https://fonts.google.com/specimen/Lato?preview.text=Reaction...
EDIT: It's called ligatures: https://developer.mozilla.org/en-US/docs/Web/CSS/font-varian.... The CSS for headings on this site turns on some extra ligatures.
(So does `font-feature-settings: "dlig" 1`, which is the low-level equivalent; the site includes both.)
https://fonts.google.com/specimen/Lato?preview.text=Reaction...
EDIT: It's called ligatures: https://developer.mozilla.org/en-US/docs/Web/CSS/font-varian.... The CSS for headings on this site turns on some extra ligatures.