Bootstrap is the more common of the two in our index: it runs on 9,426 websites against 4 for Flutter, 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.
Flutter is an open source framework by Google for building beautiful, natively compiled, multi-platform applications from a single codebase.
1 site runs both Bootstrap and Flutter. That is 25% of the sites using Flutter, 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.