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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 DTD s, 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".

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