What are CSS preprocessors?

CSS preprocessors are advanced scripting tools that extend the capabilities of standard CSS by allowing the use of features such as variables, nesting, functions, and mixins. This enhances workflow and maintainability by enabling developers to write cleaner, more modular CSS. For example, using variables allows a developer to define a color or style once and then reuse it throughout the stylesheet, making it easier to update and manage styles consistently.

While the other options describe important aspects of web development, they do not accurately capture the defining characteristics of CSS preprocessors. For instance, minimizing CSS files is an optimization process but does not relate to the foundational structure or functionality enhancements that preprocessors offer. Converting CSS to HTML is not something preprocessors do, as CSS and HTML serve different purposes—CSS styles content, while HTML structures it. Lastly, validating CSS is crucial for quality assurance, but it involves checking for errors in the code rather than enhancing its capabilities through additional features.