http://validator.w3.org/
Most people dont know how to get thier sites XHTML strict or transitional, but in the case of pornsites, I really dont think it even makes a difference. Its about selling your product, the most effective way you can, and if it turns out that your html is compliant, then all the better.
99% of all sites on the internet are not compliant to XHTML standards, and it seems like its almost impossible to even enforce some type of regulation to make people compliant.