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.
Tailwind CSS is a utility-first CSS framework for rapidly building custom user interfaces. It is a highly customizable, low-level CSS framework that gives you all of the building blocks you need to build bespoke designs without any annoying opinionated styles you have to fight to override.
Bulma is a free, open source CSS framework based on Flexbox and used by more than 100,000 developers.
Materialize is a modern responsive CSS framework based on Material Design by Google. It is a collection of CSS and JavaScript components.
Pico is a lightweight, mobile-first, responsive CSS framework. It is a collection of CSS and JavaScript components.