Core interface elements.
No opinions about anything else.
Some CSS frameworks come prepackaged with all sorts of components that are convenient at first but quickly become limiting. Utility frameworks like Tailwind are awesome, but can be difficult start using on their own.
a17t tries to get the balance right. Instead of providing all-inclusive, opinionated components (like jumbotrons, navbars, and menus), a17t provides common single-class elements in a default (but easily customizable) style.