The Nu Markup Checker uses the Validator.nu engine to check the markup of Web documents.
The Nu Markup Checker is an experimental tool and its behavior remains subject to change. In particular, because new types of error checks continue to be actively added to the checker, there is no guarantee provided that if the checker reports zero errors for a particular document at one point in time, it will report zero errors for that same document at some later point in time.
The Nu Markup Checker should not be used as a means to attempt to unilaterally enforce pass/fail conformance of documents to any particular specifications; it is intended solely as a checker, not as a pass/fail certification mechanism.
If you find any bugs, want to request a new feature, or have a question about the Nu Markup Checker, please file a bug report or send an e-mail message to firstname.lastname@example.org.
Steve Faulkner has created a WCAG 2.0 parsing error bookmarklet that you can use in conjunction with the Nu Markup Checker. It filters the messages emitted by the checker to show only those messages that are relevant to checking the guidelines in section 4.1.1: Parsing of the Web Content Accessibility Guidelines (WCAG) 2.0 specification.