[css-variables-1] Expose pending substitution value?

[andruud]

Pending-substitution values must be serialized as the empty string, if an API allows them to be observed.

https://drafts.csswg.org/css-variables/#substitution-in-shorthands

Authors do trip over this behavior sometimes (https://issues.chromium.org/issues/40804066), and it's not ideal for cases where round-tripping via text is crucial, like serializing to MHTML.

Can we somehow expose the pending-substitution value? E.g.:

border-width: shorthand(border, 1px var(--x) black);

[Loirooriol]

Duplicate of #8055 ?

[tabatkins]

Technically a separate request, but close enough to be worth deduping. I'll move the comment over to there.