CSS

CSS (Cascading Style Sheets) is the language that controls how HTML elements look in the browser — colors, typography, spacing, layout, animation, and responsive behaviour. It was first proposed by Håkon Wium Lie in 1994, with CSS1 published by the W3C in 1996.

The cascade is CSS's central mechanism: rules from multiple sources (browser defaults, author stylesheets, inline styles) are applied in a defined priority order based on specificity and source order. This layering model, while powerful, is also the source of CSS's most common debugging challenges.

CSS3 marked a shift from a monolithic specification to a modular architecture. Each capability (Flexbox, Grid, Animations, Custom Properties, Container Queries) is now its own specification, developed and released independently. The result is a continuously evolving language with no single version number.

Flexbox and CSS Grid are the two primary modern layout systems. Flexbox handles one-dimensional layouts (row or column); Grid handles two-dimensional layouts. Together they replace most uses of floats, inline-block hacks, and table-based layouts.

CSS Custom Properties (CSS variables, e.g., --primary-color: #3b82f6) allow storing and reusing values throughout a stylesheet, enabling runtime theming and dynamic values that can be updated via JavaScript.

CSS Animations and Transitions provide declarative keyframe animations and smooth property transitions entirely in the browser's compositor thread — often producing smoother 60fps results than JavaScript-driven animations.

Container Queries (widely supported since 2023) allow elements to respond to the size of their containing element rather than the viewport, solving a key limitation of media-query-based responsive design.

CSS is always used alongside HTML and JavaScript. Tooling like PostCSS, Sass/SCSS, CSS Modules, and utility frameworks (Tailwind CSS) extend or abstract CSS for larger projects.