[css-syntax][selectors] Clarify handling of invalid selectors in css-syntax.

[Ms2ger]

CC @tabatkins @SimonSapin

https://drafts.csswg.org/css-syntax/#css-stylesheets says

The prelude of the qualified rule is parsed as a selector list. If this results in an invalid selector list, the entire style rule is invalid.

The prelude of a qualified rule is a list of component values, and I can't find any prose that defines how to parse that into a selector.

(Pedantic, I know, but this is much less well-defined than the rest of the otherwise well-written spec.)

[SimonSapin]

I think the proper way to fix this is to rewrite https://drafts.csswg.org/selectors/#grammar in terms of css-syntax.

Could railroad diagrams be normative?

[tabatkins]

Yeah, there's no proper normative description yet in Syntax terms; I have a long-delayed task to rewrite the Selectors grammar accordingly.

Railroad diagrams probably could be normative, if you wanted. There's no error-correction, so they're fine. But it's also easy to just write some syntax for it.

[tabatkins]

Ah, I did get around to writing the Selectors grammar, so I can rephrase this to explicitly reference the correct productions.

[tabatkins]

Fixed.

[Ms2ger]

f02bb2e