Security and exactness of the Internet communications are now being promoted by W3C, which introduces W3C Markup Validation Service available at http://validator.w3.org/.
Providers of content are able to use this unique service for confirmation of their pages against the XHTML and HTML, and to get advices for maximization of their audience. The service is also very convenient since it supports the documents of the XHTML Family like XHTML+MathML+SVG, XHTML+MathML and some markup vocabularies including SVG.
The program developers who create XHTML and HTML editing instruments can confirm functional compatibility with other Internet software by checking if their instruments’ work corresponds with the Recommendations of W3C.
W3c's Validator is a free service that helps check the validity of Web documents.
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.