[cssom][selectors] Define or fix selectors_group #6927
Description
In CSSOM:
5.1. Parsing Selectors
To parse a group of selectors means to parse the value using the
selectors_groupproduction defined in the Selectors specification and return either a group of selectors if parsing did not fail or null if parsing did fail.
I can not find selectors_group in Selectors. Does it correspond to <selector-list>? When does parse a selector should be used?
EDIT:
I think it should use parse a selector, as well as the definition for parsing the prelude of a style rule in Syntax, which is currently:
The prelude of the qualified rule is parsed as a
<selector-list>. If this returns failure, the entire style rule is invalid.*
And it should be completed with a validation for nest-containing selector for @nest, as explained in my comment below.