Modern CSS HTML

Modern CSS & HTML Standards

Write CSS and HTML using current standards. Every pattern here has shipped in WebKit and Chrome unless explicitly noted otherwise. Avoid legacy hacks when a native solution exists.

How to use this skill

1. Read this file first for the quick-reference rules and snippets.
2. For deeper detail on a specific area, read the relevant file in references/:
   • references/snippets.md — 82 old-vs-modern comparisons with browser support percentages
   • references/properties.md — modern CSS properties reference
   • references/selectors.md — modern selectors and pseudo-classes/elements
   • references/at-rules.md — modern at-rules (@layer, @scope, @property, @container, etc.)
   • references/functions.md — modern CSS functions (color-mix, clamp, anchor, etc.)
   • references/css-2026.md — features shipping in 2026
   • references/css-2025.md — features that shipped in 2025

Browser support policy

Only use features supported in both WebKit (Safari) and Chrome/Edge (Blink). This means:

• Safe to use (Widely Available, 90%+): ship without fallback
• Use with progressive enhancement (Newly Available, 80–90%): ship with a sensible fallback via @supports
• Do not use by default (Limited, <80% or single-engine only): mention as an option but provide a fallback

When a feature is Chrome-only (e.g. some Interop 2026 items), note it explicitly and provide a cross-browser alternative.

Core rules

Layout

• Centre elements with display: grid; place-items: center — not position: absolute + transform: translate(-50%, -50%)
• Use gap for spacing in flex and grid — not margin hacks on children
• Use aspect-ratio: 16 / 9 — not the padding-top percentage hack
• Use inset: 0 — not top: 0; right: 0; bottom: 0; left: 0
• Use 100dvh for full-viewport height on mobile — not 100vh
• Use object-fit: cover for responsive images — not background-image + background-size: cover
• Use position: sticky — not JavaScript scroll listeners for sticky headers
• Use scroll-snap-type and scroll-snap-align — not carousel JS libraries
• Use overscroll-behavior: contain — not JS wheel event prevention
• Use scrollbar-gutter: stable — not overflow-y: scroll or hardcoded padding
• Use width: stretch — not calc(100% - margins) or -webkit-fill-available
• Use logical properties (margin-inline-start, padding-block-end, border-inline) — not physical left/right for direction-aware layouts
• Use grid-template-areas for named layout regions — not line number hacks
• Use subgrid to align nested grid children to parent tracks — not duplicated column definitions
• Use field-sizing: content for auto-growing textareas — not JS resize listeners
• Use anchor positioning (anchor-name, position-anchor, position-area) for tooltips — not Floating UI/Popper.js (progressive enhancement, check support)

Container queries

• Use @container (width < 400px) for component-level responsiveness — not viewport media queries for component layout
• Use container-type: inline-size on the parent

Media queries

• Use range syntax: @media (600px <= width <= 1200px) — not (min-width: 600px) and (max-width: 1200px)

Typography

• Use text-wrap: balance on headings — not manual <br> or JS
• Use clamp() for fluid type: font-size: clamp(1rem, 2.5vw, 2rem) — not multiple media query breakpoints
• Use font-display: swap in @font-face — not invisible text during load
• Use variable fonts with font-weight: 100 900 — not multiple @font-face declarations per weight
• Use initial-letter: 3 for drop caps — not float: left hacks
• Use -webkit-line-clamp / line-clamp for multiline truncation — not JS text slicing
• Use text-box: trim-both cap alphabetic for optical vertical centering — not padding tweaks (newly available)

Colour

• Use oklch() for perceptually uniform colour palettes — not hand-picked hex values
• Use color-mix(in oklch, ...) — not Sass mix() or lighten()/darken()
• Use relative colour syntax oklch(from var(--base) calc(l + 0.2) c h) for tints/shades — not hardcoded variants
• Use light-dark() with color-scheme: light dark — not duplicated @media (prefers-color-scheme) blocks
• Use accent-color for form control theming — not appearance: none + full rebuild
• Use backdrop-filter: blur() for frosted glass — not pseudo-element image blur hacks

Animation & transitions

• Use individual transform properties (translate, rotate, scale) — not the combined transform shorthand when animating independently
• Use @starting-style for entry animations — not requestAnimationFrame timing hacks
• Use transition-behavior: allow-discrete to animate display: none — not visibility + opacity + pointer-events workarounds
• Use animation-timeline: view() for scroll-driven animations — not IntersectionObserver JS (progressive enhancement)
• Use linear() easing for custom bounce/spring curves — not JS animation libraries for simple easing
• Use View Transitions API (document.startViewTransition()) for page transitions — not Barba.js
• Use interpolate-size: allow-keywords + height: auto transitions — not JS scrollHeight measurement
• Use offset-path for motion path animation — not GSAP motionPath plugin

Selectors

• Use :has() for parent selection — not JavaScript class toggling
• Use :is() and :where() for grouping — not long repetitive selector lists
• Use :focus-visible — not :focus (which triggers on mouse click too)
• Use :user-invalid / :user-valid for form validation — not JS blur + class toggling
• Use @layer for specificity control — not !important chains
• Use @scope for component-scoped styles — not BEM naming or CSS Modules (progressive enhancement)

CSS architecture

• Use native CSS nesting (& a { }) — not Sass/Less nesting
• Use CSS custom properties (var(--primary)) — not Sass variables
• Use @property for typed custom properties with animation — not untyped --var strings
• Use @supports for feature detection — not Modernizr or JS checks
• Use color-scheme: light dark for automatic dark mode defaults — not manual input/select restyling
• Use content-visibility: auto for lazy rendering of off-screen content — not IntersectionObserver

HTML

• Use <dialog> for modals — not div.modal + JS focus trap
• Use closedby="any" on <dialog> for light dismiss — not click-outside JS listeners
• Use <details> / <summary> for accordions — not JS toggle handlers
• Use <details name="group"> for exclusive accordions — not JS close-all-others logic
• Use popover attribute for dropdowns/tooltips — not JS show/hide + z-index management
• Use commandfor / command attributes for declarative button actions — not onclick handlers
• Use <input list="..." /> + <datalist> for autocomplete — not JS typeahead libraries
• Use <output for="..."> for live form output — not JS DOM writes
• Use loading="lazy" on images — not IntersectionObserver lazy loading
• Use inputmode="numeric" and enterkeyhint — not JS mobile keyboard detection
• Use appearance: base-select for customisable <select> — not Select2/Choices.js (newly available)
• Use interestfor + popover=hint for hover tooltips — not JS mouseenter/mouseleave

CSS 2026 features (use with caution — check cross-browser support)

• @mixin / @apply — native CSS mixins (Chrome 146 expected, not yet in Safari)
• Cross-document view transitions — MPA page transitions via @view-transition { navigation: auto }
• Gap decorations — column-rule / row-rule on grid/flex (Chrome 147 expected)
• reading-flow — keyboard/screen reader order matching visual order
• Masonry layout — grid-template-rows: masonry (Firefox behind flag, Chrome in development)
• contrast-color() — automatic WCAG-compliant text colour (Firefox 146, Safari 26)

What NOT to write

These patterns are outdated. If you catch yourself writing them, stop and use the modern alternative above:

• position: absolute; top: 50%; left: 50%; transform: translate(-50%, -50%) for centering
• padding-top: 56.25% for aspect ratios
• height: 100vh on mobile (use 100dvh)
• overflow-y: scroll to prevent layout shift (use scrollbar-gutter: stable)
• Sass variables, nesting, mix(), lighten(), darken() when native CSS alternatives exist
• @media (min-width: ...) and (max-width: ...) verbose syntax
• Float-based layouts
• -webkit- prefixed properties where the unprefixed version is widely available
• jQuery or JS for: accordions, modals, tooltips, dropdowns, lazy loading, scroll snapping, form validation styling, dark mode detection, feature detection