Are you a web-developer and tired of using FIREBUG on firefox to check what error message or issues on your wesbite? Atleast it is for me as i dont like the idea of installing FireBug to identify problem when developing a site.The Markup Validator is a free service by W3C that helps check the validity of Web documents.
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.
This validator can process documents written in most markup languages. Supported document types include the HTML (through HTML 4.01) and XHTML (1.0 and 1.1) family, MathML, SMIL and SVG (1.0 and 1.1, including the mobile profiles). The Markup Validator can also validate Web documents written with an SGML or XML DTD, provided they use a proper document type declaration.
This validator is also An HTML validating system conforming to International Standard ISO/IEC 15445—HyperText Markup Language, and International Standard ISO 8879—Standard Generalized Markup Language (SGML) – which basically means that in addition to W3C recommendations, it can validate according to these ISO standards.