Bug 200369 – Slanted text in variable fonts is not rendered correctly if font-style descriptor defines an upper angle that is less than 20deg

• Component: CSS
• Status: NEW

[Šime Vidas]

Steps to reproduce:

1. Open https://jsbin.com/dabetaq/edit?css,output

Expected results:

All three boxes should have the same, correct text slant.

Actual results:

In Safari, the slant is incorrect for the second and third box.

Explanation:

It appears that font-style: oblique only renders the correct slant if the upper angle in the font-style descriptor is at least 20deg in Safari. But the variable font has a 'slnt' axis range of 0 to 10*, which translates to the CSS range 0dge 10deg.

*You can check the font file’s 'slnt' range on wakamaifondue.com.

If you change this range to 0deg 20deg, Safari renders the slant correctly. Safari should let authors specify the variable font’s actual slant range. Currently, authors have to use 20deg as some sort of magic number to get correct rendering in Safari.

Note: I’m not an expert on variable fonts, but the range 0deg 10deg works in Chrome and Safari, so this seems to be a bug in Safari.

[Radar WebKit Bug Importer]

<rdar://problem/53849550>

[Stephen Nixon]

I think that the real problem may be that WebKit is applying a “faux-slant” for variable fonts when styled with font-style:oblique or font-style:italic.

I have made a test page showing the issue in a way that I think is more clear, overlapping type to show the different results:

https://arrowtype.github.io/vf-slnt-test/faux-slnt.html

[Stephen Nixon]

It is additionally worth noting that Chrome and Firefox also don't render the slnt axis correctly, and I'm hoping that WebKit doesn't make the same mistakes when fixing this issue (the differences between the OpenType & CSS specs are slightly unintuitive to most people, at first, so it's easy to make mistakes here). 

Please see https://bugs.chromium.org/p/chromium/issues/detail?id=859869 for more details.

I have made a test page for font Slant in general at:

https://arrowtype.github.io/vf-slnt-test/

[Stephen Nixon]

Created attachment 389249 [details]
Test website for faux-slant versus true Slant

Here's a test case for this issue:

https://arrowtype.github.io/vf-slnt-test/faux-slnt.html