The "errors" are just differences in opinion, I think; true errors in code would mean it wouldn't work. There's no site "police" telling you how to make your website :-) The W3C people are just a group of people who have their idea of standardization based on how their members like things and how Microsoft and other big programming groups are making new features work in browsers or not supporting old programming features, etc. There's lots of places that will check sites for you to see if they're one thing or another. I know there are lots of people who write on Web accessibility http://www.skally.net/alinks.html for those who don't see well or have other physical/computer-related problems, too.