Cascading Style Sheets (CSS) is the styling language of the web, responsible for controlling the visual presentation of HTML documents. Introduced in 1996 and continuously evolved through modular specifications, CSS enables developers to separate content from presentation, creating responsive, accessible, and visually compelling user interfaces. While HTML provides structure and JavaScript adds interactivity, CSS bridges the gap between raw markup and polished design. The key mental model: CSS is a cascade — specificity, inheritance, and source order all matter, making understanding the cascade critical to avoiding frustration and writing maintainable styles.