The Nu Html Checker (v.Nu) is an ongoing experiment in better HTML checking, 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 Html 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.


Steve Faulkner has created the following bookmarklets you can use in conjunction with the Nu Html Checker.

Reporting issues

To report bugs, request new features, or ask questions, please raise an issue or send mail to

How to to run your own copy of the Nu Html Checker

To run your own copy of the Nu Html 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:

Why validate?

The core reason to run your HTML documents through a conformance checker is simple: To catch unintended mistakes—mistakes you might have otherwise missed—so that you can fix them.

Beyond that, some document-conformance requirements (validity rules) in the HTML spec are there to help you and the users of your documents avoid certain kinds of potential problems. To explain the rationale behind those requirements, the HTML spec contains these two sections:

To summarize what’s stated in those two sections:

Validating your documents alerts you to those potential problems.