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Tools

Nu HTML Checker

Checks HTML documents.

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Mobile Checker

Be Mobile Friendly.

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CSS Validator

Checks your Cascading Style Sheets (CSS).

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Internationalization Checker

Checks level of internationalization - friendliness.

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Markup Validator

Checks the markup of your Web documents. (HTML or XHTML).

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rdf

RDF Validator

Checks and visualizes RDF documents.

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RSS feed Validator

Validator for syndicated feeds. (RSS and Atom feeds)

Unicorn

Unified validator. HTML, CSS, Links & Mobile.

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Get help

Stack Overflow has a great and active community focused around W3C Standards and W3C developer tools.
To ask a question to the W3C Community, we invite you to use the w3c tag.

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Contribute

Developers are welcome to get involved by contributing code, either to existing tools or proposed future projects.

Testing, bug reports, suggestions, or help in creating good documentation are also important!

You will find more information to get started in each project on their own web page.

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Donate

Good tools cost a lot to develop, support, host and maintain.

With your help, we can make the tools and validators better, faster, friendlier, and maintain a great, free service for all of the Web.

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