CSS validation checks your stylesheets against W3C standards to find syntax errors, deprecated properties, and browser compatibility issues. Invalid CSS can cause unpredictable rendering across different browsers, especially on mobile devices. Even a single missing semicolon or mismatched bracket can break styling for every rule that follows it in the stylesheet. Regular CSS validation helps catch these issues before your visitors do.
Enter the URL of a page whose CSS you want to validate, or upload a CSS file directly using the tabs above. Select your encoding preferences and click "Validate." The tool will analyze every CSS rule and report errors, warnings, and deprecated properties.
Common CSS validation errors include missing units on numeric values (e.g., writing "10" instead of "10px"), using vendor prefixes without the standard property, and deprecated properties that have been replaced in newer CSS specifications. Not every validation warning needs to be fixed. Vendor prefixes and CSS custom properties may generate warnings but are perfectly valid in practice.
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