CSS Validator

Errors

•  Require properties appropriate for display
•  Beware of broken box sizing
•  Disallow empty rules
•  Disallow duplicate properties
•  Require use of known properties

Compatibility

•  Disallow box-sizing
•  Disallow adjoining classes
•  Require all gradient definitions
•  Require compatible vendor prefixes
•  Require standard property with vendor prefix
•  Disallow negative text-indent
•  Disallow star hack
•  Require fallback colors
•  Bullet-proof @font-face
•  Disallow underscore hack

Performance

•  Disallow @import
•  Don't use too many web fonts
•  Disallow selectors that look like regexs
•  Disallow duplicate background images
•  Disallow unqualified attribute selectors
•  Disallow universal selector
•  Disallow overqualified elements
•  Disallow units for 0 values
•  Require shorthand properties

Maintainability & Duplication

•  Don't use too many font sizes
•  Disallow too many floats
•  Disallow !important
•  Disallow IDs in selectors

Accessibility

•  Disallow outline:none

OOCSS

•  Heading should only be defined once
•  Disallow qualified headings

Learn More About the Rules