CSS (Cascading Style Sheets) is the language that defines how web pages look: colors, fonts, spacing, layout, animations. HTML describes the content and its structure; CSS decides how it is displayed.
The "cascade" principle means that several rules can apply to the same element, with the most specific one winning. It is powerful, but a source of conflicts on large projects if the code is not organized.
Modern CSS makes complex layouts possible with Flexbox and Grid, and adapts to every screen size through media queries (responsive design). To stay organized, teams use naming conventions, preprocessors such as Sass, utility frameworks such as Tailwind CSS or styles attached to components. CSS is maintained by the W3C and evolves continuously.
Why it matters when hiring
CSS is often underestimated in front-end hiring, even though a polished, accessible and fast interface depends on it. In an interview, a good signal is the ability to explain Flexbox versus Grid, specificity or how to avoid one change breaking another page. For a role focused on the interface and design system, check this as much as JavaScript.