Definition

What Is CSS and What Does It Do?

A stylesheet language used to describe the visual presentation of HTML documents — controlling layout, colours, fonts, spacing, animations, and responsive behaviour.

Related Terms

HTML

Framework

Grid System

Responsive Design

Above the Fold

Example

CSS grid and flexbox are used to create a responsive three-column card layout that collapses to one column on mobile.

CSS is used by 97% of all websites, making it — alongside HTML — one of the two universal technologies of the web, according to W3Techs.

Synonyms:

Cascading Style Sheets, Styling language

Acronyms:

CSS

What it applies to

Every element visible on a webpage. CSS controls layout, colour, typography, spacing, animation, and responsive behaviour across all browsers and devices.

How CSS works

  • CSS rules consist of a selector (which HTML element to target) and a declaration block (the properties and values to apply): h1 { color: #1A1A2E; font-size: 48px; }
  • The "cascade" determines which rule wins when multiple rules target the same element — based on specificity, source order, and importance.
  • CSS is linked to HTML via a <link> tag in the page's <head>, or written inline using a <style> tag.

Key CSS concepts for web projects

  • Flexbox: One-dimensional layout — aligns items in a row or column. Used for navigation bars, card rows, and centring content.
  • CSS Grid: Two-dimensional layout — controls both rows and columns simultaneously. Used for page layouts, feature grids, and complex compositions.
  • Custom Properties (Variables): Define brand values once (--colour-primary: #4361EE) and reuse them throughout — making global updates instant.
  • Media Queries: Apply different styles at different viewport widths — the foundation of responsive design.

In Webflow

Webflow generates CSS automatically from the visual Designer. Understanding CSS concepts — specificity, the box model, flexbox — makes Webflow significantly more powerful and helps debug unexpected styling behaviour.

CSS: Common Questions Answered

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