Thanks for replying, Silvia. I was thinking of emailing to ask.

@kind=metadata is a good semantic, because it fits the use case perfectly.

How would you recommend linking to a png that’s too big to fit comfortably in a data url?

I recall be awed by the rendering in http://0xfe.blogspot.com/2010/05/music-notation-with-html5-canvas.html , but a little dismayed at the prospect of yet another markup for notation. I am somewhat sold on EITHER the basic approach to markup taken by Lilypond, or plain old JSON.