Validation is the process of checking web documents against the ever-evolving standards of the W3C and WHATWG. A document that follows the rules set forth by these governing bodies is said to be “valid”.

In addition, documents are checked for well-formedness, and their adherence to any data typing if specified.

Validation can be performed a-page-at-a-time with tools made available by the W3C and WHATWG, or partially automated, as in the case of HTML Validator Pro.