Resources & Documentation
MDN WebMIDI Reference
A decent overview of the WebMIDI implementation
Circuit Playground Library Reference
Documentation for the various sensors and capabilities of the Circuit Playground Express
Arduino Reference
If you've never worked with Arduino hardware before (or even if you have!) it's helpful to have the reference on hand
W3C Web MIDI API
Proposal for the Web MIDI API
Getting Started with the Web MIDI API
Good article from Smashing Magazine giving an introductory overview
Web Audio Demos
A good number of these utilize Web MIDI and are great examples of musical applications
WebMidi.js
Although the WebMIDI API is pretty straightforward to begin with, if you're going to work with preexisting MIDI controllers or have a more conventional MIDI project in mind this library makes working with the API even simpler
MIDIUtils
If you need to translate MIDI messages to musical notes and pitches for your project this collection of methods is extremely helpful