Here you will find the documentation for The W3C Markup Validation Service. We are constantly working on improving this documents and adding new elements, and eagerly welcome feedback and additions!
A quick intro and help to get you started, with answers to Frequently Asked Questions. Read this first.
This document has a lot of information on how the validator works, answers to frequent questions about "why did the validator say that...", as well as pointers to technical and policy documents.
The User's Guide and tips for the W3C Markup Validator. It notably lists and explains the available options.
Explanations of common error messages.
This list conveniently gathers all the error messages currently used by the validator when processing Web documents, and the community-contributed explanations to these error messages, whenever available.
Favelets, a.k.a Bookmarklets: Validation made fast and easy
Quick navigation with the access keys used throughout this site.
Information on source code availability and download instructions for the Markup Validator.
Step-by step guide for the installation of the Markup Validator on your server.
Use the validator from your applications with the experimental Markup Validator Web service, and its open API.
Information for programmers wanting to play with the validator's code, and/or contribute to its development.
An introduction to concepts of SGML, DTDs and formal validation, and how they apply to the Validator, with links to further information.