**Definition:**
CSS, or Cascading Style Sheets, is a language used to style and layout web pages. It controls the appearance of HTML elements, such as colors, fonts, and spacing.

**Key ideas:**
- CSS separates content from design, making it easier to manage and update styles.
- It allows for responsive design, adapting layouts to different screen sizes.
- CSS uses selectors to apply styles to specific HTML elements.
- Styles can be applied inline, internally, or externally through linked stylesheets.

**Why this topic matters in this category:**
Understanding CSS is essential for creating visually appealing and user-friendly websites in computer science.