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 wth 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 colletion of methods is extremly helpful