These web standards are established by the W3C (World Wide Web Consortium), so you should consider having a link to validate the XHTML and CSS of your website (for each page). The advantage of having the links -so people can click em- and of course to have both XHTML and CSS validated it’s like if they (the W3C) were “approving” your design and this will give you some extra points.
The links you should include are (paste them as they are):
XHTML: http://validator.w3.org/check?uri=referer
CSS: http://jigsaw.w3.org/css-validator/check/referer