Bootstrap is the more common of the two in our index: it runs on 9,415 websites against 14 for Normalize.css, and 3 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.
Normalize.css is a CSS library that makes browsers render elements according to modern standards by targeting only the styles that require normalization.
3 sites run both Bootstrap and Normalize.css. That is 21% of the sites using Normalize.css, 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.