W3C Markup Validator

The World Wide Web Consortium (W3C) is an international community that works to develop Web standards. The Markup Validation service is offered by the W3C to check the markup validity of Web documents in HTML, XHTML, SMIL, MathML, and more. Developers can use the API to access the functionality of the service and integrate it with their own applications. The API uses SOAP protocol and responses are formatted in XML.