That would make using ReST or Markdown a little more tolerable in a text area.
In the end if I have to do a lot of editing, I really want to use a real editor, ideally my editor of choice. I used to use mozex on older versions of Firefox; your post inspired me to go looking for a working solution. Someone has updated mozex to run on newer versions of Firefox: https://nic-nac-project.de/~kaosmos/mozex107-en.html
here's the original parameter docs: http://mozex.mozdev.org/arguments.html
In Extensions, Preferences, set up vim:
/usr/X11R6/bin/urxvtc -e /usr/local/bin/vim %t
(sub whatever you use as a terminal for the first chunk of the command line)
Click on the text area field when you've saved and closed the temp file et voila. Hey, I'm soaking in it right now!