W3C got too strict about two years ago, most of the errors the validation process lists now-a-days are stupid errors that don't cause any problems for any browser, it's just not prefered. Sometimes it's just people that handcode large sites by hand and don't use a program (outside of a simple text editor), so there's bound to be a few problems here and there, but nothing that causes any issues.
I have found that many WYSIWYG editors tent to make many, many errors. There are a few good ones that do a good job of both clean code and use CSS(Like dreamweaver).