Sentence case vs. title case

Two capitalization conventions for anything short: headings, buttons, menu items, form labels, table columns. Sentence case caps the first word, proper nouns, and anything else the language itself requires ('Create a new project'). Title case caps the major words and lowercases the small connectors ('Create a New Project'). Neither is ALL CAPS, which is a third thing and usually a styling choice.

Sentence case has quietly won in product UI: Google's Material, Shopify Polaris, Atlassian, and Microsoft's style guide all specify it, and Apple moved much of its own interface that way too. It reads faster, travels better across locales (English-style title case does not transfer reliably: German capitalizes all nouns whatever the context, and French title conventions differ from ours and from each other), and a writer can apply it correctly without a reference book. Title case still rules editorial headlines, book and film titles, and plenty of US marketing sites, where it reads as more formal and more deliberately 'set'.

Gotcha: title case has no single spec. AP, Chicago, APA, and MLA disagree about prepositions, 'vs', hyphenated compounds, and the word 'is', so 'consistent title case' costs someone a permanent policing job. Pick one authority in your style guide or pick sentence case. Second gotcha: if you fake caps with CSS text-transform, the underlying text is unchanged, which is good for screen readers and copy-paste, but it means your real capitalization is still whatever someone typed.