p5.js lets you create Web-based multimedia applications that can interact with other HTML5 objects, including text, input, video, webcam, and sound.
Reader Ian Watkins has been experimenting with it and says it’s a great starting point for in-browser synths. Here’s one of Watkin’s examples, with on-screen code. View with a modern browser.
If you’re interested in P5js, check out the multimedia intro video at the project site.