Documentation for the W3C Markup Validator

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!

Table of contents

For beginners and experts alike

W3C Markup Validator Help and FAQ

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.

W3C Markup Validator User's Guide

The User's Guide and tips for the W3C Markup Validator. It notably lists and explains the available options.

W3C Markup Validator Error Messages

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

Access Keys

Quick navigation with the access keys used throughout this site.

Experts only

Development Roadmap
high-level overview of past and future releases.
Download / Source

Information on source code availability and download instructions for the Markup Validator.

Installation Guide

Step-by step guide for the installation of the Markup Validator on your server.

Markup Validator Web Service API

Use the validator from your applications with the experimental Markup Validator Web service, and its open API.

Developer Information

Information for programmers wanting to play with the validator's code, and/or contribute to its development.

SGML, Validation and the Markup Validator

An introduction to concepts of SGML, DTDs and formal validation, and how they apply to the Validator, with links to further information.