Just because a JS engine is really fast doesn't mean you should do everything in JS, esp if CSS is the right tool for the job (not to say that this particular demo is a good example of using the right tool for the job).
Sure, right tool for the right job, etc.
Animations are almost always better when they can be hinted to the GPU (will-change), can be transitioned with translate2d changes, and do not cause reflow by being used on absolute/fixed positioned elements. For example.
My point is that not using JS will not make browsing experience into something "pure". It will remain an incredibly compplex stuff in itself, which JS is only a part of. And when it comes to firing off XHR's, setting some data or providing interactivity (with real states, etc), then JS is a good tool for the job.
