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?