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

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/ 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.