Allow registered custom properties to have a guaranteed-invalid initial value?

[mirisuzanne]

The guaranteed invalid value is useful when we want to provide inline fallbacks for a variable. But currently registered custom properties must have a valid initial-value. At that point, it's no longer possible to provide inline fallbacks – the variable will never again be guaranteed invalid, because it always has a value to revert to.

[alexgagnon]

I'm starting to use @Property in a design system I'm working on and basically immediately ran into what I think is related to this issue. It's probably a me not understanding the spec more than anything but here's an example:

@property --spacing-override {
  syntax: "<length>";
  inherits: false;
  initial-value: ???;
}

@property --spacing {
  syntax: "<length>";
  inherits: true;
  initial-value: 20px;
}

button {
  padding: var(--spacing-override, var(--spacing));
}

When --spacing-override has anything other than a <length>, the property isn't registered at all, so it isn't set and the fallback is used. I appear to be able to give a custom-ident in syntax like "<length> | null", and then use "null" as the initial-value, which does register the property, but then --spacing-override is set and the fallback isn't used, even though the "null" value is invalid for padding.

A "guaranteed invalid" value seems like it would address this by saying: "register this property with a given syntax, but mark it as IACT unless it's explicitly set"?

I think this might have been suggested already here or might be loosely related to the "space toggle hack"?