CSS Validator

The W3C CSS Validation Service is a free software created by the W3C to help Web designers and Web developers check Cascading Style Sheets (CSS).

It can be used on this free service on the web, or downloaded and used either as a java program, or as a java servlet on a Web server.
If you are a Web developer or a Web designer, this tool will be an invaluable ally. Not only will it compare your style sheets to the CSS specifications, helping you find errors, typos, or incorrect uses of CSS, it will also tell you when your CSS poses some risks in terms of usability.