Modern css

Modern CSS

When writing or reviewing CSS, prefer modern CSS features over legacy hacks, JavaScript workarounds, or library dependencies.

How to Use

Read individual rule files in the rules/ directory for detailed before/after code examples:

rules/layout-inset.md
rules/color-oklch.md
rules/selector-has.md

Each rule file contains a before/after comparison and browser support percentage. See rules/_sections.md for category definitions and ordering.

Treat bundled support percentages as snapshots, not current compatibility guarantees. Check the project's Browserslist or supported-browser policy first. For newly available or experimental features, verify current support in primary browser documentation before recommending production use.

Rule Categories

Priority	Category	Prefix	Support
1	Layout & Positioning	layout-	90%+
2	Colors & Theming	color-	83-96%
3	Typography & Fonts	typography-	79-96%
4	Selectors & Specificity	selector-	84-96%
5	Transitions & Animation	animation-	69-92%
6	Interactive Components	component-	72-96%
7	Performance & Rendering	performance-	90-96%
8	Forms	form-	73-93%
9	Experimental	experimental-	< 70%

Core Principles

1. CSS over JS — If a CSS property can replace JavaScript (scroll spy, sticky detection, height animation, IntersectionObserver), use it.
2. Native over libraries — Prefer native <dialog>, popover, anchor-name, scroll-snap over Popper.js, Floating UI, Swiper, etc.
3. Logical properties — Use margin-inline-start, padding-block-end, inset instead of directional properties (left/right/top/bottom).
4. oklch for color — Use oklch() for perceptually uniform colors. Use oklch(from ...) for tint/shade variants instead of Sass lighten()/darken().
5. Layers for specificity — Use @layer to control specificity instead of !important escalation.
6. Container queries over media queries — Use @container for component-level responsiveness.

Quick Reference — Use These (90%+ Support)

Instead of...	Use
position: absolute; top:0; right:0; bottom:0; left:0	inset: 0
display:grid; justify:center; align:center (verbose)	place-items: center
calc(100% - padding) width hacks	width: stretch
Padding-top aspect ratio hack	aspect-ratio: 16/9
JS scroll listeners for sticky	position: sticky; top: 0
Margin + :last-child override	gap: 16px
Sass $variables	var(--custom-property)
Media query breakpoint ladder	clamp(1rem, 2.5vw, 2rem)
Manual <br> for headline wrapping	text-wrap: balance
Sass nesting (build step)	Native CSS nesting & a { }
:focus (shows on click)	:focus-visible
Complex selectors for resets	:where() (zero specificity)
Repeating selectors	:is(h1, h2, h3) grouping
.parent.has-child class toggle	:has() parent selector
Background-image cover hack	object-fit: cover
overflow-y: scroll for shift	scrollbar-gutter: stable
JS preventDefault on scroll	overscroll-behavior: contain
JS IntersectionObserver for lazy	content-visibility: auto
Separate @font-face per weight	Variable font font-weight: 100 900
JS for dark mode form controls	color-scheme: light dark
Duplicated dark mode variables	light-dark(#111, #eee)
appearance: none + custom styles	accent-color: #7c3aed
Separate transform shorthand rewrite	translate, rotate, scale individually
JS classList.toggle for display	transition-behavior: allow-discrete + @starting-style
Sass mix()	color-mix(in oklch, color1, color2)

Support Tiers

Widely Available (90%+) — Use freely

oklch colors, backdrop-filter, scrollbar-gutter, overscroll-behavior, object-fit, inset, content-visibility, accent-color, aspect-ratio, sticky, gap, clamp(), :is(), :where(), :has(), :focus-visible, CSS variables, native nesting, @container, color-scheme, font-display, variable fonts, grid-template-areas, dialog element, scroll-snap, line-clamp, initial-letter, independent transforms, @property, @layer, logical properties, place-items, stretch, customizable select

Newly Available (80-89%) — Use with confidence

light-dark(), color-mix(), text-wrap: balance, subgrid, @starting-style, transition-behavior: allow-discrete, view transitions, :user-invalid/:user-valid, popover, anchor positioning, @scope, scroll-linked animations

Limited Support (< 80%) — Use progressively or check project targets

field-sizing, interpolate-size, scroll-state(), attr() typed, if() conditions, CSS functions, corner-shape, sibling-index(), scroll-button/scroll-marker, text-box