What is CSS?

Nearly every page you've ever seen in a browser is styled with CSS. It's one of the web's three core technologies, alongside HTML for structure and JavaScript for behavior. The W3C maintains it as an open standard. Browsers read CSS rules (plain text like p { color: navy; }) and apply them to the page's HTML elements.

What does "cascading" mean in CSS?

"Cascading" is the order a browser follows when several rules target the same element: its built-in defaults, the site's stylesheet, even the reader's own settings. The cascade ranks them by origin, then by how specific the selector is, then by source order. The browser applies the rule that wins. That's why a more specific selector beats a general one, and why a rule you add after a theme's stylesheet can override it.

How do you add CSS to a page?

There are three routes, as the diagram above shows. External stylesheets keep all your CSS in a separate .css file that any number of pages can link to. This is the standard approach: one file controls the whole site, and browsers can cache it. Internal styles live inside a <style> block in the page's HTML and only affect that page. Inline styles sit in a style attribute on a single element; they're best saved for quick tests and one-off exceptions.

What can CSS do today?

Far more than colors and fonts. Modern CSS handles page layout with flexbox and grid. Media queries adapt a design to any screen size. Transitions and animations run smoothly, no JavaScript needed. Custom properties (variables) let a site define its colors and spacing once and reuse them everywhere. Effects that once required scripts or images — gradients, shadows, rounded corners — are now a line of CSS each.