Bootstrap

https://getbootstrap.com/

Snippet from Wikipedia: Bootstrap (front-end framework)

Bootstrap is a free and open-source CSS framework directed at responsive, mobile-first front-end web development. It contains CSS- and (optionally) JavaScript-based design templates for typography, forms, buttons, navigation, and other interface components.

As of April 2021, Bootstrap is the tenth most starred project on GitHub, with more than 150,000 stars, behind freeCodeCamp (almost 312,000 stars), Vue.js framework, React library, TensorFlow and others.