[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

[tabatkins]

No, that's a special exception specifically for var() so you can substitute with an empty value (since you're allowed to define a custom property to have an empty value). It doesn't apply to anything else; the normal comma-elision rules are in force for attr().