Bootstrap is the more common of the two in our index: it runs on 9,990 websites against 96 for UnoCSS, and 1 sites use both. Counts come from real scans, not vendor claims.
Bootstrap is a free and open-source CSS framework directed at responsive, mobile-first front-end web development. It contains CSS and JavaScript-based design templates for typography, forms, buttons, navigation, and other interface components.
UnoCSS is instant on-demand Atomic CSS engine.
1 site runs both Bootstrap and UnoCSS. That is 1% of the sites using UnoCSS, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.