This is well neat
https://github.com/mochreach/midi-surf - A flexible and powerful MIDI controller in your browser!
https://www.youtube.com/watch?v=c5BpeMxs5ZU - MIDI Surf: A free, web-based, customisable MIDI controller!
Not specifically Linux, but there isn't just, like, a "MIDI" subforum (I keep thinking the subforum ontology on LM could be a lot better)