Here is a list of web site validators that you can bookmark, so not to search for them every time you need them.
http://validator.w3.org/ The W3C Markup Validation Service
http://jigsaw.w3.org/css-validator/ The W3C CSS Validation Service
http://www.w3.org/2003/12/semantic-extractor.html Semantic data extractor
If you know of any more, add them in comments.