On the topic of Markdown.. here is previewer I wrote a while back: http://marky.devel.io It includes a few extensions for my common needs (click the Show Example link in the preview area). It comes in handy for seeing how things _will_ look (like GitHub READMEs) or for creating quick syntax-highlighted code samples for copy and pasting into slide decks.
As much as I like the minimal/darkroom approach of OP's (et al.) idea, the constant swapping between preview and edit is painful, this calms me. Let's embrace reactive transparent update (BrettVictor style).
: the update delay is a tiny bit high for my taste, or maybe just lacks communications/notifications, a tiny gizmo (spinner, led, hud..).
