Organization and separation of distinctly different aspects of a web application or website is good code manners and dramatically eases maintenance and adding changes. In application development MVC frameworks separates models, views and controllers. And in web development you separate functionality from visuals by using Cascading Style Sheets.
In an article on Progressive Enhancement with CSS Aaron Gustafson at A List Apart discusses the separation and organization of stylesheets. Below is his suggested basic model which separates type, color, and layout from each other, and deals with the various types of media type layouts a website may be viewed in.
The article also discusses how to deal with oddball browsers like N4, IE6, and IE7.
