Perhaps nothing has more of an effect on the appearance of typography than the size of type used, and nothing has a greater effect on the hierarchical organization of information than the number of sizes of type used.
Before digital typography, typeface families consisted of a font for each size. A form of trust, rigid as it was, existed between the maker of type and its users, promising fonts guaranteed to produce good results under reasonable conditions at the size they were made for, until they wore out. The trust was also based on the fact that type developers knew how to scale type up and down nonlinearly to preserve readability at the bottom end of the size of use, and to more efficiently conserve space as the size of use went up.
Digital type, by the the mid-1990s, had seen Postscript introduce the concept of any font being scalable to any size and resolution. But the professional market embraced custom fonts for particular sizes or ranges of use, and Microsoft’s release of Verdana was a semi-secretly 8pt design for better screen reading at small sizes. Not much else was possible in distinguishing size of use with the font format, since it only recorded a minimum recommended size hardly any application paid attention to.
From the time the web launched, to even after downloadable webfonts became possible and today, web typography has been restricted to fonts that were easy to use at all sizes and practical to deploy. And as size-independent type became entrenched, text fonts, display fonts, and even logos have migrated to fonts that are easy to use at all sizes and practical to deploy. The typographic term for this is “sans serif,” in particular, sans-serif designs with no real distinguishing features.