Both are formatting languages/technologies, and in the early years of the web, HTML was the only language calling all the shots. But the wise people behind the web scenes decided to keep the visual formatting separated from the content, semantics, and structure. Therefore CSS was implemented.

So the HTML describes the content and its structure, and CSS will describe its appearance.

Oorspronkelijk geplaatst op Quora: Why do we need CSS? Why not just use HTML?"