tomevault-io 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:
- Always use techniques in the Widely Available tier (90%+ support) without asking. If you spot legacy patterns, refactor them to the modern equivalent.
- 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.
- Ask first before using Limited Availability techniques (<80% support). Warn about browser support and suggest a progressive-enhancement approach or fallback.
- 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
@supportsto 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.
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;
}
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;
}
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;
}
Positioning shorthand — inset
/* OLD: four separate properties */
.overlay { top: 0; right: 0; bottom: 0; left: 0; }
/* MODERN: inset shorthand */
.overlay {
position: absolute;
inset: 0;
}
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;
}
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;
}
Filling available space — stretch
/* OLD: calc workaround */
.full { width: calc(100% - 40px); }
/* MODERN: stretch keyword */
.full { width: stretch; }
/* fills container, respects margins */
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;
}
Preventing scrollbar layout shift — scrollbar-gutter
/* OLD: force scrollbar or hardcode padding */
body { overflow-y: scroll; }
/* MODERN: reserve scrollbar space */
body { scrollbar-gutter: stable; }
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; }
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() */
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;
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; }
}
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; }
Selectors
Parent selection — :has()
/* OLD: JavaScript el.closest('.parent').classList.add(...) */
/* MODERN: :has() parent selector */
.card:has(img) {
grid-template-rows: auto 1fr;
}
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; }
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;
}
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);
}
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;
}
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);
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) */
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);
}
Typography
Fluid typography — clamp()
/* OLD: multiple media queries for each breakpoint */
h1 { font-size: 1rem; }
@media (min-width: 600px) { h1 { font-size: 1.5rem; } }
@media (min-width: 900px) { h1 { font-size: 2rem; } }
/* MODERN: clamp for fluid scaling */
h1 { font-size: clamp(1rem, 2.5vw, 2rem); }
Multiline text truncation — line-clamp
/* OLD: JS to slice text by chars/words and add "..." */
/* MODERN: line-clamp */
.card-title {
display: -webkit-box;
-webkit-line-clamp: 3;
line-clamp: 3;
}
Drop caps — initial-letter
/* OLD: float hack with manual line-height */
.drop-cap::first-letter { float: left; font-size: 3em; line-height: 1; }
/* MODERN: initial-letter */
.drop-cap::first-letter { initial-letter: 3; }
Font loading — font-display: swap
/* OLD: invisible text until font loads (FOIT) */
@font-face { font-family: "MyFont"; src: url(...); }
/* MODERN: font-display swap shows fallback immediately */
@font-face {
font-family: "MyFont";
src: url("MyFont.woff2");
font-display: swap;
}
Variable fonts — single file, all weights
/* OLD: separate @font-face for each weight */
@font-face { font-weight: 400; src: url("Regular.woff2"); }
@font-face { font-weight: 700; src: url("Bold.woff2"); }
/* MODERN: one variable font file */
@font-face {
font-family: "MyVar";
src: url("MyVar.woff2");
font-weight: 100 900;
}
Animation
Independent transforms
/* OLD: rewrite entire transform shorthand to change one value */
.icon { transform: translateX(10px) rotate(45deg) scale(1.2); }
/* MODERN: individual transform properties */
.icon {
translate: 10px 0;
rotate: 45deg;
scale: 1.2;
}
/* animate any one without touching the rest */
Typed custom properties — @property
/* OLD: custom properties are strings, can't animate */
:root { --hue: 0; }
.wheel { transition: --hue .3s; } /* ignored */
/* MODERN: @property makes them animatable */
@property --hue {
syntax: "<angle>";
inherits: false;
initial-value: 0deg;
}
.wheel {
background: hsl(var(--hue), 80%, 50%);
transition: --hue .3s;
}
Responsive clip paths — shape()
/* OLD: clip-path: path() uses fixed px coordinates */
.shape { clip-path: path('M0 200 L100 0...'); }
/* MODERN: shape() uses responsive units */
.shape { clip-path: shape(from 0% 100%, ...); }
Workflow
CSS nesting — no preprocessor needed
/* OLD: requires Sass/Less compiler */
// .nav { & a { color: #888; } }
/* MODERN: native CSS nesting */
.nav {
& a { color: #888; }
}
/* plain .css file, no build step */
Cascade layers — @layer
/* OLD: specificity wars and !important */
.page .card .title.special { color: red !important; }
/* MODERN: cascade layers control order */
@layer base, components, utilities;
@layer utilities {
.mt-4 { margin-top: 1rem; }
}
Theme variables — custom properties
/* OLD: Sass variables compile to static values */
$primary: #7c3aed;
.btn { background: $primary; }
/* MODERN: CSS custom properties update at runtime */
:root { --primary: #7c3aed; }
.btn { background: var(--primary); }
Dark mode defaults — color-scheme
/* OLD: manually restyle every form control for dark mode */
@media (prefers-color-scheme: dark) { input, select, textarea { ... } }
/* MODERN: color-scheme tells the browser to handle it */
:root { color-scheme: light dark; }
Lazy rendering — content-visibility
/* OLD: IntersectionObserver to defer rendering */
// new IntersectionObserver((entries) => { /* render */ }).observe(el)
/* MODERN: content-visibility auto */
.section {
content-visibility: auto;
contain-intrinsic-size: auto 500px;
}
Newly Available (80–90% support) — Suggest to User
These techniques are newly available across major browsers. Suggest them to the user, mention the support level, and offer a fallback if needed.
Layout
Dropdown menus — [popover] (86%)
/* OLD: display:none + JS toggle + click-outside + ESC */
.menu { display: none; }
.menu.open { display: block; }
/* MODERN: popover attribute */
/* <button popovertarget="menu">Toggle</button> */
/* <div id="menu" popover>...</div> */
#menu[popover] { position: absolute; }
Subgrid — align nested grids to parent tracks (88%)
/* OLD: duplicate parent track definitions in child */
.child-grid { grid-template-columns: 1fr 1fr 1fr; }
/* MODERN: subgrid inherits parent tracks */
.child-grid {
display: grid;
grid-template-columns: subgrid;
}
Customizable selects — appearance: base-select (86%)
/* OLD: Select2 or Choices.js replacing native select */
/* MODERN: base-select unlocks full styling */
select, select ::picker(select) {
appearance: base-select;
}
Note: support is expanding rapidly — verify current status on caniuse. Reference →
Hover tooltips — popover=hint + interestfor (86%)
<!-- OLD: JS mouseenter/mouseleave + focus/blur + positioning -->
<!-- MODERN: declarative hover trigger -->
<button interestfor="tip">Hover me</button>
<div id="tip" popover=hint>Tooltip content</div>
Animation
Entry animations — @starting-style (85%)
/* OLD: requestAnimationFrame to add class after paint */
// requestAnimationFrame(() => el.classList.add('visible'))
/* MODERN: @starting-style defines entry state */
.card {
transition: opacity .3s, transform .3s;
@starting-style {
opacity: 0;
transform: translateY(10px);
}
}
Animating display: none — transition-behavior: allow-discrete (85%)
/* OLD: wait for transitionend, use visibility + opacity + pointer-events */
/* MODERN: allow-discrete enables display transitions */
.panel {
transition: opacity .2s, display .2s;
transition-behavior: allow-discrete;
}
.panel.hidden { opacity: 0; display: none; }
Page transitions — View Transitions API (89%)
/* OLD: Barba.js or React Transition Group */
/* MODERN: View Transitions API */
/* JS: document.startViewTransition(() => updateDOM()); */
.hero { view-transition-name: hero; }
/* Style with ::view-transition-old(hero) / ::view-transition-new(hero) */
Color
Color mixing — color-mix() (89%)
/* OLD: Sass mix($blue, $pink, 60%) */
/* MODERN: color-mix in CSS */
background: color-mix(in oklch, #3b82f6, #ec4899);
Color variants — relative color syntax (87%)
/* OLD: Sass lighten($brand, 20%) — compile-time only */
/* MODERN: relative color syntax — runtime */
.btn {
background: oklch(from var(--brand) calc(l + 0.2) c h);
}
/* change --brand and all derivatives update */
Dark mode colors — light-dark() (83%)
/* OLD: duplicate values in prefers-color-scheme media query */
@media (prefers-color-scheme: dark) { color: #eee; }
/* MODERN: light-dark() function */
color: light-dark(#111, #eee);
color-scheme: light dark;
Selectors
Form validation — :user-invalid / :user-valid (85%)
/* OLD: JS blur listener to add .touched class */
// el.addEventListener('blur', () => el.classList.add('touched'))
/* MODERN: :user-invalid only fires after interaction */
input:user-invalid { border-color: red; }
input:user-valid { border-color: green; }
Workflow
Scoped styles — @scope (84%)
/* OLD: BEM naming (.card__title) or CSS Modules */
/* MODERN: @scope limits selectors to a root */
@scope (.card) {
.title { font-size: 1.25rem; }
.body { color: #444; }
}
/* .title only matches inside .card */
Range style queries (88%)
/* OLD: multiple style() blocks for each value */
@container style(--p: 51%) {}
@container style(--p: 52%) {}
/* MODERN: range comparisons */
@container style(--progress > 50%) {
.bar { ... }
}
Typography
Balanced headlines — text-wrap: balance (87%)
/* OLD: manual <br> tags or Balance-Text.js */
/* MODERN: text-wrap balance */
h1, h2 { text-wrap: balance; }
Limited Availability (<80% support) — Ask Before Using
These are cutting-edge CSS features. Always ask the user before using them, warn about browser support, and suggest fallbacks or progressive enhancement strategies.
Layout
Modal controls — commandfor (72%)
<!-- OLD: onclick handler to call showModal() -->
<button
<!-- MODERN: declarative command -->
<button commandfor="dlg" command="show-modal">Open</button>
<dialog id="dlg">...</dialog>
Fallback: use a small JS onclick handler to call dialog.showModal().
Reference →
Dialog light dismiss — closedby="any" (72%)
<!-- OLD: JS click listener checking click bounds -->
<!-- MODERN: closedby attribute -->
<dialog closedby="any">Click outside to close</dialog>
Fallback: add a click listener on ::backdrop to call dialog.close().
Reference →
Anchor positioning — position-anchor + anchor() (77%)
/* OLD: Popper.js / Floating UI for tooltip positioning */
/* MODERN: CSS anchor positioning */
.trigger { anchor-name: --tip; }
.tooltip {
position-anchor: --tip;
top: anchor(bottom);
}
Fallback: use Floating UI or absolute positioning with JS. Reference →
Carousel controls — ::scroll-button / ::scroll-marker (72%)
/* OLD: Swiper.js or Slick carousel */
/* MODERN: native scroll markers and buttons */
.carousel::scroll-button(right) { content: "→"; }
.carousel li::scroll-marker { content: ''; }
Fallback: use CSS scroll-snap with custom buttons via JS. Reference →
Corner shapes — corner-shape: squircle (67%)
/* OLD: clip-path polygon with 20+ points */
/* MODERN: corner-shape property */
.card {
border-radius: 2em;
corner-shape: squircle;
}
Fallback: use standard border-radius or SVG clip-path.
Reference →
Animation
Scroll-driven animations — animation-timeline: view() (78%)
/* OLD: IntersectionObserver + GSAP or AOS.js */
/* MODERN: CSS scroll-driven animations */
@keyframes reveal {
from { opacity: 0; translate: 0 40px; }
to { opacity: 1; translate: 0 0; }
}
.reveal {
animation: reveal linear both;
animation-timeline: view();
animation-range: entry 0% entry 100%;
}
Fallback: use IntersectionObserver with a .visible class toggle.
Polyfill: scroll-timeline
Reference →
Height auto animations — interpolate-size: allow-keywords (69%)
/* OLD: JS to measure scrollHeight, set px, then transition */
/* MODERN: interpolate-size enables height:auto transitions */
:root { interpolate-size: allow-keywords; }
.accordion { height: 0; overflow: hidden; transition: height .3s ease; }
.accordion.open { height: auto; }
Fallback: use max-height with a large value, or JS measurement.
Reference →
Staggered animations — sibling-index() (72%)
/* OLD: nth-child custom property for each item */
li:nth-child(1) { --i: 0; }
li:nth-child(2) { --i: 1; }
/* MODERN: sibling-index() function */
li {
transition-delay: calc(0.1s * (sibling-index() - 1));
}
Fallback: use nth-child with CSS custom properties. Reference →
Sticky/snapped element styling — scroll-state container queries (50%)
/* OLD: JS scroll listener checking element position */
/* MODERN: @container scroll-state() */
@container scroll-state(stuck: top) {
.header { box-shadow: 0 2px 8px rgb(0 0 0 / .1); }
}
Fallback: use IntersectionObserver to toggle a class. Reference →
Typography
Vertical text centering — text-box (79%)
/* OLD: uneven padding hacks for optical centering */
.btn { padding: 10px 20px; padding-top: 8px; /* hack */ }
/* MODERN: text-box trims leading/trailing space */
h1, button {
text-box: trim-both cap alphabetic;
}
Fallback: use manual padding adjustments. Reference →
Auto-growing textarea — field-sizing: content (73%)
/* OLD: JS input listener resizing on every keystroke */
/* MODERN: field-sizing */
textarea {
field-sizing: content;
min-height: 3lh;
}
Fallback: use a JS auto-resize handler. Reference →
Selectors
Scroll spy — :target-current (48%)
/* OLD: IntersectionObserver with 15+ lines of JS */
/* MODERN: :target-current pseudo-class */
nav a:target-current {
color: var(--accent);
}
Fallback: use IntersectionObserver to toggle an .active class.
Reference →
Workflow
CSS functions — @function (67%)
/* OLD: Sass @function */
// @function fluid($min, $max) { @return clamp(...); }
/* MODERN: native CSS @function */
@function --fluid(--min, --max) {
@return clamp(...);
}
Fallback: use Sass functions or repeated clamp() expressions.
Reference →
Typed attribute values — attr() with type (42%)
/* OLD: JS reading dataset to set styles */
// el.style.width = el.dataset.pct + '%';
/* MODERN: attr() with type() */
.bar { width: attr(data-pct type(<percentage>)); }
Fallback: use JS to read data attributes and set inline styles. Reference →
Inline conditionals — if() (35%)
/* OLD: JS classList toggle */
// el.classList.toggle('primary', isPrimary)
/* MODERN: CSS if() function */
.btn {
background: if(style(--variant: primary): blue; else: gray);
}
Fallback: use CSS custom properties with class toggles. Reference →
Core Principles
When writing any CSS, follow these guiding principles:
- Native CSS over JavaScript — If CSS can do it, don't use JS. Scroll snap over carousel libraries. Popover over toggle scripts. Dialog over modal libraries.
- Logical properties over physical — Use
inline/blockinstead ofleft/right/top/bottom. Your layout will work in any writing direction. - Intrinsic sizing over fixed breakpoints — Use
clamp(),min(),max(), container queries, andflex/gridauto-sizing. Let content determine layout. - Custom properties over preprocessor variables — CSS custom properties update at runtime, work with JS, and cascade. Sass variables compile away.
- Cascade layers over specificity hacks — Use
@layerto organize your cascade. Stop using!importantor deep nesting to win specificity battles. - Progressive enhancement — Use modern features with fallbacks. Use
@supportsto gate Newly Available and Limited features behind capability checks:/* fallback for older browsers */ .card { display: flex; flex-wrap: wrap; } /* modern upgrade */ @supports (container-type: inline-size) { .wrapper { container-type: inline-size; } @container (width > 400px) { .card { flex-direction: row; } } }
Quick Decision Guide
| You want to... | Use this |
|---|---|
| Center something | display: grid; place-items: center |
| Space children evenly | gap on flex/grid |
| Maintain aspect ratio | aspect-ratio: 16 / 9 |
| Pin a header on scroll | position: sticky; top: 0 |
| Responsive component | @container query |
| Create a modal | <dialog> element |
| Create a dropdown | [popover] attribute 🟡 |
| Select parent based on child | :has() selector |
| Style keyboard focus only | :focus-visible |
| Fluid font size | clamp(min, preferred, max) |
| Truncate multiline text | line-clamp: N |
| Balance headline wrapping | text-wrap: balance 🟡 |
| Animate on scroll | animation-timeline: view() ⚠️ |
| Page transitions | View Transitions API 🟡 |
| Animate into view | @starting-style 🟡 |
| Create color palette from one color | oklch() with adjusted lightness |
| Mix two colors | color-mix(in oklch, ...) 🟡 |
| Dark mode colors | light-dark() + color-scheme 🟡 |
| Scope styles to a component | @scope (.component) 🟡 |
| Control cascade order | @layer |
| Nest selectors | Native CSS nesting & selector |
| Type a custom property | @property |
| Position a tooltip | CSS anchor positioning ⚠️ |
No marker = Widely Available (90%+, always use). 🟡 = Newly Available (80–90%, suggest to user). ⚠️ = Limited (<80%, ask first).
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