[css-values-5] Using `first-valid()` and `interpolate()` in shorthands

[cdoublev]

There are currently three <whole-value> substitution functions: first-valid(), interpolate(), toggle(). All are resolved at computed value time or later.

Using them as a shorthand value is specified for toggle(), but not for the other two: should they expand like arbitrary substitutions, with a pending-substitution value?

[Loirooriol]

The current definition of toggle() on shorthands is broken, see #6764.

This needs to use internal values akin to pending-substitution values, or #8055.

But I think first-valid() is resolved at parse time?

[tabatkins]

But I think first-valid() is resolved at parse time?

No, it's not. If you're solely providing literal arguments, then yes, it could be, but then you could just write them as separate declarations for the same effect. The point of first-valid() is to take var()s in their arguments, so in practice it's always resolved at computed value time, during substitution. For consistency, we just make it always resolve then, like the other arbitrary-substitution functions.

[Loirooriol]

then you could just write them as separate declarations for the same effect

Wasn't this the behavior that you wanted in #5055 (comment) ? There is actually #10989 about this.

[tabatkins]

Ah, I can see how my #5055 comment can be taken like that, but no. first-valid() is resolved "at parse time", but "at parse time" gets delayed when there's an arbitrary-substitution function involved. My point in that comment is that there's no special complexity involved here; you just do necessary substitution, then check parse-validity of the arguments.

But if you're not using a substitution function, then first-valid() is indeed resolved at normal parse time. For example, display: first-supported(foo, bar); from your example is indeed invalid at parse time, throwing out the declaration, because neither foo nor bar are valid display values.

[cdoublev]

Can you please clarify this, and close #10989?

<first-valid()> = first-valid( <declaration-value># )

If none of the arguments represent a valid value for the property, the property is invalid at computed-value time.

For example:

However, if none of them includes a substitution function that resolve after parsing, it is invalid at parse time.

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So the question in the op is still open for interpolate().