Explainers on how sound and tuning actually work, with the diagrams made to be played with rather than looked at. Start with synthesis if you are new to it, and read the harmonic series when you want to know why an oscillator sounds like anything at all.
2 guides
Build a classic subtractive voice from oscillator to VCA, then take it apart. Nearly every slide has live Web Audio: a clickable keyboard, a click-to-set sequencer, filter sweeps that redraw the spectrum as you drag, envelopes you can hold and release.
Build a tone one sine at a time and hear what each harmonic does, see where the series falls against the twelve notes of a keyboard, and pluck a string to hear why the sawtooth is at the bridge rather than on the string.