UX Glossary · C
CSS
Also: Cascading Style Sheets
The language that controls how HTML is presented: layout, colour, type, spacing and motion.
CSS describes how HTML should be presented. Håkon Wium Lie proposed it in 1994 while at CERN, and the W3C published CSS Level 1 as a recommendation in 1996. The “cascading” part is the rule for resolving conflicts when several rules target the same element.
Why it is worth a designer’s attention
CSS has changed enough in the last few years that design decisions once considered expensive are now trivial, and knowing which is which changes what you propose.
Grid and flexbox made complex layout straightforward. Custom properties made
design tokens native, so a colour can be defined once and
referenced everywhere. clamp() makes type scale fluidly between sizes without a stack of
breakpoints. Container queries let a component respond to its own container rather than the
viewport. :has() allows styling a parent based on its contents.
In practice
The practical value is in specifying things that are cheap and avoiding things that are not. Asking for a value that is already a token costs nothing. Asking for a layout that fights the grid model costs a lot, and usually produces something that breaks at a width nobody tested.
It is also where accessibility is won or lost: focus indicators, contrast, and honouring
prefers-reduced-motion are all CSS decisions.