CSS controls how webpage content looks and how it is arranged. It is like the paint, wallpaper, and furniture layout added after a house has its basic frame.
CSS can set text color, space between cards, or where a picture sits. The same page structure can look different on a phone and a large monitor because the styles can adjust.
CSS changes appearance rather than replacing the words and structure themselves. Keeping that separation can make a site easier to restyle and read with assistive tools.