Put XHTML 1.0 Strict and Transitional to work
XHTML 1.0 Strict
XHTML 1.0 Strict is the most demanding XHTML flavor, but it provides the cleanest structural markup. Strict code is free of any markup used to define layout. It uses cascading style sheets (CSS) to control the presentation. This separation of structure from presentation is what makes XHTML Strict flexible enough to be displayed on different devices. The reliance on CSS to control presentation can be problematic for developers, because it's not a good choice for Web content that needs to be viewed on devices or in browsers that do not recognize style sheets.

XHTML 1.0 Transitional
XHTML 1.0 Transitional is the more forgiving XHTML flavor. Unlike Strict, which completely separates structure from presentation, Transitional allows you to use tags to control the look of your markup. Its goal is bridging the gap between HTML-based pages that allow the markup to control the presentation and XHTML Strict, which does not. Its main benefit is that it overcomes Strict's CSS dependence. Transitional pages are still accessible to users who use older browsers or who are using devices that don't recognize style sheets.

How to choose?
The choice between Strict and Transitional depends on a couple of factors:

Audience. If you find that much of your audience uses older browsers that don't recognize style sheets, Strict may not be the right answer—although I would make the case that supporting standards is more important than backward browser compatibility at this point. If most of your audience is using the latest versions of Internet Explorer, Netscape, Opera, or Safari, Strict is the best long-term choice.

Current code. If you already use CSS and your HTML doesn't contain a lot of markup that controls presentation, you can make the leap to Strict.…

http://builder.com.com/5100-6371-5061538.html?fromtm=e606