[css-properties-values-api-1] Should `@property` always be invalid with CSS-wide keywords or property substitutions for `initial-value`?

[cdoublev]

There is this note about computationally independent values:

[...] 3em is not computationally independent [...]. Neither is a value with a var() function, because it relies on the value of a custom property.

Browsers make @property invalid with syntax: "<custom-ident>" and initial-value: initial (WPT) whereas initial is computationally independent. If initial cannot match <custom-ident>, neither can var(--custom). Both are valid property values, no matter the property value definition.

Similarly, syntax: "<length>" with initial-value: first-valid(1vmin, 20px) seems reasonable. But that might also not be allowed (cf. #1120).