I have thought some about rich text, but haven't implemented anything yet. My general thinking is that attribute spans lend themselves well to the OT model, but trees do not. The classic example is if you have three words, user A bolds the first two, and user B italicizes the last two. With spans, they just partially overlap, which is fine, but with trees you have an ambiguity.

Representing certain kinds of document structure (like nested bullet lists) is not obvious with spans. I want to explore the combination of a marker representing "beginning of tree" and a span, and define transformations in and out of tree form that would be robust to concurrent edits. However, this is all just ideas at this point.