CSS libraries are pre-written collections of CSS code that developers and designers can use to quickly and easily style web projects, significantly improving efficiency and maintainability. These libraries address common challenges in CSS, such as repetitive styling, unorganized code, and cross-browser compatibility issues, by providing reusable components for elements like buttons, forms, and navigation bars. As CSS is used by 97.3% of websites, these libraries are crucial for creating visually appealing, responsive, and user-friendly websites. The text highlights 20 CSS libraries, each offering unique features and use cases, from animations with Animate.css to retro UIs with 98.css and responsive layouts with Pure CSS. It also discusses key criteria for selecting CSS libraries, such as performance, compatibility, ease of use, and customization, emphasizing the importance of choosing the right library to streamline workflows and deliver high-quality web experiences.