W3C plans to stop providing the on-line unified validator service Unicorn as of 31 March 2023, due to lack of resources to maintain the project. We encourage interested people in the community to fork Unicorn. We have many other developer tools such as the markup and CSS validators, and checkers like the Internationalization checker, link and feed checkers.
W3C’s unified validator Unicorn helped improve the quality of Web pages by performing a variety of different checks, by gathering the results of the popular HTML and CSS validators as well as the Feed validator.
Unicorn, which got its name from “unified” and “Conformance Observation Report Notation”, started in April 2006 and was publicly introduced in July 2006. It took another four years before the public release of Unicorn, All-in-One Validator in July 2010.