acaprino/frontend-css

<!-- upstream: paulirish/dotfiles - agents/skills/modern-css/SKILL.md -->

Modern CSS

This skill provides a reference for writing modern, robust, and efficient CSS.

---

References Library

For deeper coverage of specific topics, consult the files under references/:

| File | Topic | |------|-------| | typography.md | Vertical rhythm, modular scales, font selection, web font loading, fluid type, OpenType features | | color-and-contrast.md | OKLCH color spaces, tinted neutrals, palette structure, WCAG, dark mode, theming | | motion-design.md | Duration rules, easing curves, premium motion materials, reduced motion, perceived performance | | heuristics-scoring.md | Nielsen's 10 heuristics with 0-4 scoring rubric and P0-P3 severity | | cognitive-load.md | Intrinsic/extraneous/germane load, working memory rule, 8 common violations | | personas.md | 5 design-critique archetypes (Alex/Jordan/Sam/Riley/Casey) | | css-patterns.md | Container queries, color manipulation, SASS, BEM, CSS Modules, focus, vendor prefixes, responsive images, plus a Responsive Design Foundations section (mobile-first, breakpoints, pointer detection, safe areas) | | layout-patterns.md | Holy Grail, Split, Bento, Editorial Asymmetry, Masonry, Card Grid recipes, plus a Spatial Design Principles section (4pt spacing, hierarchy, container queries, optical adjustments) | | ui-pattern-guide.md | Cards/list/table/gallery decisions, navigation, pagination, modals, blank slate, plus an Interaction Design section (8 states, focus rings, Popover API, anchor positioning, keyboard nav) | | ux-patterns.md | Onboarding, trust/social proof, persuasion, paywalls, chunking, plus a UX Writing section (button labels, error messages, voice/tone, i18n, terminology) | | visual-intensity.md | Amplify (bolder) vs restrain (quieter) on one axis: scale/weight/color/space moves, anti-AI-slop "bold", easing rules | | delight-and-microinteractions.md | Where/when delight is earned, micro-interactions, copy personality, loading-state copy (anti-slop), celebrations, easter eggs, libraries | | production-hardening.md | Edge-case resilience: text overflow, i18n/RTL logical properties, error/empty/loading states, validation, a11y and performance resilience | | flow-patterns.md | User flow recipes | | argyle-cacadia-2025-deck.md | Adam Argyle CSS Wrapped 2025 deck (offline copy) | | token-architecture.md | Three-layer token system (primitive -> semantic -> component), naming convention, file organization, W3C DTCG alignment | | primitive-tokens.md | Layer 1 reference: raw color scales (gray/blue/status), 4px spacing scale, type scale, radius, shadow, motion, z-index | | semantic-tokens.md | Layer 2 reference: purpose-based aliases (background/foreground, primary/secondary/muted/accent/destructive, status, border/ring), dark-mode overrides | | component-tokens.md | Layer 3 reference: component-scoped tokens (button, input, card, badge, alert, dialog, table) referencing the semantic layer | | component-specs.md | Component spec tables (button, input, card, badge, alert, dialog, table) with variants, sizes, states, anatomy | | states-and-variants.md | Interactive states (default/hover/focus/active/disabled/loading), state priority, transitions, focus rings, error states, variant patterns, ARIA | | tailwind-integration.md | CSS variables + Tailwind config recipe, HSL opacity format, component classes, shadcn/ui alignment |

Several references and merged sections are derived from pbakaus/impeccable (Apache-2.0) and nextlevelbuilder/ui-ux-pro-max-skill (MIT); see attribution headers inside each file.

---

Layout & Responsive Design

Container Queries

.card {
  container: --my-card / inline-size;
}

@container --my-card (width < 40ch) {
  /* Component-based responsive design */
}

@container (20ch < width < 50ch) {
  /* Range syntax */
}

Container units: cqi, cqb, cqw, cqh - size relative to container dimensions

Anchored container queries: Style an anchored element (and its descendants) based on which position-try fallback is currently active

/* Anchored container queries (newest container-type, 2026) */
.tooltip {
  position: absolute;
  position-anchor: --trigger;
  position-try-fallbacks: flip-inline; /* flip side if it overflows */
  container-type: anchored;
}

/* When the inline-flip fallback is in use, flip the caret to match */
@container anchored(fallback: flip-inline) {
  .tooltip__caret { translate: 100%; }
}

.trigger {
  anchor-name: --trigger;
}

Media Query Range Syntax

@media (width <= 1024px) { }
@media (360px < width < 1024px) { }

Grid Enhancements

• Subgrid: Inherit parent grid lines for nested layouts
• Masonry: Pinterest-style layouts via display: grid-lanes (CSS Grid Level 3), which packs items into the shortest lane while keeping logical tab order. This is the resolved syntax (previously proposed as grid-template-rows: masonry). Safari 26 ships it; Chrome and Firefox keep it behind a flag, so gate with @supports for progressive enhancement.

---

Color & Theming

Color Scheme & Light-Dark Function

:root {
  color-scheme: light dark;
  --surface-1: light-dark(white, #222);
  --text-1: light-dark(#222, #fff);
}

Modern Color Spaces

/* OKLCH: uniform brightness, P3+ colors */
.vibrant {
  background: oklch(72% 75% 330);
}

/* Display-P3 for HDR displays */
@media (dynamic-range: high) {
  .neon {
    --neon-red: color(display-p3 1 0 0);
  }
}

/* Better gradients with in oklch */
.gradient {
  background: linear-gradient(
    to right in oklch,
    color(display-p3 1 0 .5),
    color(display-p3 0 1 1)
  );
}

Color Manipulation

/* color-mix() */
.lighten {
  background: color-mix(in oklab, var(--brand), white);
}

/* Relative color syntax */
.lighter {
  background: oklch(from blue calc(l + .25) c h);
  background: oklch(from blue 75% c h); /* Set to specific lightness */
}

.semi-transparent {
  background: oklch(from var(--color) l c h / 50%);
}

.complementary {
  background: hsl(from blue calc(h + 180) s l);
}

Accent Color

:root {
  accent-color: hotpink; /* Tints checkboxes, radios, range inputs */
}

---

Typography

Text Wrapping

h1 {
  text-wrap: balance; /* Balanced multi-line headings */
  max-inline-size: 25ch;
}

p {
  text-wrap: pretty; /* No orphans */
  max-inline-size: 50ch;
}

Text Box Trim

h1, p, button {
  text-box: trim-both cap alphabetic; /* Optical vertical centering */
}

Fluid Typography

.heading {
  font-size: clamp(1rem, 1rem + 0.5vw, 2rem); /* Respects user preferences */
}

Dynamic Viewport Units

• dvh / dvw - Dynamic (accounts for mobile browser UI)
• svh / svw - Small (smallest possible viewport)
• lvh / lvw - Large (largest possible viewport)

---

Animations & Motion

Scroll-Driven Animation

/* Animate on scroll position */
.parallax {
  animation: slide-up linear both;
  animation-timeline: scroll();
}

/* Animate on viewport intersection */
.fade-in {
  animation: fade linear both;
  animation-timeline: view();
  animation-range: cover -75cqi contain 20cqi;
}

View Transitions

Status: Baseline Newly Available (Same-document). Cross-document transitions are in Limited Availability (Chrome/Safari 18.2+).

@view-transition {
  navigation: auto; /* Automatically animate page transitions (MPAs) */
}

nav {
  view-transition-name: --persist-nav; /* Persist specific elements */
  view-transition-class: --site-header; /* Group transitions with classes */
}

/* Style the active transition */
html:active-view-transition {
  overflow: hidden;
}

Nested View Transition Groups: Preserve 3D transforms and clipping during transitions.

Advanced Easing with linear()

.springy {
  --spring: linear(
    0, 0.14 4%, 0.94 17%, 1.15 24% 30%, 1.02 43%, 0.98 51%, 1 77%, 1
  );
  transition: transform 1s var(--spring);
}

@starting-style

.dialog {
  transition: opacity .5s, scale .5s;

  @starting-style {
    opacity: 0;
    scale: 1.1;
  }
}

---

Custom Properties & Advanced Features

@property

Type-safe, animatable custom properties:

@property --gradient-angle {
  syntax: "<angle>";
  inherits: false;
  initial-value: 0deg;
}

.animate {
  transition: --gradient-angle 1s ease;

  &:hover {
    --gradient-angle: 360deg;
  }
}

Math Functions & calc-size()

Newly Available: calc-size() allows calculations and transitions on intrinsic sizes (auto, min-content).

/* Finally: Animate to auto height! */
.accordion-content {
  height: 0;
  overflow: hidden;
  transition: height 0.3s ease;
}

.accordion-item.open .accordion-content {
  height: calc-size(auto);
}

.radial-layout {
  --_angle: calc(var(--sibling-index) * var(--_offset));
  translate:
    calc(cos(var(--_angle)) * var(--_circle-size))
    calc(sin(var(--_angle)) * var(--_circle-size));
}

Tree Counting Functions (Coming Soon)

.staggered {
  animation-delay: calc(sibling-index() * .1s);
  background-color: hsl(sibling-count() 50% 50%);
}

Conditional CSS with if() (Coming Soon)

.dynamic {
  color: if(
    style(--theme: dark),
    white,
    black
  );
}

---

Architecture & Organization

Cascade Layers

@layer reset, design-system, components, utilities;

@import "open-props/colors" layer(design-system);
@import "components/nav/base.css" layer(components.nav);

@layer components.nav.primary {
  nav {
    position: sticky;
    inset-block-start: 0;
  }
}

Benefits:

• Import third-party CSS with lower specificity
• Organize styles by concern, not selector weight
• Nested layers create clear hierarchies

---

Interactive Components

Dialog

<dialog id="modal">
  <form method="dialog">
    <button value="cancel">Cancel</button>
    <button value="confirm">Confirm</button>
  </form>
</dialog>

<button commandfor="modal" command="showModal">Open</button>
<button commandfor="modal" command="close">Close</button>

New: closedby attribute enables light-dismiss behavior

Popover

<button popovertarget="menu">Show Menu</button>
<div popover id="menu">...</div>

popover=hint: Ephemeral tooltips that don't dismiss other popovers

[popover] {
  transition:
    display .5s allow-discrete,
    overlay .5s allow-discrete,
    opacity .5s;

  @starting-style {
    &:popover-open {
      opacity: 0;
    }
  }
}

Anchor Positioning

.tooltip-anchor {
  anchor-name: --tooltip;
}

.tooltip[popover] {
  position-anchor: --tooltip;
  position-area: block-start;
  position-try-fallbacks: flip-block;
  position-try-order: most-height;
}

Pseudo-elements: anchor(), ::scroll-button(), ::scroll-marker()

Exclusive Accordion

<details name="accordion">...</details>
<details name="accordion">...</details>
<!-- Only one can be open at a time -->

Customizable Select

select {
  appearance: base-select; /* Full CSS control */
}

/* Style options with rich HTML */
select option::before {
  content: ""; /* Can include images, icons */
}

Search Element

<search>
  <form>
    <input type="search" name="q">
    <button type="submit">Search</button>
  </form>
</search>

---

Form Enhancements

Field Sizing

textarea, select, input {
  field-sizing: content; /* Auto-grow to content */
}

textarea {
  min-block-size: 3lh; /* Line-height units */
  max-block-size: 80dvh;
}

Better Validation Pseudo-Classes

/* Wait for user interaction before showing errors */
:user-invalid {
  outline-color: red;
}

:user-valid {
  outline-color: green;
}

label:has(+ input:user-invalid) {
  text-decoration: underline wavy red;
}

HR in Select

<select>
  <option>Option 1</option>
  <hr>
  <option>Option 2</option>
</select>

---

Visual Effects

Scrollbar Styling

.custom-scrollbar {
  scrollbar-color: hotpink transparent;
  scrollbar-width: thin;
}

Shape Function

.complex-clip {
  clip-path: shape(
    from 0% 0%,
    curve by 50% 25% via 25% 50%,
    line to 100% 100%
  );
}

Corner Shapes

.fancy-corners {
  corner-shape: squircle;
  corner-shape: notch;
  corner-shape: scoop;
  corner-shape: superellipse(0.7);
}

---

Progressive Enhancement Patterns

Feature Detection

@supports (animation-timeline: view()) {
  .fade-in {
    animation: fade linear both;
    animation-timeline: view();
  }
}

@supports (container-type: inline-size) {
  .responsive-card {
    container-type: inline-size;
  }
}

Respect User Preferences

@media (prefers-reduced-motion: no-preference) {
  .animated {
    animation: slide 1s ease;
  }
}

@media (prefers-color-scheme: dark) {
  :root {
    --surface: #222;
  }
}

@media (prefers-contrast: more) {
  .text {
    font-weight: 600;
  }
}

---

Checking Browser Support: Baseline

What is Baseline? A unified way to understand cross-browser feature availability. Features are marked as:

• Widely Available: Supported in the last 2.5 years of all major browsers
• Newly Available: Available in all major browsers
• Limited Availability: Not yet in all browsers

How to Check Baseline Status

0. BEST: Fetch https://web-platform-dx.github.io/web-features-explorer/groups/ and find the feature in there, then fetch its detail page.
1. Can I Use: caniuse.com shows Baseline badges at the top of each feature
2. MDN: Look for the Baseline badge in the browser compatibility table
3. web.dev: Feature articles include Baseline status

Remember: Always check Baseline status, use @supports for cutting-edge features, and respect user preferences with media queries. Modern CSS is about progressive enhancement and building resilient interfaces that work for everyone.

---

Real-World Example: Modern Component

Here's a card component using many modern CSS features:

/* Cascade layer for organization */
@layer components.card {

  /* Custom properties with @property */
  @property --card-hue {
    syntax: "<number>";
    inherits: false;
    initial-value: 200;
  }

  .card {
    /* Container for responsive design */
    container: card / inline-size;

    /* Logical properties */
    inline-size: 100%;
    padding-inline: var(--space-md);
    padding-block: var(--space-lg);

    /* Modern color system */
    background: light-dark(
      oklch(98% 0.02 var(--card-hue)),
      oklch(20% 0.02 var(--card-hue))
    );

    /* Border with relative color */
    border: 1px solid oklch(from var(--surface) calc(l * 0.9) c h);

    /* Smooth corners */
    border-radius: var(--radius-md);

    /* View transition */
    view-transition-name: --card;

    /* Scroll-driven animation */
    animation: fade-in linear both;
    animation-timeline: view();
    animation-range: entry 0% cover 30%;

    /* Anchor for tooltips */
    anchor-name: --card-anchor;

    /* Transition custom property */
    transition: --card-hue 0.5s var(--ease-spring-3);

    &:hover {
      --card-hue: 280;
    }

    /* Responsive typography in container */
    @container card (width > 30ch) {
      .card__title {
        font-size: clamp(1.5rem, 3cqi, 2.5rem);
        text-wrap: balance;
      }
    }

    @container card (width < 30ch) {
      .card__image {
        aspect-ratio: 16 / 9;
        object-fit: cover;
      }
    }
  }

  .card__title {
    /* Text box trim for optical alignment */
    text-box: trim-both cap alphabetic;
    text-wrap: balance;

    /* Logical margin */
    margin-block-end: var(--space-sm);
  }

  .card__body {
    text-wrap: pretty;
    max-inline-size: 65ch;
  }

  .card__cta {
    /* Inherit font */
    font: inherit;

    /* Accent color */
    accent-color: var(--brand);

    /* Field sizing */
    field-sizing: content;

    /* Logical properties */
    padding-inline: var(--space-md);
    padding-block: var(--space-sm);

    /* Modern color with relative syntax */
    background: oklch(from var(--brand) l c h);
    color: oklch(from var(--brand) 95% 0.05 h);

    &:hover {
      background: oklch(from var(--brand) calc(l * 1.1) c h);
    }

    &:user-invalid {
      outline: 2px solid light-dark(red, #ff6b6b);
    }
  }

  /* Popover tooltip anchored to card */
  .card__tooltip[popover] {
    position-anchor: --card-anchor;
    position-area: block-start;
    position-try-fallbacks: flip-block;

    /* Entry animation */
    @starting-style {
      opacity: 0;
      scale: 0.9;
    }

    transition:
      opacity 0.2s,
      scale 0.2s,
      display 0.2s allow-discrete,
      overlay 0.2s allow-discrete;
  }

  /* Scroll state container queries */
  @supports (container-type: scroll-state) {
    .card__sticky-header {
      container-type: scroll-state;
      position: sticky;
      inset-block-start: 0;

      @container scroll-state(stuck: top) {
        box-shadow: 0 2px 8px oklch(0% 0 0 / 0.1);
      }
    }
  }

  /* Respect user preferences */
  @media (prefers-reduced-motion: reduce) {
    .card {
      animation: none;
      transition: none;
    }
  }

  @media (prefers-contrast: more) {
    .card {
      border-width: 2px;
    }
  }
}

/* Keyframes for scroll animation */
@keyframes fade-in {
  from {
    opacity: 0;
    scale: 0.95;
  }
  to {
    opacity: 1;
    scale: 1;
  }
}

HTML for the Example

<article class="card">
  <img
    class="card__image"
    src="image.jpg"
    alt="Description"
    loading="lazy"
  >

  <h2 class="card__title">Card Title</h2>

  <p class="card__body">
    Card description with pretty text wrapping that avoids orphans.
  </p>

  <button
    class="card__cta"
    popovertarget="card-tooltip"
  >
    Learn More
  </button>

  <div
    class="card__tooltip"
    popover="hint"
    id="card-tooltip"
  >
    Additional information appears here
  </div>
</article>

Canonical Resources

• CSS Wrapped 2025 - The year's CSS features
• The Coyier CSS Starter - Opinionated modern baseline
• Adam Argyle's CascadiaJS 2025 Deck - (markdownified locally in references/argyle-cacadia-2025-deck.md)
• Modern CSS in Real Life - Practical applications

Usage Guidelines

1. Prioritize Stability:

   • Recommend Newly Available or Widely Available features for production code.
   • Use Limited Availability features with progressive enhancement, graceful degradation, or @supports. Or ask the user how they want to handle it.

2. Use the web platform:

   • Always prefer standard CSS solutions over JavaScript libraries for layout, animation, and interaction (e.g., use CSS Masonry instead of Masonry.js, Popover API instead of custom tooltip scripts).

3. Code Style:

   • Use modern color spaces (oklch) for new palettes.