Multiple selectors not supported in :is() pseudo

[PedroGuedesDT]

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I'm exploring this library to develop a POC for a possible solution. Yet, in my tests, when i try to parse a valid CSS file that have a selector like this:

:is(p, a)

It complains that it don't expects the comma. But when i check some documentation, it is expected to support multiple values.

Check this: Mozilla documentation

Am i doing something wrong, or is something that is not previewed to happen here?