Glossary · Internet & Web

CSS (Cascading Style Sheets)

CSS is the language that controls the visual presentation of HTML, including layout, colors, fonts and spacing across screen sizes.

Updated October 6, 2026

CSS (Cascading Style Sheets) is a stylesheet language that describes how HTML elements should look. It sets colors, fonts, spacing, layout and responsive behavior.

Why it matters

Separating style from structure lets one stylesheet control hundreds of pages. CSS is also render-blocking: the browser waits for it before painting, so large or slow stylesheets delay what visitors see and affect performance measures such as Largest Contentful Paint.

Inlining a small amount of critical CSS and loading the rest asynchronously is a common technique to speed up first paint on slow connections.

Example

body { font-family: system-ui, sans-serif; }
.button { background: #0a66c2; color: #fff; padding: 0.5rem 1rem; }
@media (max-width: 600px) { .button { width: 100%; } }

The media query changes the button on narrow screens.

Common mistakes

  • Shipping one huge stylesheet with unused rules, slowing the first paint.
  • Using !important to win specificity fights instead of organizing selectors.
  • Forgetting to set a viewport and testing only at desktop width.

Get notified when this term updates

Plus occasional practical tips on uptime and monitoring, nothing else. Unsubscribe anytime.

Keep reading

Glossary · SEO & MarketingCore Web VitalsCore Web Vitals are Google's page experience metrics, LCP, INP and CLS, that measure loading speed, responsiveness and visual stability for real users.Glossary · Web DevelopmentCritical CSSCritical CSS is the minimal set of styles needed to render the visible part of a page, inlined in the HTML so content appears without waiting for stylesheets.Glossary · PerformanceFCP (First Contentful Paint)FCP is the time from navigation start until the browser renders the first piece of DOM content, such as text, an image or a non-white canvas.Guide · PerformanceCore Web Vitals Guide: Improve LCP, INP and CLS Step by StepLearn what LCP, INP and CLS measure, how lab and field data differ, and the practical fixes for images, fonts, JavaScript and layout shifts.9 min readGuide · PerformanceHow to Check Your Website Speed for Free (and What to Fix First)Test your website speed with free tools, understand the numbers in plain language, and fix the biggest problems first: images, hosting, scripts and caching.7 min readGuide · PerformanceImage Optimization Guide: WebP, AVIF, Sizing, srcset, Lazy LoadingChoose between JPEG, PNG, WebP and AVIF, size images correctly, use srcset and lazy loading, and batch convert with cwebp, avifenc and sharp.6 min read

This page is general information written by our editorial team and updated when documentation changes. Servers and providers differ, so test changes safely first. See our editorial policy and disclaimer.