CSS

Cascading Style Sheets — the presentation language of the web: rules that select elements and declare how they look and lay out, resolved by a cascade that decides which declaration wins when several apply

Maintained by the W3C CSS Working Group as a set of modules rather than one spec. The cascade is the part people fight and the part that makes a design system possible: custom properties carry tokens, cascade layers order the sources, container queries let a component respond to its own space, and OKLCH describes color the way the eye does. This site's nine themes are CSS alone — the same HTML under a different data-theme.