Electric Telepathy has a nice set of interactive tutorials that use Web MIDI/Web Audio widgets to explain synthesis concepts, like filter cutoff and envelope decay and sustain. You can play the keyboard of each widget using the bottom row of your computer’s keyboard (the z-m keys) and experiment with the controls to hear their effect on the sound.
All of the widgets are eventually combined into a complete subtractive synthesizer that can be controlled via computer keyboard or a MIDI controller to explore the full set of concepts.
The project highlights the educational potential of the Web MIDI and Audio APIs – by making audio apps that are accessible without specialized hardware or software it’s possible to reach a much broader audience than otherwise would be possible.
Electric Telepathy also develops a VST plugin called Fluoresynth, which you can try in the browser, too. Thanks to them for developing and publishing the teaching materials and wish them well with their audio development efforts!