Sentence case

Sentence case is the rule of ordinary prose applied to interface text: the first word takes a capital, proper nouns keep theirs, everything else stays down. “Export as PDF” is sentence case, because PDF is an initialism and not a decision the writer made, and so is “Mentions in Harbour”, because Harbour is a name. The rule is grammatical rather than mechanical, which is exactly the part CSS cannot do for you: text-transform: capitalize raises every word including the ones that should stay down, and lowercase flattens the names that earned their capitals. Sentence case is written into the string, never transformed onto it.

Most of the large systems now ask for it. Material calls it sentence-style capitalisation and applies it to buttons, dialog titles and menu items; GOV.UK writes it into its content style guide; Carbon asks for it in headings, labels and body copy alike. The arguments they give are consistent. Lowercase letters carry more silhouette, so a scanned label resolves faster, which is the same shape argument that makes all caps slow to read. Capitals in the middle of a line are a signal that a proper noun is arriving, and spending them on ordinary words spends the signal. And sentence case survives translation: French, Spanish and most other languages have no title-case convention at all, so a string written this way needs no separate decision in every locale it reaches.

Title case is the alternative, and between the two the difference is tone rather than correctness. What is not optional is picking one. Case is a system decision applied to every string at once, because a screen where half the headings capitalise their nouns and half do not reads as unfinished rather than as varied, and reviewers will spend their attention on the inconsistency instead of on the work. Write the rule down where the copy is written, not where it is styled.

The arguments that remain are about what counts as a name. A feature is a proper noun only when the organisation has genuinely made it one, and most have not, so “turn on dark mode” beats “turn on Dark Mode” in almost every product that ships the sentence. The same rule runs through the smaller places people forget: column headings in a table, the heading of an empty state, the first word after a colon, and the labels on a segmented control are all sentences of one or two words, and they get the capital the first word of a sentence gets, and no others.