I know that we've decided we want Lilypond support on Music.SE. I've found a competitor to Lilypond that looks like it already has some good JS libraries called Vextab. Would Vextab be a good tool for us, or do we want to ask for Lilypond support in particular?
Vextab has an interactive tutorial you can use to see what kinds of features it has and how it works: http://www.vexflow.com/vextab/tutorial.html
Briefly, I think the pros of Vextab are that it already has an implementation that the SE devs might be able to easily work into the website. The biggest downside I can think of is that Lilypond is a better known and more feature-complete tool, and if we ask for Vextab, we might not get Lilypond later. Also, while it's open-source, its developer wants to be paid for commercial usages, so we'd have to ask the devs to spend some money for us.