6.3. Kerning: the font-kerning property
https://drafts.csswg.org/css-fonts-4/#font-kerning-prop

When kerning is enabled, the OpenType kern feature is enabled (for vertical text runs the vkrn feature is enabled instead).

I'm guessing that the vkrn feature is only applied to upright characters in a line of vertically-set text, whereas Latin text (say) that runs down the line on its side is not affected. Is this taken care of by the font only having vkrn settings for upright characters in vertical lines, or do we have an issue here?