For example VexFlow (http://www.vexflow.com/) or, even with music, abcjs (https://abcjs.net/). Both of those use their own language.
A cursory look at the lilypond docs makes it seem like the language is pretty simple and straightforward... although, the source code looks nuts. How hard do you think a JS parser/vm for it would be?
Edit: been looking at some large lily projects from mutopiaproject linked below. Oof. There's more there than the docs let on. Still, would be a very interesting project...
