I was just having a quick read of an article called The Craft of HTML. It mentions in this post the article created by Bruno Pedro called Top blogs fail W3C markup validation. This brings up the interesting question of signing over a website to a client. After all your hard work ensuring that you have a valid site, six months down the line you check it again only to find out that the client has buggered it up and it no longer validates.

Does it really make a difference? of course it does but the problem lies in that the client probably doesn’t know what valid code is and what is not. All they care about is adding content to their site.

If these top blogs don’t spend the time making sure that they are valid then why would you expect your client to care about checking validity after they change or add content. It could be as simple as adding an image and not specifying an alt attribute, six months of this and the site would be littered with errors, never mind making it a little more difficult for screen readers. 

Mostly it seems on this list that the developers have not bothered to check the sites or simply don’t care about having a valid site.  Or as stated by Bruno, maybe they only care that the site looks right.