[Requirement] Use valid (X)HTML [Details] Using valid HTML means having an HTML code that follows one of the DTDs of the HTML specification. Your best choice is valid XHTML strict markup: XHTML means that the code will benefit from all the XML power, and strict ensures your site won't mix presentation markup with the structure. [Rationale] Valid HTML code is a first step toward more compatibility with browsers and better rendering on the latest ones. Using only strict HTML (that is, the Strict HTML DTD) makes the site easier to maintain and evolve. [Test tool] 
The W3C HTML Validator is one of the many options available. The LogValidator is especially helpful in maintaining a valid Web site.