[css-fonts-4] Clarify meanings of variation range values in @font-face property descriptors

[drott]

In the font property descriptors section, which describes style, weight, and stretch font-stretch has a notion of percentage and percentage range.

I may be missing something, but I believe we should clarify what a percentage corresponds to. Does 100% correspond to normal stretch and what percentage ranges are permitted. Are percentages > 100% allowed? How are these mapped to the 'wdth' axis?

Similarly what should be allowed values for angle and number for style and weight respectively? Or if we don't restrict those with intervals up front, what should be the mapping, including out-of-range values that cannot be mapped to variation axis parameters for the font?

CC @litherum @PeterCon

[drott]

I assume the intention is to map these to the registered Variation Axis Tags, described on https://www.microsoft.com/typography/otspec/fvar.htm

So "Style" would be matching to 'slnt' or 'ital'?
"Weight" matching to 'wght'?
"Stretch" matching to 'wdth'?

So that would mean percentage values can be larger than 100%, and 100% would take on the same meaning as in this part of the OpenType spec, referring to "whatever the font designer considers “normal” for that font design".

"Weight" taking values from 1 to 1000, where the normal value would be 400, according to OpenType.

How do we map "Style"?

[drott]

Okay, I missed that there is already more information on those in sections 2.2 to 2.4 which we should perhaps reference here.

[drott]

Now I found: "If specified values are out of range of the accepted values of the property of the same name, the descriptor is treated as a parse error." - that's what I was looking for.

[litherum]

The mapping for GX fonts is not straightforward; I had to study a font with many different weights to come up with a mapping (since these fonts use a different scale than CSS).

You have to remember whether “italic” or “oblique” was specified to font-style to make sure you trigger the correct axis.

[litherum]

(But you’re right about the mapping for non-GX fonts.)

[PeterCon]

I don't understand the references to percentage and percentage range for font-style and font-stretch in 4.4 since 2.2 and 2.4 indicate that percentages are "n/a" for those properties.

Also, in 2.3 it says "Percentages: Not resolved." It's not clear to me what that means. In the linked CSS Values and Units spec, it only offers (in 8.1.3), "Where percentages are not resolved at computed-value time, they are not resoved in 'calc()' expressions...". Perhaps a little clarification in Fonts/2.3 would help?

[litherum]

I don't understand the references to percentage and percentage range for font-style

Where do you see percentages mentioned with regards to font-style in 4.4? Font-style doesn't accept percentages; only font-stretch does. These percentages are not resolved against anything; they remain as percentages and used directly when performing font selection and variation axis application. Section 2.3 mentions that these percentages are "not resolved," which is correct.

Because font-weight and font-style don't accept percentages, they are marked n/a.

Maybe I'm not understanding your question?

[PeterCon]

I don't understand the references to percentage and percentage range for font-style

Sorry, pre-coffee remark; please ignore.