the CSS fonts spec has a couple (new to me) generic font families, like ui-serif and ui-sans-serif, aimed at providing finer-grained controls for specifying OS-level fonts. This allows developers the power to integrate their UIs with the look and feel of the underlying operating system. To suggest Apple user agents render text on screen with the “New York” serif font, developers can specify: font-family: ui-serif.
Information can often be divided into data and annotations. A web form needs a way to distinguish entry fields from labels; a graph needs not only labels for its x and y axes, but most crucially a verifiable reference for the source of its data. (#fakenews, I’m looking at you.) The most familiar and obvious way to establish this hierarchy is through type size, using palpably smaller type to distinguish the content from its notes. But at smaller-than-text sizes, even the most lucid typefaces can become difficult to read, their spacing overly tight, their counters congested, and their x-heights measly. Compare the tiny type in these two examples.