In my experience, they all seem to fall down with:

(1) Lists, especially indenting and nesting properly. (2) Tables. Everything about tables. (3) Pasting rich text from other apps. (4) Accumulating invisible formatting data.

From what I've read, if your library uses "contentEditable" and tries to clean up after it, you're in for a bad time. Instead, the best thing is to write your own engine, like Google Docs did (this is of course very difficult and no one has mastered it).

I'll give yours a shot!