About the Markup Validator

What is the Markup Validator and what does it do?

The Markup Validator is a free tool and service that checks the syntax of (X)HTML documents.

The Validator is sort of like lint for C. It compares your HTML document to the defined syntax of HTML and reports any discrepancies.

Learn more about the Markup Validator.

Why should I validate my HTML pages?

One of the important maxims of computer programming is: Be conservative in what you produce; be liberal in what you accept.

Browsers follow the second half of this maxim by accepting Web pages and trying to display them even if they're not legal HTML. Usually this means that the browser will try to make educated guesses about what you probably meant. The problem is that different browsers (or even different versions of the same browser) will make different guesses about the same illegal construct; worse, if your HTML is really pathological, the browser could get hopelessly confused and produce a mangled mess, or even crash.

That's why you want to follow the first half of the maxim by making sure your pages are legal HTML. The best way to do that is by running your documents through one or more HTML validators.

Who owns/maintain the Markup Validator?

The Markup Validator is Maintained at W3C by W3C staff and benevolent collaborators, who receive a lot of help from contributors. (Read the full credits)

What other validators are there?

We're doing our best to provide clear and reliable results as well as a good interface with the Markup Validator, but for some reason you may want to check other validators. Here are a few choices:

Page Valet, part of the Site Valet suite.
WDG's HTML validator

Looking for validators at W3C, but not the Markup Validator? Check out the list of validators at W3C, including well-known CSS validator, link checker, etc.

How does The Validator work?

The Validator is based on James Clark's nsgmls SGML parser. The Validator itself is a CGI script that fetches your URL, passes it through nsgmls, and post-processes the resulting error list for easier reading.

How do I send feedback/bug reports about the Markup Validator?

Read the instructions on our Feedback page.

Using this service

How do I use this service?

Most probably, you will want to use the online Markup Validation service. Read the user's manual for further help with this service.

If, for some reason, you prefer running your own instance of the Markup Validator, check out our developer's documentation.

What are these error messages?

The output of the Markup Validator may be hard to decypher for newcomers and experts alike, so we are maintaining a list of error messages and their interpretation, which should help.

Many error messages? Don't panic.

Don't panic. Did The Validator complain about your DOCTYPE declaration (or lack thereof)? Make sure your document has a syntactically correct DOCTYPE declaration, as described in the section on DOCTYPE, and make sure it correctly identifies the type of HTML you're using. Then run it through The Validator again; if you're lucky, you should get a lot fewer errors.

If this doesn't help, then you may be experiencing a cascade failure — one error that gets The Validator so confused that it can't make sense of the rest of your page. Try correcting the first few errors and running your page through The Validator again.

Be patient, with a little time and experience you will learn to use the Markup Validator to clean up your HTML documents in no time.

I don't want error messages, I want you to clean up my page!

The Markup Validator can not do this for you. You may want to have a look at tools such as HTML Tidy.

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