Comment by philsnow

There are even checkboxes on the page to simultaneously disable ligatures and enable texture healing. They're just controlling whether various subfeatures of font-feature-settings are off/on.

I have a bunch of font-feature-settings subfeatures turned off explicitly in my userChrome.css, so it took me a little while to realize that that was what was messing with this site. I couldn't figure out what texture healing was supposed to do because toggling it wasn't helping, but it was because of my userChrome.