David,
I am not sure I 100% follow -- are you saying that you want to just apply consistent stylesheet, or that you want to parse textarea feed, in a manner similar to that done by txt2html (http://txt2html.sourceforge.net/), and then apply CSS/HTML to it?
I suppose the latter.
In which case it's a question of doing something that is similar to Wiki process, is it not?
On a subject of consistency in look and browser behavior -- I don't agree that the same style on a different HTML may give you different results on the same browser. No. The only varioable part in this whole equation is the browser -- other things are constants. If IE 5 is brain-dead about something -- it is consistently brai-dead about it.
Also, using the @import trick helps to "enrich" a basic CSS1 layout that maybe safer unfitting for an older ot non-compliant browser. Same goes for overloading as in:
P { color: red; }
DIV.notice P { color green; }
