[css-fonts] Improve implementation guidance for font-kerning #7958
Description
The CSS Fonts spec says that when kerning is enabled (i.e. with font-kerning: normal, or with the initial auto value "at the discretion of the user agent"), the OpenType 'kern' (horizontal) or 'vkrn' (vertical) feature should be applied.
In https://bugzilla.mozilla.org/show_bug.cgi?id=1797431, the reporter points out that the OpenType feature registry says that
If 'kern' is activated, 'palt' must also be activated if it exists.
Similarly, 'vpal' is supposed to be applied along with 'vkrn'.
Current browser engines appear to have overlooked this, and by default apply 'kern' but not 'palt' in the reporter's testcase.
I'd suggest it would be helpful to mention these associated features in the CSS Fonts description of how kerning is implemented when using OpenType fonts.