Limits on Animating Unregistered CSS Custom Properties

Goal: Assess if CSS custom properties declared with a double‑hyphen prefix can be animated solely through transition or animation properties when no @property rule is present, relying only on the default behavior defined in the CSS Custom Properties specification.

Constraints: The CSS Properties and Values API Level 1 makes registration optional for animation, leaving the decision to individual browsers; as a result, Chrome, Edge and Safari may animate unregistered properties only behind experimental flags, while Firefox currently does not, and the spec does not require uniform behavior. This inconsistency creates uncertainty for authors who depend on smooth transitions across environments.

Does the CSS specification permit animation of unregistered custom properties without @property registration? Are browsers required to treat such properties as animatable, or may they ignore the animation entirely? What fallback behavior should authors expect when animation is unsupported?