There are many popular CSS frameworks that are widely used for building responsive, mobile-first websites and web applications. Some of the most popular CSS frameworks include:

1. Bootstrap: Bootstrap is the most widely used CSS framework, with a large user base and a strong ecosystem of themes, templates, and plugins. It provides a set of pre-designed UI components and layout tools that make it easy to create professional-looking interfaces with minimal effort.

2. Foundation: Foundation is a responsive CSS framework that provides a set of layout and UI components for building mobile-first websites and applications. It is known for its flexible grid system and responsive design features.

3. Materialize: Materialize is a CSS framework based on Google’s Material Design guidelines. It provides a set of UI components and layout tools that are designed to look and feel modern and consistent across different devices and platforms.

4. Bulma: Bulma is a lightweight and modern CSS framework that uses a flexbox-based grid system and provides a set of responsive UI components. It is known for its simplicity and ease of use.

5. Semantic UI: Semantic UI is a CSS framework that provides a set of UI components and layout tools that are designed to be semantic and intuitive. It is known for its large collection of themes and its flexible design

---

There are many popular CSS frameworks to choose from, and the most suitable one for a particular project will depend on the specific requirements and preferences of the developer. Some of the most popular CSS frameworks include:

1. Bootstrap: Bootstrap is a widely-used, open-source CSS framework that provides a set of predefined styles for creating responsive, mobile-first layouts. It is one of the most popular CSS frameworks, with a large and active community of developers.

2. Foundation: Foundation is a responsive CSS framework that provides a range of styles and layout options for building web applications. It is designed to work well on a variety of devices, including mobile phones and tablets.

3. Bulma: Bulma is a modern CSS framework based on Flexbox that provides a set of styles and layout options for creating responsive, mobile-first layouts. It is designed to be easy to use and customize, with a focus on simplicity and clean design.

4. Semantic UI: Semantic UI is a CSS framework that provides a range of styles and layout options for building web applications. It is designed to be easy to use and customize, with a focus on providing intuitive and human-friendly HTML class names.

5. Tailwind CSS: Tailwind CSS is a utility-first CSS framework that provides a large set of utility classes for styling HTML elements. It is designed to be easy to use and customize, with a focus on providing a wide range of styling options without the need for complex design systems.

These are just a few examples of the many CSS frameworks that are available. There are many other frameworks to choose from, and the right choice will depend on your specific needs and preferences.