[css-fonts] font property descriptors for variable fonts #2485
Description
Re: https://drafts.csswg.org/css-fonts-4/#font-prop-desc
According to my reading of the current spec text, in particular:
If these descriptors are omitted, initial values are assumed.
Where a single value is specified, it has the same meaning as a range with identical startpoint and endpoint
variation values applied to fonts defined with '@font-face' will be clamped to [both] the values specified in these descriptors [...]
it seems that a "naïve" @font-face rule such as:
@font-face {
font-family: MyVariableFont;
src: url(fonts/MyVar.ttf);
}
will result in a face that has a font-weight/stretch/style of normal, and will not use variations to implement rendering for other values of these properties. (It might use synthetic bolding and/or obliquing, just like any other non-variable font face.)
This is what Chrome currently implements, AFAICT.
However, it's not what Safari implements; if MyVar.ttf in the above declaration has a 'wght' axis, my understanding (from a colleague's testing) is that Safari will use this to render arbitrary values of the font-weight property. Only if the @font-face rule includes an explicit font-weight descriptor will it clamp the variation values applied by the font-weight property to that range.
ISTM that the Safari behavior is perhaps more author-friendly, but it conflicts with the spec text as it currently stands, and I'm not sure if it makes the interaction of @font-face descriptors with font selection (as opposed to rendering) trickier to understand/define?
So should this be considered a Safari bug (and will it be fixed to conform to the spec?), or should the spec be revised?
A further question: where the spec says
variation values applied to fonts defined with '@font-face' will be clamped to [both] the values specified in these descriptors [...]
does this clamping apply to values specified with font-feature-settings, or only to values specified in the higher-level properties that correspond to these descriptors? On the face of it, I would understand the spec text to mean that all variation values for the axis corresponding to such a descriptor are clamped (regardless of whether they're specified in font-feature-settings or as a higher-level property like font-weight); but that does not appear to be the behavior of any current browser (and I'm not sure it would actually be desirable).