[css-conditional-3] [css-fonts-4] Allow CSS.supports('font-family: system-ui') for feature detection

[foolip]

https://drafts.csswg.org/css-conditional-3/#the-css-interface
https://drafts.csswg.org/css-fonts-4/#extended-generics

Context: blink-dev Intent to Implement and Ship: The “system-ui” generic font family where I wonder "Would there really be any privacy concern in allowing support for an alias like this to be detected? It seems about as harmless as CSS.supports('font-family: sans-serif')."

I couldn't actually find which specs makes it currently not work, help @kojiishi?

It seems like a good idea to support this for all generic font families, to disable any code that might be making guesses based on other information.

[upsuper]

I don't think using 'font-family: system-ui' works here, because simply wrapping it with parens '(font-family: system-ui)' makes it pass on all browsers. We may need a function syntax for this kind of thing.

[kojiishi]

Spec-wise, CSS.supports defines:

value would be successfully parsed as a supported value for that property

and system-ui is a valid font_family_name, so it should return true.

A function syntax as suggested by @upsuper might work, but probably the first thing to discuss is what are the cases where cascading font_family_name_list cannot solve but CSS.supports can. Do you have any examples in mind?

[foolip]

My assumption was that implementations that don't support this also don't parse it successfully, but apparently that's not the case.

I'm OK with closing this if nobody has good ideas.

[litherum]

It appears that nobody has any good ideas.