Video decoding in JS is very impressive - really highlights the speed of modern interpreters. I especially love that a Björk track from the 90's is featured.

I recently worked on a personal project which had to play back .webm files, and I used a similar utility:

https://github.com/brion/ogv.js/

It decodes .webm files and plays them in the web. I believe it's also used by Wikipedia to play bag Ogg files.