W3c org html validator

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

Validator.nu

Link checker

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

Mobile OK Checker

Internationalization Checker

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

CSS Validator

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.

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Markup Validation Service

The W3C Markup Validator provides Perl/CGI/SGML/XML/DTD-based validation of a variety of document types. SGML and DTDs are older technologies that never found wide use on the Web, so for checking of HTML documents using modern technologies, you probably want to instead use the W3C HTML Checker. To do that,

• Download the latest release version.
• Read the usage guide.

If for some reason rather than using the W3C HTML Checker, you want to use the W3C Markup Validator, this page provides the following information:

Running your own instance of the W3C Markup Validator

To provide your own service for checking HTML documents, you probably want to run an instance of the W3C HTML Checker. Instructions on how to do that are available in the following guide:

Using the W3C Markup Validator API

For programmatic checking of modern HTML documents, use the API provided by the W3C HTML Checker. To do that, call https://validator.w3.org/nu/ or the URL for another instance of the W3C HTML Checker, and consult the following documentation:

• W3C HTML Checker GET interface
• W3C HTML Checker POST interface
• W3C HTML Checker parameters
• W3C HTML Checker out=json format
• W3C HTML Checker out=xml format
• W3C HTML Checker out=gnu format

Controlling the behavior of the W3C Markup Validator

This section describes the behavior of each of the options exposed in the W3C Markup Validator user interface, and names of the corresponding query parameters (shown in parentheses) you can use with the W3C Markup Validator API.

This allows you to override the character encoding information about your document. You may use this option for test purposes, but you will eventually have to serve your document with the correct character encoding, or the validator will complain about it and you document will not be valid.

Use Fallback instead of Override ( fbc )

Uses the character encoding override mechanism described above, but only does it as a fall back mechanism if the actual document is not served with character encoding information. Think of this as a gentler override mechanism.

This allows you to override the DOCTYPE declaration for your document. You may use this option for test purposes, but you will eventually have to serve your document with the correct DOCTYPE declaration, or the validator will complain about it and you document will not be valid.

Use Fallback instead of Override ( fbd )

Uses the Doctype override mechanism described above, but only does it as a fall back mechanism if the actual document does not have a Doctype declaration. Think of this as a gentler override mechanism.

Displays the HTML source of the document you validated and links error messages directly to lines in this output. Makes it easy to see what’s wrong.

Will generate an outline of your document from the H1 — H6 elements. For a properly formed document, this will be a nicely nested tree structure. The visualization of your document’s structure makes it easier to see where you’ve skipped a heading.

If you want to examine the semantic structure of your documents, beyond the outline, try the Semantic data extractor.

Validate error pages ( No200 )

The W3C Markup Validator will usually tell you if the page you tried to validate could not be retrieved (for example, if the server gave a «404 not found» message. In some circumstances you may want to be able to validate the error page sent by the server. This is the option to use then.

This option triggers verbose output. Verbose output adds more explanations and suggestions to the validation results, and gives more information on the resource validated. This makes it a useful option if you prefer to be given as much help as possible; if you prefer more concise reports, leave this option unset.

For Content-Negotiated resources, set a specific Accept Header ( accept )

This option (experimental, as of 0.8.2) is useful if your Web server is set up to use format negotiation, serving different content based on the preferred/accepted media types of the user agent. The validator can then emulate different HTTP Accept behaviors.

For example, append » accept=application%2Fxhtml%2Bxml%2C* » and the validator will send the HTTP Header » Accept: application/xhtml+xml,* «.

For Content-Negotiated resources, set a specific Accept-Language Header ( accept-language )

This option (experimental, as of 0.8.2) is useful if your Web server is set up to use language negotiation, serving content in different languages based on the preferred/accepted language setup of the user agent. The validator can then emulate different HTTP Accept-Language behaviors.

For example, append » accept-language=ja%2Cfr » and the validator will send the HTTP Header » Accept-Language: ja,fr «.

Set a specific Accept-Charset Header ( accept-charset )

This option (experimental, as of 0.8.3) makes the validator send an Accept-Charset HTTP header, specifying the character encodings which it will accept from server. This option is mainly used to interface the W3C Markup Validator for Mobile Web Best Practices checking.

Set a specific User-Agent Header ( user-agent )

This option (experimental, as of 0.8.3) makes the validator send a custom User-Agent HTTP header instead of the usual W3C_Validator/xx.xxxx . If the value of this parameter is mobileok , the validator will output a User-Agent string as defined by the Mobile Web Best Practices spec.

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.

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