CSS VALIDator

The W3C CSS Validator, developed by the same organization that created and standardized the CSS language, is the most used and most reliable CSS validator available. It's a free and open source web-based tool that will scan your CSS source and explain any lexical or syntactic errors it finds.

The W3C CSS Validator checks compliance with the CSS 2 standard by default. At the time of writing, most browsers are targetting the upcoming CSS 2.1 specification. There are some well-supported features of CSS 2.1 that didn't exist in CSS 2, so the W3C CSS Validator may give you errors on those.

On the results page, the validator will often give you a number of warnings even if there are no errors. These warnings do not imply that your page is invalid or even written poorly. They are basically tips for things to be aware of. Most of them deal with the fact that users may customize their default stylesheet, and if they, for example, choose to have a black background and white text by default and your page only specifies a light background color, the page may be unreadable for those users. The validator may recommend that you specify a text color along with your background color to prevent these potential problems.