Definition
CSS, short for Cascading Style Sheets, is the language that controls how a web page looks, including colors, fonts, spacing, and layout. HTML provides the structure of a page, and CSS decides how that structure is presented to the visitor. It lets one set of style rules shape an entire site at once.
Use Cases
CSS is used to make sites readable and attractive, to adapt layouts for phones and desktops, and to add animation and visual polish. It also controls which parts of a page appear first, which affects how fast a page feels to load.
Why It Matters for SEO
CSS matters for SEO because heavy or poorly ordered styles can block a page from showing content quickly, which hurts page speed and Core Web Vitals. The browser often has to read CSS before it can paint the page, so trimming and ordering it well speeds up HTML rendering. Layout shifts caused by late loading styles can also frustrate visitors and lower scores.
Related Concepts
Related concepts include HTML rendering, server side rendering, and render-blocking resources. Each describes part of the journey from raw code to a finished page that both people and crawlers can read.