ogv.js

This is a little different than codec support, though as other replies indicate, that can be done too. This is just demuxing the stream in one format, re-muxing it in a different format, and feeding it into <video> to actually do the decoding and playback. On the other hand, something like https://github.com/brion/ogv.js/ is actually doing all of the decoding and playback in JavaScript/asm.js, using C libraries compiled with Emscripten. Anyhow, yes, it probably would be possible to package up extensions that would apply one of these automatically to <video> tags with unsupported codecs or container formats. It's just a simple matter of programming.

Yes, in fact it's even deployed on Wikipedia: https://github.com/brion/ogv.js/

As for good applications, how about running DOS, MESS and MAME in your browser? It's a nice archive.org feature: https://archive.org/details/softwarelibrary_msdos_games https://archive.org/details/sega_sms_library https://archive.org/details/internetarcade Or video and audio codecs as used by Wikipedia to support those browsers which don't support those codecs natively: https://github.com/brion/ogv.js/ https://brionv.com/log/2015/08/09/video-decoding-in-the-java... Also try Bejeweled. A simple game but well executed: http://bejeweled.popcap.com/html5/0.9.12.9490/html5/Bejewele...