It changes different settings in the font. In GitHub's case, they use the font "Alliance No.1". ss02 doesn't appear to have a visible effect, but ss01 changes the letter a from a rounded body with a stem on top to a rounded body with a straight stem on the right. You may have custom fonts disabled—these feature settings do the right thing on GitHub's and ProtonMail's ("Arizona") fonts, but make your system fonts do weird things. Both sites appear just fine for me on Firefox on Linux.