[css-values-5] Trailing comma in `attr()`

[cdoublev]

Is a trailing comma allowed in attr() like in var()?

attr() = attr( <q-name> <attr-type>? , <declaration-value>?)

var() = var( <custom-property-name> , <declaration-value>? )

In an exception to the usual comma elision rules, which require commas to be omitted when they’re not separating values, a bare comma, with nothing following it, must be treated as valid in var(), indicating an empty fallback value.

Related: #6345