The Markup Validation Service by the World Wide Web Consortium (W3C) allows Internet users to check HTML documents for conformance to HTML or XHTML standards. It also provides a quick method for web page authors to check their posted pages for mark-up errors.

Method of operation

HTML validators operate by comparing the mark-up on a web page to the W3C standards. The standards vary depending upon the declared version and so the validator will start by reading the DOCTYPE declaration to see which set of standards to apply.

Once the validator has read the page and determined the applicable standards it looks for such things as missing opening or closing tags, missing quotation marks and other hand-coding errors.

The validator then provides a report signifying that the coding is correct or not. Errors are noted in a list. One error, such as neglecting to close a tag, can cause a cascade of errors through the page, producing dozens or even hundreds of noted errors. However when the page author addresses the first error listed it will also eliminate the "cascade errors".

Browser accomomdation

Many major browsers are often tolerant of certain types of error, and may display a document successfully even if it is not syntactically correct. Certain other XML documents can also be validated if they refer to an internal or external DTD.

W3C also offers validation tools for web technologies other than HTML/XHTML, such as MathML or CSS.

Validator criticism

All mark-up validators suffer from in inability to see the "big picture" on a web page. However they excel at picking up missed closing tags and other technicalities. This does not mean that the page will display as the author intended in all browsers. Even if validated, all web pages should be tested in as many different browsers as possible to ensure that the boundaries of the validator are compensated for and that the page works correctly.

CSS validation

While the W3C and other HTML and XHTML validators will assess pages coded in those formats, a separate validator is needed to conform that there are no errors in the associated Cascading Style Sheet. CSS validators work in a similar manner to HTML and XHTML validators and apply current CSS standards to referenced CSS documents.