Validate CSS
Check CSS for invalid syntax, unknown properties, malformed selectors and deprecated constructs.
Based on: W3C CSS Specifications
About this validation
Checks CSS for unknown properties, invalid values, and malformed selectors against the W3C CSS specifications.
- Protocol / standard:
- Validation engine:
- CSSTree Validator
- Rules reviewed:
- Aug 31, 2026
Standards and vendor-documented behavior can change. This reflects the source as last reviewed above, not a permanent guarantee.
Ctrl/Cmd + Enter