Firstly, a big Thank You to everyone for the comments and advice to my previous post.
I went through almost all of the links that were posted, and glanced over the various options for languages, rendering, and formatting.
Question 1. Canvas or SVG?
This is actually a no-brainer. The library was designed such that alternate rendering backends could be easily plugged in. I went over some of the SVG specification documents, and it appears (at first glance) that I could add SVG support pretty trivially.
So, the answer to this is: Yes, there will be SVG support.
Now, that said, I had two primary goals for this project. One was the ability to render pleasing-to-the-eye musical scores, and the other was to be able to support interactive scores (to allow for building tools such as score editors.)
These goals are potentially conflicting, because certain aspects of rendering perfect scores are actually computationally expensive, requiring multiple passes on the notation. This could affect the snappy-factor of interactive scores. There's also the question of whether SVG is flexible enough for animations and interactive graphics.
Anyhow, I'll dig into this some more and report my findings. Worst case, SVG will be used for rendering static scores, and Canvas for interactivity.
Question 2. MusicXML or ABC Notation or JSON or Lilypond?
I eschew XML. It's clunky. It's difficult to parse. It's difficult to read. It makes my head hurt. So I'll let someone else build a MusicXML importer for the API. (Yes, I do realize that SVG is XML too, but generating XML is far simpler than parsing it.)
ABC Notation. Love it. It's going in. I'll have to figure out a way to lump in easy tablature support.
JSON. This is a good idea in theory, but getting the schema right is the tricky part. I'm going to hold off on this for now and defer to the API.
Lilypond. Although the generated scores are beautiful, I'm not particularly a fan of the TeX-based language. Like I mentioned before, I'd like to be accessible to non-geeks.
Nevertheless, I'm working on cleaning up the API such that it is friendly to compiler-writers, and interested parties can write their own front-ends. A fringe benefit of a compiler-friendly API is the ability to run layout and positioning algorithms on top of generated scores to fine tune the presentation. Kind of like a -O3 for music notation.
Note that these opinions aren't very strong (except for maybe the MusicXML one), and I'm open to other thoughts and suggestions you may have. So... suggest away.