We have the blessing to try out timidity for a Cassette add-on which would provide midi playback as an integrated feature. Not sure how this would work yet.

One option would be an augmented version of the HTMLVideoElement which supports midi via timidity. The advantage here is that the midi integration would be flexible beyond Cassette.

Another option wouldn't mess with the media element, but would conditionally use Timidity instead of the media element if the file extension matches midi criteria. This would require internal changes to Cassette.

In either case we need to be mindful of how things that rely on media element state would react.. particularly external stuff like the media session API.