If you run into problems developing with CSS, check the CSS Support Charts here to determine if there is a problem with the browser(s) you are using.
W3C's Recommended DTDs
http://www.w3.org/QA/2002/04/valid-dtd-list.html
Assigning the right DOCTYPE to a web page helps in establishing the correct manner in which browsers will render your web page and validators will check your code. All that's on this web page is a listing of the most commonly used DOCTYPEs.
W3C's CSS Page
http://www.w3.org/Style/CSS/
This is the official site for CSS. At this site you can learn about the history of CSS, investigate learning resources and authoring tools, and read current CSS news.
CSS 2.1 Specification
http://www.w3.org/TR/CSS21/
Browser implementations of the CSS specification are sometimes a confusing mess. When you're tracking down how to achieve a certain look or an implementation bug, go here to check the specification (as well as the CSS Support Charts).
HTML 4.01 Specification
http://www.w3.org/TR/html4/
To make the most out of using CSS for web design, you need to create your web documents with structured markup instead of using workarounds and hacks. Furthermore, you need to mark up your documents with elements to imply an inherent presentational meaning. For example, you need to highlight important words by using the em element and not the b element. If you need to change your production methods, dig into the HTML specification at this site and get to know the elements all over again.
XHTML 1.0 Specification
http://www.w3.org/TR/xhtml1/
Extensible HyperText Markup Language (XHTML) is a restructuring of HTML 4 in XML 1.0. Although XHTML markup is stricter than that of HTML 4, the benefits are simple: more logical markup, increased interoperability, and enhanced accessibility.