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name: css-at-property description: "Use when implementing animated gradients, complex CSS transitions that involve custom property values, or building a typed design token system where custom property misuse should produce visible errors." metadata: category: css priority: low difficulty: advanced estimatedTime: "30" source: frontendchecklist.io url: https://frontendchecklist.io/rules/css/css-at-property

Register CSS custom properties with @property for animation and type safety

CSS custom properties are powerful but limited by default: they are untyped string substitutions that the browser cannot interpolate during transitions or animations. Registering a property with @property gives the browser the information it needs to animate between values, validate token assignments, and scope inheritance — turning custom properties from string macros into first-class typed values. This unlocks gradient animations, typed design tokens, and scoped component theming that are impossible with unregistered properties.

Quick Reference

• Unregistered custom properties cannot be animated — they transition as discrete strings
• @property specifies syntax (color, length, number), initial-value, and inherits
• Registered properties enable smooth interpolation for gradients and complex values
• inherits: false creates scoped tokens that do not cascade to children

Check

Review the CSS custom properties in this file. Identify any properties that are used in transitions or animations — these require @property registration to animate correctly.

Fix

Add @property registrations for the custom properties used in transitions or animations in this CSS. Include the correct syntax descriptor, an initial-value, and the appropriate inherits value.

Explain

Explain how @property works, why unregistered custom properties cannot be animated, what the syntax descriptor values mean, and how inherits controls cascade behaviour.

Code Review

Inspect all CSS transitions and animations that reference custom properties. Flag any custom property used in a transition or animation that is not registered with @property, and flag any @property registration that is missing initial-value.

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For full implementation details, code examples, and framework-specific guidance, see references/rule.md.

Rule page: https://frontendchecklist.io/rules/css/css-at-property