CSS

CSS (Cascading Style Sheets) is the W3C standard style-sheet language that describes how documents written in HTML and other markup should look - its layout, color, typography, spacing, and responsive behavior. Its core idea is separation of concerns: markup carries content and structure while CSS carries presentation, so the same document can be restyled without touching its meaning. Modern CSS is developed as a large family of modular specifications rather than a single monolithic document.

• The cascade - Rules from author, user, and browser stylesheets combine and resolve by specificity and order, which is where the “cascading” comes from.
• Selectors - Patterns that target elements by tag, class, id, attribute, state, or relationship so styles apply exactly where intended.
• The box model - Every element is a box of content, padding, border, and margin, the foundation of all layout.
• Layout and responsiveness - Flexbox, Grid, and media queries let a single stylesheet adapt to any screen or device.

In an API context CSS is less about the wire format and more about the human surfaces around an API - documentation portals, developer dashboards, interactive reference explorers, and status pages all lean on it for a usable, on-brand developer experience. It rarely touches the API contract itself, but it heavily shapes how approachable and trustworthy a provider’s developer area feels, which matters more to adoption than teams often admit.