I am also building a piano-learning game. I remember using WebMIDI for the interface, TonalJS for chord/theory. From another comment, https://github.com/0xfe/

I am also building a piano-learning game. I remember using WebMIDI for the interface, TonalJS for chord/theory. From another comment, https://github.com/0xfe/vexflow https://github.com/0xfe/vexflow looks cool, I could use it together with PIXI.js for the rendering.