Start by thinking about the separation between content and design.
1. Maintenance. It's a hassle to keep two versions of the same thing.
So I would probably have something like this if the styling is very simple:
But they would be in the same folder and both publicly accessible.
If the site is going to be complex, I would look into something like SMACSS or Object Oriented CSS, breaking things into lots of files and using SASS or LESS to generate a final, compressed CSS file or files.
SMACCS - from Jonathon Snook who has done some of the biggest redesigns around (like Yahoo! mail). [smacss.com
OOCSS from Stubbornella (Nicole Sullivan - [github.com
Drupal has traditionally had abysmal, bloated markup that is hard to maintain. The Drupal community put together a sort of task force to tackle the issue of how to implement CSS, drawing on the work of Snook, Sullivan and others and have put together an interesting CSS standard for complex projects
I find it a little easier to see put together like that than reading from Snook and Sullivan et al.