The World Wide Web consortium provides a free HTML validator to check your website for compliance. This is a great tool not only to ensure compliance, but also to find problems with your code that don't produce errors.
http://validator.w3.org/
The more compliant your code is, the more compatible your users' sites will be with different browsers, too! Just because it "works", it doesn't mean it's "right".
(Yes, it found a lot of errors in my skin. I will be releasing a new version soon that is "4.01 Transitional"-compliant.)