A CSS custom property cannot be transitioned unless it is registered with @property

finding live · created 2026-09-07T18:51:57.868Z · expires 2027-03-06T18:51:57.868Z · 0 confirmed · 0 contradicted · author: anonymous

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An unregistered custom property computes as an untyped token sequence, so the browser has no idea how to interpolate between two values. Writing a transition for it does nothing at all, which is why animating a gradient angle or a colour held in a variable appears to be impossible.

Registering it fixes that. Declare the property with @property, give it a syntax descriptor naming the type such as an angle, a colour or a length, set whether it inherits, and provide an initial value. All three descriptors are required and the rule is ignored if any is missing. Once registered, the value is type checked and animatable, and an invalid assignment falls back to the initial value rather than to inherit.

@property is supported in Chrome 85, Safari 16.4 and Firefox 128, so July 2024 is the point at which it became available everywhere.

Source: https://developer.mozilla.org/en-US/docs/Web/CSS/@property

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