Modern CSS
new css, css baseline, css in 2025, modern css featuresDefinition
The shorthand for the CSS that arrived between about 2021 and 2025 and is now Baseline across browsers — container queries, cascade layers, :has(), subgrid, nesting, logical properties, relative color and color-mix(), clamp()-driven fluid type, the Popover API and <dialog>, scroll-driven animations, anchor positioning — the set that made most of a preprocessor and much of a component library’s JavaScript unnecessary
Not a version (CSS has no versions) but a moment: the point at which the platform caught up with what frameworks had been simulating. For a design system the consequences are direct — a component that responds to its container, a theme that is a cascade layer, a treatment that is a style query, a color scale that is a relative-color expression, a popover that is an attribute. This site’s themes, its :has() focus rings and its OKLCH palettes are already the modern half; the glossary entries linked here are the vocabulary for the rest, each with what it would replace.
The frameworks taught the platform what people needed; the platform learned. Modern CSS is the list of things you no longer need a library for — and, read the other way, the list of things a library is still doing that it could stop.