W3C Markup Validation Service, a free service that checks Web documents in formats like HTML and XHTML for conformance to W3C Recommendations and other standards.
A. Ceroni, and M. Fisichella. Proceedings of the 36th European Conference on IR Research, ECIR 2014, volume 8416 of Lecture Notes in Computer Science, page 605-611. Springer International Publishing, (2014)