This is a master bug for implementing Web MIDI API in WebCore.
W3C TR: http://www.w3.org/TR/webmidi/
Latest editor's draft: http://webaudio.github.com/web-midi-api/
This component will be implemented in "WebCore/Modules/webmidi" with ENABLE(WEB_MIDI) flag.
This feature becomes much more urgent because of the change in Technology Preview 52 disabling NPAPI extensions other than Flash, because the NPAPI Jazz-MIDI plugin was previously the principal way of accessing MIDI in Safari. Music notation related sites such as Noteflight and artusi.xyz are likely to start recommending users move to Chrome to use MIDI input in the browser.
The Web MIDI API brings tremendous value to the music world. Chrome and Edge already have support. Firefox support is in the works. It just feels natural for Safari to also provide support for this great API.
It would be wonderful to see WebMIDI implemented in Safari. Especially if it came paired with an implementation of AudioWorklet. This would allow developers to create musical instruments and experiences in the browser on iOS with low-latency and a variety of musical input devices. Personally, I'd find an initial implementation without sysex (this is a security concern for some browser vendors) to be incredibly useful.