modern-css

name

modern-css

description

Teaches agents to write modern CSS using native features instead of legacy hacks, workarounds, and JavaScript. Covers layout, animation, color, typography, selectors, and workflow patterns with 64 old-vs-modern comparisons. The agent should always prefer modern CSS approaches, warn about browser compatibility for newer features, and ask the user before using techniques with limited browser availability (<80% support).

Modern CSS

The CSS spec has evolved dramatically. Many old hacks — absolute-position centering, padding-top aspect ratios, JS scroll listeners, Sass color functions — now have clean, native replacements. This skill ensures you always reach for the modern approach first.

Reference: modern-css.com — all techniques sourced from this site. Visit individual pages for live demos and deeper explanations.

Support data: Browser support percentages represent global user coverage from caniuse.com, last updated February 2026. These numbers shift as browsers release updates — verify on caniuse if precision matters for a project.

How to Use This Skill

When writing or reviewing CSS:

1. Always use techniques in the Widely Available tier (90%+ support) without asking. If you spot legacy patterns, refactor them to the modern equivalent.
2. Suggest techniques in the Newly Available tier (80–90% support). Mention they are newly available and let the user decide. Offer a fallback if relevant.
3. Ask first before using Limited Availability techniques (<80% support). Warn about browser support and suggest a progressive-enhancement approach or fallback.
4. If the user has legacy browser requirements (e.g. older Safari, corporate browsers), ask before using any technique — even Widely Available ones — since support percentages assume modern browser versions. Use @supports to provide fallbacks.

When you see old CSS patterns in existing code, point them out and suggest the modern replacement with a brief explanation of why it's better.

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Widely Available (90%+ support) — Always Use

These techniques are well-established and work across all modern browsers. Use them without hesitation.

Layout

Centering elements — place-items: center

/* OLD: absolute + transform hack (5 declarations, 2 selectors) */
.parent { position: relative; }
.child {
  position: absolute;
  top: 50%; left: 50%;
  transform: translate(-50%, -50%);
}

/* MODERN: grid centering (2 declarations, 1 selector) */
.parent {
  display: grid;
  place-items: center;
}

Reference →

Spacing elements — gap

/* OLD: margin hacks with :last-child override */
.grid > * { margin-right: 16px; }
.grid > *:last-child { margin-right: 0; }

/* MODERN: gap on the container */
.grid {
  display: flex;
  gap: 16px;
}

Reference →

Aspect ratios — aspect-ratio

/* OLD: padding-top percentage hack */
.wrapper { padding-top: 56.25%; position: relative; }
.inner { position: absolute; inset: 0; }

/* MODERN: aspect-ratio property */
.video-wrapper {
  aspect-ratio: 16 / 9;
}

Reference →

Positioning shorthand — inset

/* OLD: four separate properties */
.overlay { top: 0; right: 0; bottom: 0; left: 0; }

/* MODERN: inset shorthand */
.overlay {
  position: absolute;
  inset: 0;
}

Reference →

Sticky headers — position: sticky

/* OLD: JS scroll listener + getBoundingClientRect */
// add/remove .fixed class based on scroll position

/* MODERN: sticky positioning */
.header {
  position: sticky;
  top: 0;
}

Reference →

Responsive images — object-fit

/* OLD: background-image with background-size */
.card-image {
  background-image: url(...);
  background-size: cover;
  background-position: center;
}

/* MODERN: object-fit on an img element */
img {
  object-fit: cover;
  width: 100%;
  height: 200px;
}

Reference →

Filling available space — stretch

/* OLD: calc workaround */
.full { width: calc(100% - 40px); }

/* MODERN: stretch keyword */
.full { width: stretch; }
/* fills container, respects margins */

Reference →

Preventing scroll chaining — overscroll-behavior

/* OLD: JS wheel event preventDefault */
// modal.addEventListener('wheel', e => e.preventDefault(), { passive: false })

/* MODERN: overscroll-behavior */
.modal-content {
  overflow-y: auto;
  overscroll-behavior: contain;
}

Reference →

Preventing scrollbar layout shift — scrollbar-gutter

/* OLD: force scrollbar or hardcode padding */
body { overflow-y: scroll; }

/* MODERN: reserve scrollbar space */
body { scrollbar-gutter: stable; }

Reference →

Grid template areas — named areas

/* OLD: grid line numbers or floats */
.item { grid-column: 1 / 3; grid-row: 2; }

/* MODERN: named grid areas */
.layout {
  display: grid;
  grid-template-areas:
    "header header"
    "sidebar main"
    "footer footer";
}
.header { grid-area: header; }

Reference →

Modal dialogs — <dialog> element

/* OLD: fixed overlay + JS open/close/ESC/focus-trap */
.overlay { position: fixed; z-index: 999; }

/* MODERN: native dialog with ::backdrop */
dialog { padding: 1rem; }
dialog::backdrop { background: rgb(0 0 0 / .5); }
/* JS: dialog.showModal() / dialog.close() */

Reference →

Direction-aware layouts — logical properties

/* OLD: physical properties + RTL overrides */
margin-left: 1rem;
padding-right: 1rem;
[dir="rtl"] .box { margin-right: 1rem; }

/* MODERN: logical properties */
margin-inline-start: 1rem;
padding-inline-end: 1rem;
border-block-start: 1px solid;

Reference →

Responsive components — @container

/* OLD: viewport-based media queries */
@media (min-width: 768px) {
  .card { grid-template-columns: auto 1fr; }
}

/* MODERN: container queries */
.wrapper { container-type: inline-size; }
@container (width > 400px) {
  .card { grid-template-columns: auto 1fr; }
}

Reference →

Scroll snapping — scroll-snap

/* OLD: Slick/Swiper carousel or touch handlers */
// $('.carousel').slick({ ... })

/* MODERN: CSS scroll snap */
.carousel {
  scroll-snap-type: x mandatory;
  overflow-x: auto;
}
.carousel > * { scroll-snap-align: start; }

Reference →

Selectors

Parent selection — :has()

/* OLD: JavaScript el.closest('.parent').classList.add(...) */

/* MODERN: :has() parent selector */
.card:has(img) {
  grid-template-rows: auto 1fr;
}

Reference →

Grouping selectors — :is()

/* OLD: repeat the full selector for each match */
.card h1, .card h2, .card h3, .card h4 { margin-bottom: 0.5em; }

/* MODERN: :is() grouping */
.card :is(h1, h2, h3, h4) { margin-bottom: 0.5em; }

Reference →

Low-specificity resets — :where()

/* OLD: class-based resets with specificity issues */
.reset ul, .reset ol { margin: 0; }

/* MODERN: :where() has zero specificity */
:where(ul, ol) {
  margin: 0;
  padding-inline-start: 1.5rem;
}

Reference →

Keyboard-only focus — :focus-visible

/* OLD: :focus fires on mouse click too, so people remove outlines (a11y fail) */
:focus { outline: 2px solid blue; }

/* MODERN: :focus-visible only shows for keyboard navigation */
:focus-visible {
  outline: 2px solid var(--focus-color);
}

Reference →

Color

Styling form controls — accent-color

/* OLD: appearance: none + rebuild the entire control */
input[type="checkbox"] { appearance: none; /* 20+ lines of custom styles */ }

/* MODERN: accent-color */
input[type="checkbox"],
input[type="radio"] {
  accent-color: #7c3aed;
}

Reference →

Perceptually uniform colors — oklch()

/* OLD: hex/rgb with guess-and-check for each shade */
--brand: #4f46e5;
--brand-light: #818cf8;
--brand-dark: #3730a3;

/* MODERN: oklch — adjust lightness, keep perceived hue */
--brand: oklch(0.55 0.2 264);
--brand-light: oklch(0.75 0.2 264);
--brand-dark: oklch(0.35 0.2 264);

Reference →

Wide-gamut colors — display-p3 / oklch

/* OLD: sRGB only — washed out on P3 displays */
.hero { color: rgb(200, 80, 50); }

/* MODERN: oklch or display-p3 for vivid colors */
.hero { color: oklch(0.7 0.25 29); }
/* or: color(display-p3 1 0.2 0.1) */

Reference →

Frosted glass — backdrop-filter

/* OLD: pseudo-element with blurred background image */
.card::before { content: ''; background-image: url(bg.jpg); filter: blur(12px); z-index: -1; }

/* MODERN: backdrop-filter */
.glass {
  backdrop-filter: blur(12px);
  background: rgba(255, 255, 255, .1);
}