The Nu Markup Checker (v.Nu) checks the markup of HTML 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.

To run your own copy of the Nu Markup Checker, get the latest release and read the following instructions on how you can use it both for:

Alternatively, consider automating your HTML checking with a frontend such as:

Reporting issues

If you find any bugs, want to request a new feature, or have a question about the Nu Markup Checker, please report an issue or send an e-mail message to


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.