The W3 Consotium has announce on its website that its W3CValidator Suite which checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML is now OPEN SOURCE!. The source code is available on GitHub under the W3C Software license. Anyone can run it for themselves, fork it. Enjoy!
W3C Validator Suite allows you to check an entire Web site and evaluate conformance with W3C open standards, including HTML, CSS and Internationalization checking. By integrating Web standards conformance tools, W3C Validator Suite makes keeping your entire Web site compliant with the latest Web standards easy and quick.
Important: The Premium W3C Validator Suite service, operated by the World Wide Web Consortium at https://validator-suite.w3.org will be discontinued at the end of 2015 when W3C stops maintenance and customer support. All current customers will be able to use remaining credits until
December 31, 2015 February 29, 2016 but not acquire more. Unused credits are non-refundable. We are not accepting additional customers.
W3C will continue to operate its other validators.