I have a problem with kerning in Firefox. I decided to give the letters with vertical caron negative kerning against round x-height letters (which works as expected), and then I thought it would be nice to include a slight negative kerning against space. The thing is—when I rely on browser defaults for font-feature-settings, Firefox applies the kerning for the first case but ignores the case with space. When I explicitly turn on kerning, space is kerned with the caron letters correctly. Which means the kerning for my font in Firefox can be: on, off, or ‘just a bit off’
In Chrome the space is not kerned at all even when kerning is explicitly enabled.
Everything works fine in Word, InDesign, Edge (which doesn't turn kerning on by default, so no tri-state experiments are possible).
I suppose this has to do with space being a multiscript character?