.0.2 (OS X, 10.8.2) is 453. Once it hits that, I can drag it all the smaller I like, and it is stuck at 453.
