[css-counter-styles] Exclude 'none' from <counter-style-name>

[SimonSapin]

https://drafts.csswg.org/css-counter-styles/#typedef-counter-style-name

The general form of an @counter-style rule is:

@counter-style <counter-style-name> { <declaration-list> }

where <counter-style-name> is a <custom-ident>.
If a counter style’s name is an ASCII case-insensitive match for "decimal" or "none", the @counter-style rule is invalid.

We’ve clarified in #1285 that the last sentence only applies to at-rules, and not other uses of <counter-style-name>. This makes sense for decimal which is a style that always exists (it just can’t be overridden). Allowing none however is useless since it can’t ever refer to an existing counter style.

I suggest changing the spec to make none (ASCII-case-insensitive) an invalid value of <counter-style-name> in all contexts, like CSS-wide keywords already are.

CC @upsuper, @tabatkins

[upsuper]

Gecko currently treats none as a predefined counter style in that case, but I think excluding none from <counter-style-name> makes sense, given that it can never be a counter style, and can never be referred anywhere.

Strictly speaking, this proposal would make system: extends none and fallback: none invalid, but I don't think that would cause any real problem.

[tabatkins]

Yeah, this sounds reasonable.

[therealglazou]

+1