It's all SVG. Given that the canvas music notation source will be published, it would probably also not be hard to use that - either directly in Dan's Lively Canvas version or as a canvas element embedded in the SVG DOM. But I think for this particular application SVG is the better choice.
What's missing, of course, is support to assemble the notation. You can currently just drag and drop those objects. Maybe some simple language could be used for that or maybe there are even better ways, like playing and "recording" with a piano -- I leave that for you :-)