W3C CSS Validation Service

The W3C CSS Validation Service is a free software tool used to verify that a website's Cascading Style Sheets (CSS) code correctly adhere to official Web Standards Project specifications. It is highly effective for pinpointing syntax errors that might break a website's layout. Designers and developers use it to ensure their styling is robust and accessible.

While search engines like Google are generally forgiving of minor CSS syntax errors, severely broken code can negatively impact user experience. If invalid CSS causes a layout to break on mobile devices, it will likely hurt your mobile-first indexing and rankings. Running your code through the W3C validator is a best practice to ensure cross-browser compatibility.

You can use the service by visiting the W3C website and entering your website's URL, uploading a CSS file, or directly pasting the CSS code into the text box. The tool will then process the input and generate a report detailing any errors or warnings. It provides specific line numbers to help developers quickly fix their code.

The tool was created and is maintained by the World Wide Web Consortium (W3C), the primary international standards organization for the internet. The W3C is led by web inventor Tim Berners-Lee and provides the foundational frameworks for how web pages operate. Their validation tools exist to promote open, accessible, and standard-compliant web development.