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 (DTD). 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. Though W3C validation is important for browser compatibility and site usability, it has not been confirmed what effect it has on search engine optimization.