Yeah it's a really hard problem (content editing in the browser). I am working on something like this now (for HTML content, not markdown specifically) and all

Yeah it's a really hard problem (content editing in the browser). I am working on something like this now (for HTML content, not markdown specifically) and all I can say is it's prickly. The browser's content editing APIs are terrible so you have to do most everything yourself. But then there are some things that are very difficult to emulate, like selections. Still I think it's worth doing because I am tired of hand-coding my HTML pages all the time, and I think someone who can figure this might make a few dollars.