As of 2023, the most popular CSS frameworks are Bootstrap, Foundation, Bulma, Tailwind CSS, and Materialize.
Bootstrap:
Created by Twitter, it is an open-source framework that is widely used for responsive, mobile-first web development.
Foundation:
It is an open-source CSS framework that is widely used for responsive, mobile-first web development.
Bulma:
It is a modern CSS framework based on Flexbox, which makes it easy to create responsive and flexible layout.
Tailwind CSS:
A utility-first CSS framework that is designed to be highly customizable and easy to use.
Materialize:
A CSS framework that is based on Google’s Material Design guidelines. It is widely used for creating responsive and modern-looking websites.
All of the above-mentioned frameworks are open-source and have a large community of developers who contribute to them and provide support.