https validator w3 org

Check the markup (HTML, XHTML, …) of Web documents

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

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.

Quick navigation with the access keys used throughout this site.

Experts only

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.

This service runs the W3C Markup Validator, v 1.3+hg.

Copyright © 1994-2013 W3C ® (MIT , ERCIM , Keio, Beihang), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply. Your interactions with this site are in accordance with our public and Member privacy statements.

Providing standards conformance checking services to the Web community

W3C Val >

1. About
2. Misuse
3. Blocking
4. Validation Services

About W3C Validation services

W3C provides various free validation services that help check the conformance of Web sites against open standards.

You are most likely here because this address appeared in logs for your website. This means someone used one of our services to assess content on your site.

Misuse

While these services were created for the purpose of helping Web developers and designers there is potential like many online services for use other than intended.

Modest traffic from these services does not consitute abuse against your website. Third parties using this service to review content you make publicly available is not substantially different from browsing your site. Web designers frequently evaluate techniques of other websites as a means to learn.

Blocking W3C Validators

Before considering blocking W3C Validator services you should ensure that nobody in your organization or perhaps contracted by them is requesting our services to make the assessments.

Should you wish to block all or some W3C Validation services from assessing your site you may do so based on our IP addresses or user-agent header string. How to do so varies based on specific operating systems, firewalls and webserver software.

Blocking on User-Agent

As these services commonly include the link https://validator.w3.org/services in their user-agent you can filter them all based on presence of that string in user-agent header. You can instead opt to block specific Validators based on the unique portion of their user-agents. If you wish to block them individually it would be best not to include the version numbers as those are subject to change.

Blocking on IP Address

Traffic from W3C Validator services will be coming from 128.30.52.0/24 subnet and you may firewall or block that in your web server configuration. You should only firewall incoming port 80 and 443 from this subnet so as not to block your users from assessing W3C website or ability to participate in mailing lists.

W3C Validation Services

Below is a listing of W3C’s various Validation services, links to the services themselves, the user-agent header being sent and how to find out more information on each.

Markup Validator

• Service
• User-Agent: W3C_Validator/1.3
• About

Validator.nu

• Service
• User-Agent: Validator.nu/LV
• About

Link checker

• Service
• User-Agent: W3C-checklink
• About
• * as a crawling service this honors robots.txt directives

Mobile OK Checker

• Service
• User-Agent: W3C-mobileOK/DDC-1.0
• About

Internationalization Checker

• Service
• User-Agent: W3C_I18n-Checker/1.0
• About

Validator Suite

• Service
• User-Agent: NING/1.0
• About
• * this service invokes other W3C Validators
• * as a crawling service this honors user supplied directives

Feed Validation

• Service
• User-Agent: FeedValidator/1.3
• About

CSS Validator

• Service
• User-Agent: Jigsaw/2.3.0 W3C_CSS_Validator_JFouffa/2.0
• About

Unicorn

• Service
• User-Agent: W3C_Unicorn/1.0
• About
• * this service invokes other W3C Validators

Copyright © 1994-2013 W3C ® (MIT , ERCIM , Keio), Beihang All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply. Your interactions with this site are in accordance with our public and Member privacy statements.

Check the markup (HTML, XHTML, …) of Web documents

Note: file upload may not work with Internet Explorer on some versions of Windows XP Service Pack 2, see our information page on the W3C QA Website.

Validate by direct input

This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc. If you wish to validate specific content such as RSS/Atom feeds or CSS stylesheets, MobileOK content, or to find broken links, there are other validators and tools available. As an alternative you can also try our non-DTD-based validator.

This service runs the W3C Markup Validator, v 1.3+hg.

Copyright © 1994-2013 W3C ® (MIT , ERCIM , Keio, Beihang), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply. Your interactions with this site are in accordance with our public and Member privacy statements.