**Definition:**
HTML (HyperText Markup Language) is the standard language used to create and structure content on the web, while CSS (Cascading Style Sheets) is used to style and layout that content.

**Key ideas:**
- HTML uses elements and tags to define different parts of a webpage, like headings, paragraphs, and links.
- CSS allows you to control the appearance of HTML elements, including colors, fonts, and spacing.
- Together, HTML and CSS form the foundation of web development, enabling the creation of visually appealing and organized websites.
- Learning HTML and CSS is essential for anyone interested in web design or development.

**Why this topic matters in this category:**
Understanding HTML and CSS is crucial for building and designing effective websites in the field of computer science.