A Web MIDI implementation would be great as well (and in keeping with Apple's usual level of attention to MIDI and other audio production niceties) but it's admittedly a pretty niche feature with very few people active in the spec outside of Chromium.
When I was at Mozilla, there were a ton of folks interested in WebMIDI, but nobody had the time to work on it. A lot of folks were hoping that someone would build a polyfill but it never really came.