# What is CSS? — CSS Fundamentals: Layout, Flexbox, Grid and Responsive Design Source: https://www.skillbyai.com/en/css/css-intro > Learn what CSS is and the three ways to attach styles to HTML: inline, internal and external stylesheets. ## CSS in one line **CSS (Cascading Style Sheets)** describes how HTML elements look — color, spacing, layout, fonts, motion. HTML gives structure; CSS gives appearance. ## Three ways to add CSS **Inline** (`style="..."` on a tag), **internal** (a ` ``` ## Prefer external External stylesheets are cached by the browser and reused across pages — inline styles are the hardest to maintain and override.