CSS Rules

Below the list of rules supported by Biome, divided by group. Here’s a legend of the emojis:

• The icon indicates that the rule is part of the recommended rules.
• The icon indicates that the rule provides a code action (fix) that is safe to apply.
• The icon indicates that the rule provides a code action (fix) that is unsafe to apply.
• The icon indicates that the rule has been implemented and scheduled for the next release.

a11y

Rule name	Description	Properties
useGenericFontNames	Disallow a missing generic family keyword within font families.

complexity

Rule name	Description	Properties
noImportantStyles	Disallow the use of the !important style.

correctness

Rule name	Description	Properties
noInvalidDirectionInLinearGradient	Disallow non-standard direction values for linear gradient functions.
noInvalidGridAreas	Disallows invalid named grid areas in CSS Grid Layouts.
noInvalidPositionAtImportRule	Disallow the use of @import at-rules in invalid positions.
noMissingVarFunction	Disallow missing var function for css variables.
noUnknownFunction	Disallow unknown CSS value functions.
noUnknownMediaFeatureName	Disallow unknown media feature names.
noUnknownProperty	Disallow unknown properties.
noUnknownPseudoClass	Disallow unknown pseudo-class selectors.
noUnknownPseudoElement	Disallow unknown pseudo-element selectors.
noUnknownTypeSelector	Disallow unknown type selectors.
noUnknownUnit	Disallow unknown CSS units.
noUnmatchableAnbSelector	Disallow unmatchable An+B selectors.

nursery

Rule name	Description	Properties
noEmptySource	Disallow empty sources.

style

Rule name	Description	Properties
noDescendingSpecificity	Disallow a lower specificity selector from coming after a higher specificity selector.
noValueAtRule	Disallow use of @value rule in css modules.

suspicious

Rule name	Description	Properties
noDuplicateAtImportRules	Disallow duplicate @import rules.
noDuplicateCustomProperties	Disallow duplicate custom properties within declaration blocks.
noDuplicateFontNames	Disallow duplicate names within font families.
noDuplicateProperties	Disallow duplicate properties within declaration blocks.
noDuplicateSelectorsKeyframeBlock	Disallow duplicate selectors within keyframe blocks.
noEmptyBlock	Disallow CSS empty blocks.
noImportantInKeyframe	Disallow invalid !important within keyframe declarations
noIrregularWhitespace	Disallows the use of irregular whitespace characters.
noShorthandPropertyOverrides	Disallow shorthand properties that override related longhand properties.
noUnknownAtRules	Disallow unknown at-rules.
noUselessEscapeInString	Disallow unnecessary escapes in string literals.

Recommended rules

• useGenericFontNames (Severity: error)
• noImportantStyles (Severity: warning)
• noInvalidDirectionInLinearGradient (Severity: error)
• noInvalidGridAreas (Severity: error)
• noInvalidPositionAtImportRule (Severity: error)
• noMissingVarFunction (Severity: error)
• noUnknownFunction (Severity: error)
• noUnknownMediaFeatureName (Severity: error)
• noUnknownProperty (Severity: error)
• noUnknownPseudoClass (Severity: error)
• noUnknownPseudoElement (Severity: error)
• noUnknownTypeSelector (Severity: error)
• noUnknownUnit (Severity: error)
• noUnmatchableAnbSelector (Severity: error)
• noDescendingSpecificity (Severity: warning)
• noDuplicateAtImportRules (Severity: error)
• noDuplicateCustomProperties (Severity: error)
• noDuplicateFontNames (Severity: error)
• noDuplicateProperties (Severity: error)
• noDuplicateSelectorsKeyframeBlock (Severity: error)
• noEmptyBlock (Severity: warning)
• noImportantInKeyframe (Severity: error)
• noIrregularWhitespace (Severity: warning)
• noShorthandPropertyOverrides (Severity: error)
• noUnknownAtRules (Severity: error)
• noUselessEscapeInString (Severity: warning)

Missing a rule? Help us by contributing to the analyzer or create a rule suggestion here.