Safari/WebKit not rendering glyph substitution (GSUB) correctly
Move the slider to change Slant. The example letters below should shift to a "single story" version when Slant is greater than -7.5.
As of Nov 1, 2019, this test fails in:
- Safari on macOS, Version 12.1.1 (14607.2.6.1.1)
- Safari Technology Preview, Release 95 (Safari 13.1, WebKit 14609.1.7)
- Safari in iOS 13.1.3
ag
GSUB working with multiple characters
a
GSUB *not* working with single character
a
One weird trick: add 'ZERO WIDTH SPACE' (HTML ​) before or after the glyph
(Obviously, this shouldn't be necessary, but is a useful hack until this bug is fixed in production Safari on iOS & macOS.
What is GSUB?
Glyph substitution (GSUB) in variable fonts occurs when a character's shape is changed in a non-compatible way. Put simply, GSUB is when shape changes dramatically, rather than gradually, along an axis.
One example of this is the "Q" in Skia: its form simplifies in the bolder weights to retain readability.
For a full description of this, see the OpenType spec: tag: rvrn.