I haven’t worked on a web project in a while. So to keep my abilities up to snuff, I am working on a waveform generator for Teensy Audio Library. Written in HTML5, using jQuery and jQuery mobile. It allows touch events and mouse events, and works pretty well on mobile. You can view it here: Waveform Generator, if you are using the Teensy Audio Library you should be able to plug in the waveform into an oscillator that will oscillate that waveform at a specific frequency, there are some notes in my web application. This is a work in progress project.
Also available Apache 2.0 licensed synth originally by me: iridescentCoconutSynth