CSS

CSS, short for Cascading Style Sheets, is the code that controls how a web page looks: its layout, color, spacing, and typography. Where HTML provides the structure of a page, CSS provides the presentation layered on top of it.

Keeping CSS separate from HTML allows for adjustment to design and interactions without touching the struture of the site or the content. It is also where responsive design lives, since CSS is what reshapes a layout at each breakpoint to fit different screens.