CSS Validation Issues — Reference

Unclosed or orphan bracesMissing or unmatched { } Missing selectorA rule block with no selector before it Multiple ID selectorsChaining several #ids in one selector Deep selector chainsLong descendant-combinator selectors Universal selectorUsing * broadly in a selector Duplicate propertySame property declared twice in one rule Invalid hex colorA #color value with the wrong digit count Invalid declarationsA rule body that isn't valid property: value pairs !important overuseSeveral !important flags in one rule Unrecognized at-ruleAn @-rule outside the common, well-supported set Empty or ruleless stylesheetNo CSS, or no rules detected Unrecognized propertyA property name that isn't valid CSS Missing unitA non-zero length value with no unit Unrecognized pseudo-class/elementA :pseudo name that isn't valid CSS Empty ruleA selector with no declarations inside it Vendor prefix without standardA -webkit-/-moz- property with no unprefixed fallback Color value out of rangeAn rgb()/rgba() channel outside its valid range Duplicate selectorThe same selector defined in more than one rule block Shorthand after longhandA shorthand property silently resetting an earlier longhand Unit on a zero value0px, 0em, and 0% are all redundant — just use 0 Missing var() fallbackA CSS variable used with no fallback value Invalid media queryAn @media condition that's empty or has unbalanced parens Unquoted font familyA multi-word font name with no surrounding quotes Empty url() valueurl() with nothing inside it Pseudo-element missing content::before/::after with no content declaration