[css-fonts-5] Consider 'extends' descriptor to reduce duplicate declarations

[fantasai]

CSS Counter Styles has an extends descriptor that basically copies all the descriptors of a different @counter-style rule and lets this @counter-style rule override specific ones rather than repeating them all. See https://www.w3.org/TR/css-counter-styles-3/#extends-system

It might make sense to copy that system over to @font-face, to make it easy to tweak the definitions of a common font face into multiple variants?

[svgeesus]

Avoiding the maintenance headache of almost-the-same descriptor blocks sounds good in principle.

Should that be added as a general feature of @-rules or is it okay to just define it in @counter-style and @font-face (and potentially @ont-feature-values and @font-palette-values )?

[svgeesus]

As a very weak counter-argument, SVG at one time had a feature in gradients to make them extend other gradients; which could lead to lengthy chains of gradients extending other gradients extending other gradients, and was not liked by implementers especially when animated.

(Of course, descriptors can't be animated)

[tabatkins]

I don't think this should be a general feature; it's additional impl complexity that isn't particularly needed for most rules. But font-faces do the "multiple at-rules that are nearly identical" thing a lot more than counter-styles do, especially as we add more of these controlling descriptors.

[astearns]

Is there any evidence that authors have found descriptor duplication in font-face rules annoying enough to have created tooling for this case?

[faceless2]

There was none found for #3691, which was something I floated a while back that had a similar goal.

[css-meeting-bot]

The CSS Working Group just discussed [css-fonts-5] Consider 'extends' descriptor to reduce duplicate declarations.

The full IRC log of that discussion

<dael> Topic: [css-fonts-5] Consider 'extends' descriptor to reduce duplicate declarations
<dael> github: https://github.com//issues/6104
<dael> fantasai: The @counter-style rules can get complicated. @extend lets you go off previous definition. As @font-face gets more complicated might be might to have same mech so a font-face rule can extend another one and add some descriptors
<dael> myles: I think I agree with astearns comment where we should wait for things like Sass to come up with it. A bit early to add b/c don't know we need it
<dael> astearns: And a lot of things we're adding to complicate rule are for specific fonts and not really shared. Don't want to get to state where someone uses a value they tested on one font on a lot where they don't know correct
<dael> fantasai: Yeah. When have variable font youll have a bunch of common stuff to core. might want to set some things to set named variants
<dael> astearns: Agree probably useful for variation fonts, but should wait until we know it'll be used
<dael> leaverou_: May not have seen sign complaints b/c authors copy/paste generated @font-face rules. Duplication is there
<fantasai> +1
<dael> myles: I understand we're discussing this bc adding rules. If we give them time to start using new things we'll hear noise about needing this. If not nec we won't
<dael> Rossen_: Hearing folks leaning toward waiting to see how and when it's needed
<dael> Rossen_: We'll put this on pause until have stronger signals