Like the DTD-based W3C markup validation service, the W3C Nu Markup Validator checks the markup validity of Web documents, but instead uses the backend from the Validator.nu engine, which provides non-DTD-based validation support for a number of markup languages. As with the existing W3C DTD-based validator, code contributions are welcome and are publicly acknowledged. So if you’re a developer interested in contributing to the work, you’re encouraged to get involved (a good place to start is by checking out the Validator.nu source code).
If you find any bugs, want to request a new feature, or have a question about the service, please do one of the following:
Steve Faulkner has created a WCAG 2.0 parsing error bookmarklet that you can use in conjunction with the W3C Nu Markup Validator. It filters the messages emitted by the validator to show only those messages that are relevant to checking for conformance with the guidelines in section 4.1.1: Parsing of the Web Content Accessibility Guidelines (WCAG) 2.0 specification.