An instrument is a physics experiment you can hold. This area starts with the everyday problems — what is that note, is this string in tune, where do my fingers go, am I keeping time — and shows the working: pitch found from how long a waveform takes to repeat, target frequencies from equal temperament, chord shapes and scale positions searched out from the tuning rather than looked up, and beats placed on the audio hardware's own clock because the browser's is not good enough for music.
Every inversion on a keyboard drawn to the proportions of a real one — black keys off-centre where they belong — with a fingering for each hand, the alternative names a set of notes can have, and a progression voiced so your hand barely moves.
Every shape searched out from the tuning rather than looked up, so capos, DADGAD, open G, ukulele and bass all work the same way. Strum it, or slow the strum until you can name each string.
Every note on the stave in any of the four clefs, drawn rather than photographed — with the mnemonics, the landmarks that eventually replace them, and a drill that works out which notes you are slow on rather than only which ones you get wrong.
Any scale or mode drawn across the whole neck, spelled the way it would be written — F♯ major really does contain an E♯ — with the position boxes searched out rather than memorised, the chords that come out of it, and a drone to hear it against.
Two sounds, at any distance, added together properly — because 60 dB plus 60 dB is 63, not 120, and twice the distance costs 6 dB from a drill but only 3 from a road. Everything on one scale, with how long each level is safe for, and a microphone meter that is honest about being relative rather than calibrated.
Beats handed to the audio hardware before they are due, so a busy page cannot make it late. Editable accents, subdivisions, swing, odd meters, and a trainer that speeds you up or drops out whole bars to find out whether you are keeping time or just following it.
Listen through the microphone and read the pitch to a fraction of a cent — or ignore the microphone entirely and read the exact frequency every string should be at.