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.