[vespakoen]

You probably have already seen this, but I just found this ffmpeg library compiled with emscripten and thought it was pretty cool

http://bgrins.github.io/videoconverter.js/demo

[simcop2387]

Yea, stuff like that makes me think that while doing all codecs will be hard, a large useful subset will be not that difficult to do.

[acqq]

I've just tried webm to mp4, the native is at least 10 times faster.

It seems just very low-res stuff will be interactively playable if the speed difference remains. Maybe it's enough for the very old stuff, however.