[css-properties-values-api] Fallbacks for registered properties

[andruud]

Not necessarily a spec issue, I just need a clarification: do fallbacks (as specified by the var() reference) for registered properties ever trigger? E.g. can foo ever been substituted here?

--unreg: var(--reg-length, foo);

And what about:

--reg-length: 10em;
font-size: var(--reg-length, 100px);

In Blink, I've implemented the latter such that the fallback (100px) is triggered, but now I wonder if it's more consistent that var(--reg-length, 100px) is substituted with the initial/inherited value of --reg-length. I.e. the computed value of font-size becomes the initial/inherited value of --reg-length, and the computed value of --reg-length becomes 10 x initial/inherited.

[tabatkins]

Yes, registered properties can be invalid (and thus trigger fallback in var()). Properties registered with a "*" grammar and no initialValue is invalid by default, and any custom property becomes invalid if it's involved in a cyclic reference.