[css-fonts-4] <generic-family> should not accept <string> and unrestricted <custom-ident>+

[cdoublev]

Follow-up on #6770 (comment).

<generic-family> went from accepting a restricted list of keywords (generic family names) to accepting either generic(<custom-ident>+), <string>, or (unrestricted) <custom-ident>+.

But from reading across the issue, I suspect the intent was for it to only acquire generic(<custom-ident>+). Therefore in the comment, I suggested <generic-family> = generic(<custom-ident>+) | <generic-family-name>.

Generic family names would be defined for <generic-family-name> (to allow deriving <generic-family-name> = serif | sans-serif | ... from scraped <dfn>s).

[svgeesus]

It should really have been generic(<custom-ident>) i.e. a single identifier inside the function. Like generic(kai)

But there is a need to do two things: define the syntax for the function, in general, and also define the current list of generic font families. The older ones don't use the functional syntax while the new ones will.

[svgeesus]

Relevant on string or not:

• [meta] [css-fonts] Criteria for generic font families #4910 (comment)
• [meta] [css-fonts] Criteria for generic font families #4910 (comment)

[cdoublev]

Fixed by 69f7f7a.