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What is Markup Validation?

Most pages on the World Wide Web are written in computer languages (such as HTML) that allow Web authors to structure text, add multimedia content, and specify what appearance, or style, the result should have.

As for every language, these have their own grammar, vocabulary and syntax, and every document written with these computer languages are supposed to follow these rules. The (X)HTML languages, for all versions up to XHTML 1.1, are using machine-readable grammars called DTDs, a mechanism inherited from SGML.

However, Just as texts in a natural language can include spelling or grammar errors, documents using Markup languages may (for various reasons) not be following these rules. The process of verifying whether a document actually follows the rules for the language(s) it uses is called validation, and the tool used for that is a validator. A document that passes this process with success is called valid.

With these concepts in mind, we can define "markup validation" as the process of checking a Web document against the grammar (generally a DTD) it claims to be using.
Is validation some kind of quality control? Does "valid" mean "quality approved by W3C"?

Validity is one of the quality criteria for a Web page, but there are many others. In other words, a valid Web page is not necessarily a good web page, but an invalid Web page has little chance of being a good web page.

For that reason, the fact that the W3C Markup Validator says that one page passes validation does not mean that W3C assesses that it is a good page. It only means that a tool (not necessarily without flaws) has found the page to comply with a specific set of rules. No more, no less. This is also why the "valid ..." icons should never be considered as a "W3C seal of quality".

Is validity the same thing as conformance?

No, they are different concepts. Markup languages are defined in technical specifications, which generally include a formal grammar. A document is valid when it is correctly written in accordance to the formal grammar, whereas conformance relates to the specification itself. The two might be equivalent, but in most cases, some conformance requirements can not be expressed in the grammar, making validity only a part of the conformance.
What is the Markup Validator and what does it do?

The Markup Validator is a free tool and service that validates markup: in other words, it checks the syntax of Web documents, written in formats such as (X)HTML.

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 and the languages it can validate.

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.

A lengthier answer to this question is also available on this site if the explanation above did not satisfy you.

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

How does The Validator work?

The Validator is based on OpenSP, an SGML parser based on James Clark's SP SGML parser. The Validator itself is a CGI script that (basically) fetches your document, passes it through the parser, 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. The simple way to use this service to validate a Web page is to paste its address into the text area on the validator's home page, and press the "Check" button.

There are other possible uses and a few usage options, please 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 decipher for newcomers and experts alike, so we are maintaining a list of error messages and their interpretation, which should help.

Source: Official WC website