I recently went to the trouble of trying to make sure my site's pages were "valid" XHTML and CSS. I have heard several reasons why validation is important. But why do many big-time websites not worry about validation if it is so important. Check the Google, Yahoo, and the MSN homepages.

http://validator.w3.org/check?uri=www.google.com (300 errors)
http://validator.w3.org/check?uri=www.yahoo.com (50 errors)
http://validator.w3.org/check?uri=www.msn.com (30 errors)

Google and Yahoo are unbelievably invalid and don't even use a DOCTYPE tag. MSN does inlcude DOCTYPE (XHTML 1.0 strict) but that page blatantly contains several tags that are invalid in strict XHTML and also includes uppercase tags (a no no in XHTML). The big boys obviously don't bother with validation. Do they know something we don't?

Look at it this way, those are million/billion dollar compainies and already have a well known site that is a household name; so they dont have to worry (as much) as we do about valid HTML/CSS.