I saw many valid html sites but none valid css. It`s really exist a site that is valid css? Thank you.
Just concurring with what others mentioned above. : ) But, I also wanted to add that what often makes a stylesheet "invalid" are CSS hacks and using the wrong syntax. If you use the Web Developer plugin for FF there are three buttons near the top of the browser. One of those is to tell you if the site you're browsing has CSS errors. (If there are any, it tells you what they are.) The w3c validator belkocrnic mentioned is good because you can link to it from within your site so users can check your validation.