OpenType features

They are stored as four-letter tags — smcp for small capitals, onum for old-style figures, tnum for tabular figures, ss01 for a stylistic set — and turned on in CSS with font-feature-settings or, better, with the higher-level properties like font-variant-numeric that browsers understand without you memorising the tags.

Two of them are worth reaching for on almost any project. Tabular figures make every digit the same width, which is the difference between a column of numbers that lines up and one that does not. Old-style figures sit at x-height with ascenders and descenders like lowercase letters, which stops a date in the middle of a sentence from shouting.

A feature only works if the font contains it. Asking for small capitals from a font with none gets you nothing, or worse, gets you the browser shrinking the capitals for you — which is not the same thing and looks it.