The W3C Compliance Validator is a service that OmniUpdate offers, which 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. 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.