Typography is the atomic element of good interface design as it defines branding, layout, composition & rhythm. Note that it’s not just about choosing the right typeface, it’s also about tone of voice, microcopy. Head over to IBM Design Language for an in-depth overview on the elements of typography.
Good reading experiences have the right amount of characters per line. If a line is too long users will have a hard time focusing, if a line is too short the eye will have to travel back too often, breaking the reader’s rhythm. An optimal line length is 50-75 characters, which is about 10-12 words or 25-33 × base font-size (1em). To increase readability of typographic elements, default letter-spacing values for headings & small text are adjusted.