What is a CSS framework?

A CSS framework is fundamentally a collection of CSS styles and components that are pre-defined, allowing developers to create web pages more efficiently. This framework typically includes a grid system, typography rules, colors, button styles, and other UI elements that streamline the design process. By using a CSS framework, developers can save time and maintain consistency in their designs, as they need not rewrite common styles from scratch for each project.

Using a pre-prepared library of styles means that developers can focus on the functionality of a website and the unique features of their design, rather than spending excessive time on the foundational styling. Frameworks often come with responsive design capabilities, making it easier to create web applications that look good on various screen sizes.

Other options don't align with the definition of a CSS framework. Debugging CSS and optimizing website performance relate to different aspects of web development, while a guide for writing HTML code does not pertain to CSS at all. Hence, the focus on predefined styles and components makes this choice the most accurate definition of a CSS framework.