zMIDI uses the Web MIDI API, which is currently supported in Google Chrome, Microsoft Edge and Opera, making it a niche functionality, though support could grow over time.
Who / what is using it ?
zMIDI was written specifically for WebSID and later integrated in Efflux. In both apps zMIDI listens to incoming note messages allowing you to instantly play and record notes with a connected keyboard. Additionally, Efflux also maps CC controller messages to allow tweaking of various sound shaping parameters.
Available via npm
npm install zmidi