There are three important parts to a Web page: Content, Style, and Behavior. And the best Web developers know what they are and how to keep them separate. But it’s easy to get hung up in any one of the three layers – in fact, most developers prefer one layer over another when developing pages. But if you can keep them separate you’ll build pages that will be more effective in the long term and easier to maintain no matter which layer you prefer.
Read this web design article at About.com