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.
World Wide Web Consortium (W3C) allows internet users to check HTML and XHTML documents for well-formatted markup. Markup validation is an important step towards ensuring the technical quality of web pages.
Most Web documents are written using markup languages, such as HTML or XHTML. These languages are defined by technical specifications, which usually include a machine-readable formal grammar (and vocabulary). The act of checking a document against these constraints is called validation, and this is what the Markup Validator does.
Validating Web documents is an important step which can dramatically help improving and ensuring their quality, and it can save a lot of time and money (read more on why validating matters). Validation is, however, neither a full quality check, nor is it strictly equivalent to checking for conformance to the specification.
The W3C is an organisation set up by the major players on the internet (particularly the web browser developers) to decide on standards for the different web languages, such as HTML. For anybody who remembers creating websites in the early days of the web, before the standards, you will realise how important these standards are.