I'm not convinced supporting a comma-separated list for the :heading pseudo-class alone is worth it. If it shared <An+B># with another pseudo-class it'd be more persuasive, but for a pseudo-class that can match at most six levels (perhaps nine in the near future) it's really not compelling enough to add the additional parsing and serialization complexity.

The argument @keithamus gave me is that :heading(1, 2, 3) is easier than :heading(-n+3), but you can also write :heading(1), :heading(2), :heading(3) and that will work just as well.

My suggestion is that we use <An+B>, matching :nth-of-type.

cc @zcorpan @tabatkins @nt1m @fantasai