What is CSS?
Cascading Style Sheets, the language browsers use to style HTML: colors, typography, layout, animation, and responsive behavior.
Also known as: Cascading Style Sheets
CSS pairs selectors (.button, nav > a, body[data-theme="dark"]) with declarations (color: #2563eb; padding: 8px 16px;). The cascade decides which rule wins when several apply: specificity, source order, and !important all factor in. Layout primitives like Flexbox and Grid replaced the float-and-clear hacks that dominated CSS for a decade.
Modern CSS includes custom properties (CSS variables: --primary: #2563eb;), container queries (style based on parent size, not viewport), :has() (parent-selector logic), nesting, color functions (oklch, color-mix), and animations that no longer need JavaScript. Most of these features ship in evergreen browsers without polyfills now.
For brand intelligence, CSS is a goldmine. Inline <style> tags and linked stylesheets carry the actual color palette a site uses, often expressed as variables that are easy to recover. Font stacks reveal which web fonts the brand has chosen. Extracting these is one of the cleanest ways to recover a styleguide from a public site.
In the wild
- Reading
:root { --color-primary: #635bff; }from a site's CSS to recover its primary brand color - A design audit pulling every unique color value declared across a stylesheet to spot drift
- A responsive layout that switches from a single column to a three-column grid using a container query