Is there something that can render Lilypond content to an image, such as mathjax?
Also, in my mind it would be nice to open a Stack Exchange forum about music. Such a thing would bring nice functionality to it, like LaTeX in http://math.stackexchange.com and so on.
Musically educated people hate ABC notation, but apparently it is ideal for forums just as TeX for math fomulas (e.g. via MathJax). Lilypond is very advanced in its integration (TeX, HTML) and platform support (Windows, Linux, Mac). It is also recommended by pedants who are crazy about producing beautiful note sheets (see http://lilypond.org/web/switch/advantages).
Here is a nice review of note editors, so Lilypond is not limited to TeX integration only - it also produces nice .PS and .SVG files.
Vexflow is a nice javascript engine based on Raphael for programmatic drawing of music notation, but it is really for programmers. It has also a web-based SaaS http://static.vexflow.com/ as described by the 0xfe blog.
Other javascript rendering libraries supporting ABC notation are abcjs and abcnode. IMO abcjs is the closest to MathJax in the sense of HTML5 js rendering.
MusicXML implements a non-ABC approach. This XML notation has become really well-supported since 2012, but essentially it is very cumbersome (just like MathML vs. TeX). There are also a number of JS renderers supporting MusicXML such as score-library.
It seems there was proposal for Lilypond typesetting, but it was deleted. Such questions can be asked on https://music.stackexchange.com/
http://0xfe.blogspot.ch/2010/05/music-notation-with-html5-canvas.html
This is another interesting task, i.e. to render the notation into MIDI or HTML5 audio element, but it is a topic for a different question.