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 | | --- | --- | --- | | noDeprecatedMediaType | Disallow deprecated media types. | | | noDuplicateSelectors | Disallow duplicate selectors. | | | noExcessiveLinesPerFile | Restrict the number of lines in a file. | | | noExcessiveSelectorClasses | Limit the number of classes in a selector. | | | noHexColors | Disallow hex colors. | | | useBaseline | Disallow CSS properties, values, at-rules, functions, and selectors that are not part of the configured Baseline. | |

`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. | | | noEmptySource | Disallow empty sources. | | | 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)

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