My issue is probably quite rare and not very serious, but I would like to optimize it nevertheless.
Not counting a few FontAwesome icons, I only use Open Sans on the site.
I set up Open Sans everywhere in Typography Options.
I thought everything was fine, until I tested the site in MS Edge.
I frequently use non-English characters, such as Č, Ř, Ě – and Edge inserted these letters from some funky font instead of Open Sans, which looked quite bad. Opera/Firefox don’t have this issue.
So, here is what I did: In GP Hooks I imported this to the head: