W3C Markup Validation Service

Tag certifying that a website has been checked for well-formed XHTML (above) and CSS (below) markup

The Markup Validation Service is a validator by the World Wide Web Consortium (W3C) that allows Internet users to check pre-HTML5 HTML and XHTML documents for well-formed markup against a document type definition. Markup validation is an important step towards ensuring the technical quality of web pages. However, it is not a complete measure of web standards conformance.^[1] Though W3C validation is important for browser compatibility and site usability, it has not been confirmed what effect it has on search engine optimization.

As HTML5 has removed the use of DTD in favor of a "Living Standard", the traditional Markup Validation Service is not applicable to these formats. Validation is instead performed using an open-source "Nu Validator", an instance of which is provided by W3C.^[2]