I echo the other voices saying canvas and pure JS (or, in my case, CoffeeScript or ClojureScript). The canvas API is extremely intuitive — aside from the boilerplate setup, perhaps — and you can make some really neat things with it very quickly.

The home page for my personal site has a starfield made entirely with canvas: http://ivanish.ca — best to view in a dark room so you can see all the pseudo-nebulae.

Source code (if it helps): https://github.com/ivanreese/ivanish/blob/master/source/scri...

You might also want to consider Elm, but I have very little first-hand experience with it: http://elm-lang.org