This is great!
But what is it? I just see a black page with two buttons to create or join a room...
>A free peer-to-peer platform for playing piano with others in real time, which features an audio synchronization engine.
creator has started a series of blog posts about building it: https://d-buckner.org/blog/p2piano-pt1/
After creating a room and selecting display name you get a collaborative piano/synth/bass.
Didn't test if it supports Web MIDI, though.