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A validator is a computer program used to check the validity or syntactical correctness of a fragment of code or document. The term is commonly used in the context of validating HTML, CSS and XML documents or RSS feeds though it can be used for any defined format or language.
HTML Validator 
In the context of web documents, the term HTML validator technically refers to validation using DTDs by SGML tools. However, the common (non technical) use of the term refers to checking an HTML document for various problems using some type of computerized checker, and not necessarily by using a DTD with SGML tools. Furthermore, the term HTML validation for HTML5 cannot mean validation using a DTD because HTML5 is not SGML-based and does not use a DTD.
See also 
- Lemay, Laura (1995). Teach Yourself More Web Publishing with HTML in a Week. Indianapolis, Ind: Sams.net Pub. p. 198.
- Tittel, Ed, and Mary C. Burmeister. HTML 4 for Dummies. --For dummies. Hoboken, NJ: Wiley Pub, 2005.