[selectors-4] Clarify how to parse S in <:nth-child()>

[cdoublev]

The value definitions of <:nth-child()> and <:nth-last-child()> includes S defined as a parsed as a forgiving selector list. But the first step of parsing as a forgiving selector list is:

1. Parse a list of s from input, and let selector list be the result.

I assume that :nth-child(odd of ul > li) should be invalid because ul > li is a , but :nth-child(odd of li.foo, li.bar) is valid because li.foo, li.bar is a .

I guess that this situation comes from the resolution and the commits mentionned in #1292 and #3760.

I suggest to replace S by and to define it as is identical to , except it parses its components as rather than . Selectors 5 can replace by if s become valid.

[cdoublev]

Actually the resolution is to allow as S, sorry. I'm closing this duplicate and I assume that S will be replaced by .