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Modern CSS organization with native @import, @layer cascade control, CSS nesting, design tokens, and element-focused selectors. AUTO-INVOKED when editing .css files.
Generate standardized static site structures. Use when creating new websites, setting up demos, or need consistent site structure with SEO, PWA, and accessibility foundations.
Write Node.js CLI tools with zero dependencies. Use when creating command-line tools, argument parsing, colored output, or interactive prompts.
Define and use custom HTML elements. Use when creating new components, defining custom tags, or using project-specific elements beyond standard HTML5.
Umbrella coordinator for image handling. Coordinates responsive-images, placeholder-images, and automation scripts. Use when adding images to any page, optimizing existing images, or setting up image pipelines.
Generate SVG placeholder images for prototypes. Use when adding placeholder images for layouts, mockups, or development. Supports simple, labeled, and brand-aware types.
Modern responsive image techniques using picture element, srcset, sizes, and modern formats. Use when adding images that need to adapt to different screen sizes, resolutions, or support modern image formats.
| name | css-author |
| description | Modern CSS organization with native @import, @layer cascade control, CSS nesting, design tokens, and element-focused selectors. AUTO-INVOKED when editing .css files. |
| allowed-tools | Read, Write, Edit |
This skill provides patterns for organizing CSS in modern, maintainable ways without build tools. We leverage native CSS features: @import for modularization, @layer for cascade control, and nesting for readability.
CSS should be:
styles/
├── main.css # Entry point - imports everything
├── _reset.css # CSS reset/normalize
├── _tokens.css # Design tokens (custom properties)
├── _layout.css # Site-wide layout (grid, body structure)
├── _components.css # Shared components (buttons, cards)
├── sections/
│ ├── _header.css # Site header/nav
│ ├── _footer.css # Site footer
│ └── _sidebar.css # Sidebar patterns
├── pages/
│ ├── _home.css # Homepage-specific styles
│ ├── _blog.css # Blog listing/post styles
│ └── _contact.css # Contact page styles
└── components/
├── _gallery.css # Gallery grid component
├── _tag-list.css # Tag component styles
└── _data-table.css # Table wrapper styles
_reset.css): Partial files, imported by main.cssmain.css): Entry point, linked in HTML_tag-list.css, _data-table.cssmain.css)The main stylesheet declares layers and imports partials:
/* Layer declaration - controls cascade order */
@layer reset, tokens, layout, sections, components, pages, responsive;
/* Reset (lowest priority) */
@import "_reset.css" layer(reset);
/* Design system tokens */
@import "_tokens.css" layer(tokens);
/* Site-wide layout */
@import "_layout.css" layer(layout);
/* Recurring sections */
@import "sections/_header.css" layer(sections);
@import "sections/_footer.css" layer(sections);
@import "sections/_sidebar.css" layer(sections);
/* Shared components */
@import "_components.css" layer(components);
@import "components/_gallery.css" layer(components);
@import "components/_tag-list.css" layer(components);
@import "components/_data-table.css" layer(components);
/* Page-specific styles */
@import "pages/_home.css" layer(pages);
@import "pages/_blog.css" layer(pages);
@import "pages/_contact.css" layer(pages);
/* Responsive overrides (highest priority) */
@layer responsive {
@media (max-width: 768px) {
/* Mobile overrides */
}
}
Design tokens are CSS custom properties that provide consistent, themeable values across your design system.
Design tokens provide:
| Category | Purpose | Examples |
|---|---|---|
| Colors | Brand, semantic, surface colors | --primary, --error |
| Spacing | Consistent gaps and padding | --spacing-sm, --spacing-lg |
| Typography | Font sizes, weights, heights | --font-size-lg, --line-height-normal |
| Effects | Shadows, transitions, borders | --shadow-md, --transition-normal |
| Layout | Widths, breakpoints | --content-width, --sidebar-width |
Use OKLCH instead of hex/RGB. OKLCH provides:
| Format | Use Case | Example |
|---|---|---|
oklch() | Primary format for all colors | oklch(55% 0.22 260) |
light-dark() | Theme-aware tokens | light-dark(oklch(20% 0 0), oklch(95% 0 0)) |
color-mix() | Blending, opacity | color-mix(in oklch, var(--primary), transparent 50%) |
| Relative colors | Variations from base | oklch(from var(--primary) calc(l + 0.2) c h) |
/* oklch(lightness chroma hue) */
--primary: oklch(55% 0.22 260); /* Blue */
--success: oklch(65% 0.2 145); /* Green */
--warning: oklch(75% 0.18 85); /* Orange */
--error: oklch(55% 0.22 25); /* Red */
Generate color variations programmatically from a base color:
:root {
--primary: oklch(55% 0.22 260);
/* Lighter: increase lightness */
--primary-light: oklch(from var(--primary) calc(l + 0.2) c h);
/* Darker: decrease lightness */
--primary-dark: oklch(from var(--primary) calc(l - 0.15) c h);
/* Muted: reduce chroma */
--primary-muted: oklch(from var(--primary) l calc(c - 0.1) h);
/* Hover: slightly darker and more saturated */
--primary-hover: oklch(from var(--primary) calc(l - 0.08) calc(c + 0.02) h);
}
light-dark()Single declarations for both light and dark themes:
:root {
color-scheme: light dark; /* Required for light-dark() */
/* Single token handles both themes */
--text: light-dark(oklch(20% 0 0), oklch(95% 0 0));
--surface: light-dark(oklch(100% 0 0), oklch(15% 0.02 260));
--border: light-dark(oklch(90% 0.01 260), oklch(30% 0.02 260));
}
/* Semi-transparent overlays */
--overlay-light: color-mix(in oklch, black, transparent 95%);
--overlay-medium: color-mix(in oklch, black, transparent 90%);
/* Elevated surfaces */
--surface-elevated: color-mix(in oklch, var(--surface), white 5%);
/* Blend two colors */
--accent-blend: color-mix(in oklch, var(--primary), var(--secondary) 30%);
Specify color space to prevent muddy midtones:
/* Vibrant gradient interpolation */
background: linear-gradient(in oklch, var(--primary), var(--secondary));
/* For hue transitions, use longer path */
background: linear-gradient(in oklch longer hue, oklch(65% 0.25 0), oklch(65% 0.25 360));
For older browsers, provide hex fallback first:
:root {
--primary: #2563eb; /* Fallback for older browsers */
--primary: oklch(55% 0.22 260);
}
contrast-color()The contrast-color() function automatically selects black or white text based on background:
/* Button with any background color */
button {
background: var(--primary);
color: contrast-color(var(--primary));
}
/* Dynamic accent backgrounds */
[data-accent] {
background: var(--accent);
color: contrast-color(var(--accent));
}
Combining with light-dark():
.badge {
--bg: light-dark(var(--primary-light), var(--primary-dark));
background: var(--bg);
color: contrast-color(var(--bg));
}
Limitations:
#000) or white (#fff)Best practice: Use contrast-color() for dynamic/user-selected colors. For design system colors, manually define text colors to ensure optimal readability.
/* _tokens.css */
@layer tokens {
:root {
/* Enable light-dark() function */
color-scheme: light dark;
/* ==================== COLORS (OKLCH) ==================== */
/* Hue palette - define once, reuse everywhere */
--hue-primary: 260; /* Blue */
--hue-secondary: 250; /* Slate */
--hue-success: 145; /* Green */
--hue-warning: 85; /* Orange */
--hue-error: 25; /* Red */
--hue-info: 200; /* Cyan */
/* Brand colors with relative variations */
--primary: oklch(55% 0.22 var(--hue-primary));
--primary-hover: oklch(from var(--primary) calc(l - 0.08) calc(c + 0.02) h);
--primary-light: oklch(from var(--primary) calc(l + 0.35) calc(c - 0.12) h);
--secondary: oklch(50% 0.03 var(--hue-secondary));
--secondary-hover: oklch(from var(--secondary) calc(l - 0.1) c h);
/* Semantic colors */
--success: oklch(60% 0.18 var(--hue-success));
--success-light: oklch(from var(--success) calc(l + 0.32) calc(c - 0.1) h);
--warning: oklch(75% 0.16 var(--hue-warning));
--warning-light: oklch(from var(--warning) calc(l + 0.2) calc(c - 0.08) h);
--error: oklch(55% 0.2 var(--hue-error));
--error-light: oklch(from var(--error) calc(l + 0.38) calc(c - 0.12) h);
--info: oklch(55% 0.14 var(--hue-info));
--info-light: oklch(from var(--info) calc(l + 0.38) calc(c - 0.08) h);
/* Theme-aware surface colors */
--background: light-dark(oklch(100% 0 0), oklch(12% 0.02 var(--hue-primary)));
--background-alt: light-dark(oklch(98% 0.005 var(--hue-primary)), oklch(16% 0.02 var(--hue-primary)));
--surface: light-dark(oklch(100% 0 0), oklch(16% 0.02 var(--hue-primary)));
--surface-elevated: light-dark(oklch(100% 0 0), oklch(22% 0.02 var(--hue-primary)));
/* Theme-aware text colors */
--text: light-dark(oklch(20% 0.02 var(--hue-primary)), oklch(96% 0.01 var(--hue-primary)));
--text-muted: light-dark(oklch(45% 0.02 var(--hue-primary)), oklch(65% 0.02 var(--hue-primary)));
--text-inverted: light-dark(oklch(100% 0 0), oklch(10% 0 0));
/* Theme-aware border colors */
--border: light-dark(oklch(90% 0.01 var(--hue-primary)), oklch(28% 0.02 var(--hue-primary)));
--border-strong: light-dark(oklch(82% 0.01 var(--hue-primary)), oklch(38% 0.02 var(--hue-primary)));
/* Theme-aware overlays using color-mix */
--overlay-light: light-dark(
color-mix(in oklch, black, transparent 95%),
color-mix(in oklch, white, transparent 95%)
);
--overlay-medium: light-dark(
color-mix(in oklch, black, transparent 90%),
color-mix(in oklch, white, transparent 90%)
);
--overlay-strong: light-dark(
color-mix(in oklch, black, transparent 80%),
color-mix(in oklch, white, transparent 80%)
);
/* ==================== SPACING ==================== */
--spacing-xs: 0.25rem; /* 4px */
--spacing-sm: 0.5rem; /* 8px */
--spacing-md: 1rem; /* 16px */
--spacing-lg: 1.5rem; /* 24px */
--spacing-xl: 2rem; /* 32px */
--spacing-2xl: 3rem; /* 48px */
--spacing-3xl: 4rem; /* 64px */
/* ==================== TYPOGRAPHY ==================== */
/* Font families */
--font-sans: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
--font-mono: ui-monospace, "Cascadia Code", "Source Code Pro", Menlo, monospace;
--font-serif: Georgia, Cambria, "Times New Roman", Times, serif;
/* Font sizes */
--font-size-xs: 0.75rem; /* 12px */
--font-size-sm: 0.875rem; /* 14px */
--font-size-base: 1rem; /* 16px */
--font-size-lg: 1.125rem; /* 18px */
--font-size-xl: 1.25rem; /* 20px */
--font-size-2xl: 1.5rem; /* 24px */
--font-size-3xl: 1.875rem; /* 30px */
--font-size-4xl: 2.25rem; /* 36px */
/* Font weights */
--font-weight-normal: 400;
--font-weight-medium: 500;
--font-weight-semibold: 600;
--font-weight-bold: 700;
/* Line heights */
--line-height-tight: 1.25;
--line-height-normal: 1.5;
--line-height-relaxed: 1.625;
/* ==================== EFFECTS ==================== */
/* Shadows */
--shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.05);
--shadow-md: 0 4px 6px rgba(0, 0, 0, 0.1);
--shadow-lg: 0 10px 15px rgba(0, 0, 0, 0.1);
--shadow-xl: 0 20px 25px rgba(0, 0, 0, 0.1);
/* Transitions */
--transition-fast: 0.15s ease;
--transition-normal: 0.3s ease;
--transition-slow: 0.5s ease;
/* Border radius */
--radius-sm: 0.25rem;
--radius-md: 0.375rem;
--radius-lg: 0.5rem;
--radius-xl: 0.75rem;
--radius-full: 9999px;
/* ==================== LAYOUT ==================== */
--content-width: 65ch;
--content-width-wide: 80rem;
--sidebar-width: 16rem;
/* Z-index scale */
--z-dropdown: 100;
--z-sticky: 200;
--z-modal: 300;
--z-tooltip: 400;
}
}
Recommended: Use light-dark() in the Complete Token System above. This eliminates the need for duplicate token definitions.
For sites with theme toggle UI that override system preference, use CSS :has() to scope token overrides:
/* Force dark mode when user selects dark */
:root:has(#theme-dark:checked) {
color-scheme: dark; /* Triggers light-dark() to use dark values */
}
/* Force light mode when user selects light */
:root:has(#theme-light:checked) {
color-scheme: light; /* Triggers light-dark() to use light values */
}
/* Auto follows system preference (default behavior) */
:root:has(#theme-auto:checked) {
color-scheme: light dark;
}
:root {
/* Form tokens */
--form-border: var(--border);
--form-focus: var(--primary);
--form-invalid: var(--error);
--form-input-padding: var(--spacing-sm) var(--spacing-md);
--form-input-radius: var(--radius-md);
/* Button tokens */
--button-padding: var(--spacing-sm) var(--spacing-lg);
--button-radius: var(--radius-md);
--button-primary-bg: var(--primary);
--button-primary-text: var(--text-inverted);
/* Card tokens */
--card-padding: var(--spacing-lg);
--card-radius: var(--radius-lg);
--card-shadow: var(--shadow-sm);
--card-bg: var(--surface);
}
| Pattern | Example | Purpose |
|---|---|---|
--{category} | --primary, --error | Base tokens (no -color suffix) |
--{category}-{variant} | --primary-hover, --success-light | Token variations |
--{element}-{modifier} | --text-muted, --border-strong | Semantic element tokens |
Use semantic names, not literal values:
| Avoid | Prefer |
|---|---|
--blue, --primary-color | --primary |
--red, --error-color | --error |
--16px | --spacing-md |
#2563eb (hex in code) | var(--primary) |
@layer)Layers provide explicit cascade control regardless of selector specificity:
@layer base, theme, utilities;
@layer utilities {
.hidden { display: none !important; }
}
@layer base {
button { display: inline-block; }
}
/* utilities wins over base, even with lower specificity */
| Layer | Priority | Purpose |
|---|---|---|
reset | Lowest | Normalize browser defaults |
tokens | Low | CSS custom properties |
layout | Medium-Low | Body grid, main structure |
sections | Medium | Header, footer, sidebar |
components | Medium-High | Buttons, cards, form elements |
pages | High | Page-specific overrides |
responsive | Highest | Media query adjustments |
Custom elements (hyphenated tags like <product-card>) have specific display quirks that require understanding.
Browsers treat unknown elements as display: inline, breaking block-level layouts:
<!-- Renders INLINE by default! -->
<product-card>
<img src="product.jpg" alt="..." />
<h3>Product Name</h3>
</product-card>
This causes layout issues because the element doesn't create a block formatting context.
:not(:defined) SolutionThe :not(:defined) pseudo-class matches custom elements that haven't been registered with customElements.define():
/* In reset layer - catches ALL unregistered custom elements */
:not(:defined) {
display: block;
}
This is ideal for CSS-only custom elements that will never be registered as Web Components.
Critical: Unlayered browser defaults beat layered CSS. Even with @layer reset { ... }, browser defaults can override your styles.
/* May NOT work - layer has lower priority than browser default */
@layer reset {
product-card {
display: block;
}
}
/* Solution: :not(:defined) has higher specificity */
:not(:defined) {
display: block;
}
:defined Pseudo-ClassFor registered Web Components, use :defined to style after JavaScript loads:
/* Hide until component is defined */
product-card:not(:defined) {
visibility: hidden;
}
/* Show when registered */
product-card:defined {
visibility: visible;
}
Not all custom elements should be block. Consider the content model:
| Element Type | Display | Examples |
|---|---|---|
| Container/Section | block | product-card, hero-section, card-grid |
| Badge/Indicator | inline-flex | status-badge, tag-item |
| Icon | inline-flex | x-icon |
Elements with phrasing: true in elements.json are designed to be inline.
Styling lists reliably requires understanding browser defaults and specificity.
The most reliable pattern for navigation and card lists:
/* In reset layer */
ul, ol {
list-style: none;
padding: 0;
margin: 0;
}
Warning: list-style-type: none alone may not work in all contexts. Use the shorthand list-style: none for reliability.
When you remove bullets from a list, screen readers may not announce it as a list in Safari/VoiceOver. Add role="list" to preserve semantics:
<ul role="list">
<li>Item with no bullet but announced as list</li>
</ul>
For custom bullets, use the ::marker pseudo-element:
li::marker {
color: var(--primary);
content: "→ ";
}
/* For specific lists */
ul[data-style="checkmarks"] li::marker {
content: "✓ ";
color: var(--success);
}
ol {
counter-reset: list-counter;
list-style: none;
}
ol li {
counter-increment: list-counter;
}
ol li::before {
content: counter(list-counter) ". ";
color: var(--primary);
font-weight: var(--font-weight-semibold);
}
| Pattern | Use Case |
|---|---|
list-style: none | Navigation, card grids, tab lists |
::marker | Prose lists with custom bullet style |
counter() | Numbered steps, ordered lists with custom numbers |
@scope)The @scope at-rule limits selector reach to a specific DOM subtree without increasing specificity. While @layer controls cascade order, @scope controls where selectors can match.
@scope?| Without @scope | With @scope |
|---|---|
| Selectors leak globally | Selectors limited to subtree |
| Need long descendant chains | Short selectors, explicit boundaries |
| High specificity for isolation | Low specificity preserved |
@scope (product-card) {
/* These only match inside <product-card> */
img {
border-radius: var(--radius-md);
}
h3 {
font-size: var(--font-size-lg);
}
}
The scoping root (product-card) doesn't add to selector specificity—img remains (0,0,1).
:scope Pseudo-ClassReference the scoping root itself:
@scope (blog-card) {
:scope {
/* Styles the <blog-card> element */
display: grid;
gap: var(--spacing-md);
}
h3 {
/* Styles <h3> inside <blog-card> */
margin: 0;
}
}
Exclude nested sections with a lower boundary using to:
/* Style card chrome, but not user content inside */
@scope (blog-card) to (.card-content) {
img {
/* Only matches images in card header/footer, not in content */
border: 2px solid var(--border);
}
}
Use cases for donut scope:
@scope with @layerCombine scope and layers for full control:
@layer components {
@scope (product-card) {
:scope {
container-type: inline-size;
padding: var(--spacing-lg);
}
img {
width: 100%;
aspect-ratio: 4/3;
object-fit: cover;
}
@container (min-width: 400px) {
:scope {
display: grid;
grid-template-columns: 200px 1fr;
}
}
}
}
@scope vs Element SelectorsBoth work for our custom element approach:
/* Element selector (our typical pattern) */
product-card {
display: grid;
}
product-card img {
border-radius: var(--radius-md);
}
/* @scope equivalent - cleaner for many child rules */
@scope (product-card) {
:scope {
display: grid;
}
img {
border-radius: var(--radius-md);
}
h3 { }
p { }
footer { }
}
When to use @scope:
When element selectors suffice:
In component HTML, scope without a selector:
<product-card>
<style>
@scope {
:scope { display: grid; }
img { border-radius: var(--radius-md); }
}
</style>
<img src="..." alt="..." />
<h3>Product Name</h3>
</product-card>
The scope automatically targets the parent element.
@scope limits selector reach, not inheritance. Inherited properties like color still cascade into excluded donut holes:
@scope (.card) to (.content) {
:scope {
color: blue; /* .content still inherits blue! */
}
}
To prevent inheritance, reset properties explicitly on the excluded element.
Modern browsers support CSS nesting, reducing repetition:
/* Without nesting */
nav { }
nav ul { }
nav a { }
nav a:hover { }
/* With nesting */
nav {
& ul {
display: flex;
gap: var(--spacing-lg);
}
& a {
padding: var(--spacing-sm) var(--spacing-md);
&:hover {
background: var(--overlay-light);
}
&[aria-current="page"] {
background: var(--overlay-strong);
}
}
}
& for clarity - Always prefix nested selectors with &Media queries can be nested inside selectors:
header {
padding: var(--spacing-lg);
@media (max-width: 768px) {
padding: var(--spacing-md);
}
}
Instead of inventing classes, style semantic elements:
/* Avoid */
.header-nav { }
.nav-list { }
.nav-link { }
/* Prefer */
header nav { }
header nav ul { }
header nav a { }
Custom elements provide semantic styling targets without classes:
/* Instead of .form-group { } */
form-field { }
/* Instead of .product-card { } */
product-card { }
/* Instead of .table-wrapper { } */
table-wrapper { }
Use classes sparingly for:
| Use Case | Example |
|---|---|
| Multi-variant components | .card, .card-featured |
| View transition names | .vt-card-1 (when data-* insufficient) |
| Third-party integration | Classes required by libraries |
Never use classes for state. Use data-* attributes instead.
| Level | Scope | Contents |
|---|---|---|
| Tokens | Entire site | Colors, spacing, typography, effects |
| Layout | Body structure | Grid areas, view transitions, body rules |
| Sections | Recurring site parts | Header, footer, sidebar, navigation |
| Components | Reusable blocks | Cards, buttons, forms, tables, tags |
| Pages | Single page types | Homepage hero, blog post, contact form |
| Scenario | Action |
|---|---|
| New custom element | Create components/_element-name.css |
| New page type with unique styles | Create pages/_page-name.css |
| New recurring section | Create sections/_section-name.css |
| New design token category | Extend _tokens.css |
/* components/_gallery.css */
@layer components {
gallery-grid {
display: grid;
gap: var(--spacing-md);
&[data-columns="2"] { grid-template-columns: repeat(2, 1fr); }
&[data-columns="3"] { grid-template-columns: repeat(3, 1fr); }
&[data-columns="4"] { grid-template-columns: repeat(4, 1fr); }
}
}
/* In main.css, add to appropriate section */
@import "components/_gallery.css" layer(components);
Every partial should follow this structure:
/* components/_example.css */
@layer components {
/* Element styles */
example-element {
/* Base styles */
/* State variants via data attributes */
&[data-state="active"] { }
/* Nested elements */
& .inner { }
/* Responsive adjustments */
@media (max-width: 768px) { }
}
}
Modern browsers handle @import efficiently:
@layer before @importFor very high-traffic sites, you may want to concatenate CSS:
# Simple concatenation for production
cat styles/_reset.css styles/_tokens.css styles/_layout.css > styles/bundle.css
But for most projects, native imports work well.
We use desktop-first with max-width queries, grouped in the responsive layer:
@layer responsive {
@media (max-width: 1024px) {
/* Tablet adjustments */
}
@media (max-width: 768px) {
/* Mobile adjustments */
}
@media (max-width: 480px) {
/* Small mobile adjustments */
}
}
Define breakpoints as documentation (CSS can't use variables in media queries):
/* _tokens.css */
:root {
/* Breakpoints (for reference - use literal values in @media) */
/* --breakpoint-xl: 1280px; */
/* --breakpoint-lg: 1024px; */
/* --breakpoint-md: 768px; */
/* --breakpoint-sm: 480px; */
}
@container)Container queries enable component-scoped responsive design. Unlike media queries (which respond to viewport size), container queries respond to the size of a parent container.
| Media Queries | Container Queries |
|---|---|
| Respond to viewport | Respond to container |
| Global breakpoints | Component-specific |
| Same component, same layout everywhere | Same component adapts to context |
Use case: A card component that displays horizontally in a wide sidebar but stacks vertically in a narrow sidebar—without knowing where it's placed.
Use container-type to establish a containment context:
/* Any element can be a container */
sidebar-panel {
container-type: inline-size; /* Width-based queries */
container-name: sidebar; /* Optional: name for targeting */
}
/* Shorthand */
main-content {
container: content / inline-size; /* name / type */
}
| Type | Queries On | Use When |
|---|---|---|
inline-size | Width only | Most common - responsive layouts |
size | Width and height | Rare - when height matters |
normal | No size queries | Style queries only |
Recommendation: Use inline-size for 99% of cases.
/* Query any ancestor container */
@container (min-width: 400px) {
blog-card {
display: grid;
grid-template-columns: 200px 1fr;
}
}
/* Query a specific named container */
@container sidebar (max-width: 300px) {
blog-card {
flex-direction: column;
}
}
Container-relative units for truly fluid components:
| Unit | Meaning |
|---|---|
cqw | 1% of container width |
cqh | 1% of container height |
cqi | 1% of container inline size |
cqb | 1% of container block size |
cqmin | Smaller of cqi or cqb |
cqmax | Larger of cqi or cqb |
blog-card h3 {
/* Font scales with container width, respects user zoom */
font-size: clamp(1rem, 0.875rem + 0.5cqi, 1.5rem);
}
Combine container units with lh (line-height) units for vertical rhythm:
blog-card {
/* Gap scales with container but rounds to quarter-line increments */
--gap: round(up, 2cqi, 0.25lh);
gap: var(--gap);
}
The round() function ensures spacing aligns to the typographic grid.
Container units cannot measure the element they're applied to. This would create a circular dependency. Use nested elements or wrapper patterns:
/* WRONG - card can't size based on its own container */
product-card {
container-type: inline-size;
padding: 2cqi; /* Measures parent, not self! */
}
/* CORRECT - children measure the card container */
product-card {
container-type: inline-size;
}
product-card > * {
padding: 2cqi; /* Now measures product-card */
}
Container queries integrate naturally with the layer system:
@layer components {
/* Define containers at the component wrapper level */
card-container {
container-type: inline-size;
}
/* Base card styles */
blog-card {
display: flex;
flex-direction: column;
gap: var(--spacing-md);
}
/* Container-responsive layout */
@container (min-width: 500px) {
blog-card {
flex-direction: row;
}
blog-card img {
width: 40%;
flex-shrink: 0;
}
}
}
Make components that adapt without external configuration:
/* components/_product-card.css */
@layer components {
product-card {
/* The card IS its own container */
container-type: inline-size;
display: grid;
gap: var(--spacing-md);
padding: var(--spacing-lg);
}
/* Compact layout (narrow) */
@container (max-width: 299px) {
product-card {
text-align: center;
& img {
margin-inline: auto;
max-width: 150px;
}
}
}
/* Standard layout (medium) */
@container (min-width: 300px) and (max-width: 499px) {
product-card {
grid-template-columns: 1fr;
}
}
/* Wide layout (large) */
@container (min-width: 500px) {
product-card {
grid-template-columns: 200px 1fr;
grid-template-rows: auto 1fr auto;
& img {
grid-row: 1 / -1;
}
}
}
}
Use both—they serve different purposes:
@layer components {
blog-card {
container-type: inline-size;
}
/* Container query: responds to where card is placed */
@container (min-width: 400px) {
blog-card {
grid-template-columns: 150px 1fr;
}
}
}
@layer responsive {
/* Media query: global layout changes */
@media (max-width: 768px) {
.card-grid {
grid-template-columns: 1fr; /* Stack cards on mobile */
}
}
}
Container queries can be nested inside element selectors:
sidebar-panel {
container-type: inline-size;
& blog-card {
padding: var(--spacing-md);
@container (min-width: 350px) {
padding: var(--spacing-lg);
display: grid;
grid-template-columns: 100px 1fr;
}
}
}
When implementing container queries:
container-type: inline-size on the containing elementcontainer-name when multiple containers need targetingmin-width for progressive enhancementcqi, cqw) for fluid typography/spacinground() with lh units for rhythm-aligned spacingSubgrid allows nested elements to participate in their parent's grid, enabling alignment across nested structures without duplicating track definitions.
| Without Subgrid | With Subgrid |
|---|---|
| Nested grids are independent | Child inherits parent's tracks |
| Must duplicate track sizes | Single source of truth |
| Alignment breaks across nesting | Perfect alignment across levels |
/* Parent grid */
.card-grid {
display: grid;
grid-template-columns: repeat(3, 1fr);
gap: var(--spacing-lg);
}
/* Card spans parent columns, subgrids rows */
.card {
display: grid;
grid-template-rows: auto 1fr auto; /* header, content, footer */
}
/* With subgrid: all cards align their internal rows */
.card-grid {
display: grid;
grid-template-columns: repeat(3, 1fr);
grid-template-rows: auto 1fr auto; /* Define rows at parent level */
gap: var(--spacing-lg);
}
.card {
display: grid;
grid-row: span 3;
grid-template-rows: subgrid; /* Inherit parent's row tracks */
}
Align labels and inputs across form fields:
form {
display: grid;
grid-template-columns: max-content 1fr;
gap: var(--spacing-md);
}
form-field {
display: grid;
grid-column: span 2;
grid-template-columns: subgrid;
}
form-field label {
grid-column: 1;
}
form-field input {
grid-column: 2;
}
Cards with aligned headers, content, and footers:
/* Define consistent structure at grid level */
product-grid {
display: grid;
grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
grid-auto-rows: auto 1fr auto; /* image, details, actions */
gap: var(--spacing-lg);
}
product-card {
display: grid;
grid-row: span 3;
grid-template-rows: subgrid;
gap: var(--spacing-md);
}
product-card img { grid-row: 1; }
product-card .details { grid-row: 2; }
product-card .actions { grid-row: 3; }
Inherit both column and row tracks:
.parent {
display: grid;
grid-template-columns: 1fr 2fr 1fr;
grid-template-rows: auto 1fr auto;
}
.child {
grid-column: 1 / -1;
grid-row: 1 / -1;
display: grid;
grid-template-columns: subgrid;
grid-template-rows: subgrid;
}
Named lines pass through to subgrid:
.layout {
display: grid;
grid-template-columns:
[full-start] 1fr
[content-start] minmax(0, 60ch)
[content-end] 1fr
[full-end];
}
.content {
grid-column: full-start / full-end;
display: grid;
grid-template-columns: subgrid;
}
/* Child can use parent's named lines */
.content h1 {
grid-column: content-start / content-end;
}
.content .full-bleed {
grid-column: full-start / full-end;
}
| Use Case | Benefit |
|---|---|
| Card grids | Aligned headers/footers across cards |
| Form layouts | Labels and inputs align vertically |
| Data tables | Column alignment in complex cells |
| Multi-level navigation | Consistent column widths |
| Article layouts | Full-bleed elements with named lines |
Subgrid has good modern browser support (90%+). For older browsers, the fallback is a regular nested grid which may not align perfectly but remains functional.
Logical properties replace physical direction properties (left, right, top, bottom) with flow-relative alternatives. This enables layouts that automatically adapt to different writing modes and text directions.
| Physical Properties | Logical Properties |
|---|---|
| Fixed to screen edges | Adapt to text direction |
| Break in RTL languages | Work in any writing mode |
| Require RTL overrides | Automatically flip |
margin-left: 1rem | margin-inline-start: 1rem |
Benefits:
CSS logical properties use two axes:
| Axis | Direction | Physical Equivalent |
|---|---|---|
| Block | Vertical (in LTR/RTL) | Top ↔ Bottom |
| Inline | Horizontal (in LTR/RTL) | Left ↔ Right |
Each axis has two edges:
| Edge | Block Axis | Inline Axis (LTR) | Inline Axis (RTL) |
|---|---|---|---|
| Start | Top | Left | Right |
| End | Bottom | Right | Left |
| Physical | Logical |
|---|---|
margin-top | margin-block-start |
margin-bottom | margin-block-end |
margin-left | margin-inline-start |
margin-right | margin-inline-end |
Shorthand properties:
/* Two values: start and end */
margin-block: 1rem 2rem; /* top: 1rem, bottom: 2rem */
margin-inline: 1rem 2rem; /* left: 1rem (LTR), right: 1rem (RTL) */
/* Single value: both start and end */
margin-block: 1rem; /* top and bottom */
margin-inline: 1rem; /* left and right */
Same pattern as margins:
padding-block: var(--spacing-lg);
padding-inline: var(--spacing-md);
/* Individual sides */
padding-block-start: var(--spacing-lg);
padding-inline-end: var(--spacing-sm);
| Physical | Logical |
|---|---|
width | inline-size |
height | block-size |
min-width | min-inline-size |
max-height | max-block-size |
blog-card {
inline-size: 100%;
max-inline-size: 40rem;
min-block-size: 200px;
}
| Physical | Logical |
|---|---|
top | inset-block-start |
bottom | inset-block-end |
left | inset-inline-start |
right | inset-inline-end |
Shorthand:
/* All four sides */
inset: 0; /* Same as top: 0; right: 0; bottom: 0; left: 0; */
/* Block and inline axes */
inset-block: 0; /* top and bottom */
inset-inline: 0; /* left and right */
/* Border on one logical side */
border-inline-start: 3px solid var(--primary);
/* Border radius */
border-start-start-radius: var(--radius-lg); /* top-left in LTR */
border-end-start-radius: var(--radius-lg); /* bottom-left in LTR */
| Physical | Logical |
|---|---|
text-align: left | text-align: start |
text-align: right | text-align: end |
/* Center horizontally (works in RTL) */
blog-card {
margin-inline: auto;
max-inline-size: 40rem;
}
/* Space between icon and text, flips in RTL */
button svg {
margin-inline-end: var(--spacing-sm);
}
/* Sidebar on the start edge (left in LTR, right in RTL) */
main-layout {
display: grid;
grid-template-columns: 250px 1fr;
}
sidebar-panel {
border-inline-end: 1px solid var(--border);
padding-inline-end: var(--spacing-lg);
}
/* Accent border on start edge */
blog-card[data-featured] {
border-inline-start: 4px solid var(--primary);
padding-inline-start: var(--spacing-lg);
}
When converting existing CSS:
/* Before */
.card {
margin-left: 1rem;
margin-right: 1rem;
padding-top: 2rem;
padding-bottom: 1rem;
border-left: 3px solid blue;
text-align: left;
}
/* After */
.card {
margin-inline: 1rem;
padding-block: 2rem 1rem;
border-inline-start: 3px solid blue;
text-align: start;
}
Some properties should remain physical:
| Property | Keep Physical When |
|---|---|
top, left, etc. | Fixed position relative to viewport |
transform | Animations that shouldn't flip |
box-shadow | Light source should stay consistent |
background-position | Image positioning shouldn't flip |
/* Physical: shadow direction stays consistent */
blog-card {
box-shadow: 2px 2px 8px oklch(0% 0 0 / 0.15);
}
/* Logical: border flips with text direction */
blog-card {
border-inline-start: 3px solid var(--primary);
}
Define spacing tokens and use them with logical properties:
/* _tokens.css */
:root {
--spacing-xs: 0.25rem;
--spacing-sm: 0.5rem;
--spacing-md: 1rem;
--spacing-lg: 1.5rem;
--spacing-xl: 2rem;
}
/* Component using logical properties with tokens */
article {
padding-block: var(--spacing-xl);
padding-inline: var(--spacing-lg);
margin-block-end: var(--spacing-lg);
}
Logical properties have excellent browser support (95%+). For older browsers:
/* Fallback pattern (only if supporting very old browsers) */
blog-card {
margin-left: 1rem; /* Fallback */
margin-inline-start: 1rem; /* Modern browsers */
}
/* components/_blog-card.css */
@layer components {
blog-card {
display: grid;
gap: var(--spacing-md);
padding: var(--spacing-lg);
background: var(--surface);
border-radius: var(--radius-lg);
box-shadow: var(--shadow-sm);
transition: box-shadow var(--transition-normal);
/* Hover effect */
&:hover {
box-shadow: var(--shadow-md);
}
/* Featured variant */
&[data-featured] {
border-inline-start: 4px solid var(--primary);
}
/* Child elements */
& h3 {
margin: 0;
font-size: var(--font-size-lg);
}
& time {
color: var(--text-muted);
font-size: var(--font-size-sm);
}
& p {
margin: 0;
line-height: var(--line-height-relaxed);
}
/* Responsive */
@media (max-width: 768px) {
padding: var(--spacing-md);
}
}
}
Baseline defines which CSS features are available across all major browsers. Our linter warns when using features outside Baseline Newly available status.
| Status | Meaning | Our Approach |
|---|---|---|
| Widely available | 30+ months in all browsers | Use freely |
| Newly available | Recently in all browsers | Use freely (our threshold) |
| Limited availability | Not in all browsers | Requires @supports |
Features outside Baseline must be wrapped in @supports:
/* Base: Baseline-safe fallback */
p {
word-break: break-word;
}
/* Enhancement: non-Baseline feature */
@supports (text-wrap: pretty) {
p {
text-wrap: pretty;
}
}
The linter allows non-Baseline CSS inside @supports blocks.
Some features we document may not yet be Baseline. Always check and use @supports:
/* contrast-color() - Safari Tech Preview only */
@supports (color: contrast-color(red)) {
.dynamic-bg {
color: contrast-color(var(--bg));
}
}
/* text-wrap: pretty - recently Baseline */
@supports (text-wrap: pretty) {
article p {
text-wrap: pretty;
}
}
npm run lint:css - Linter warns on non-Baseline featuresWhen setting up or reviewing CSS:
layer() syntaxdata-* attributes)responsive layer_tokens.css@scope for components with many child rules or donut patternscolor-scheme: light dark declared in :rootlight-dark() functionlinear-gradient(in oklch, ...)container-type when children need to adaptmargin-inline / padding-block instead of physical directionstext-align: start instead of text-align: left@supportsnpm run lint:css passes without baseline warningsWhen authoring CSS, consider invoking these related skills:
| CSS Feature | Invoke Skill | Why |
|---|---|---|
| Animations, transitions | animation-motion | Proper keyframes, scroll-driven effects, reduced-motion |
| Print styles (@media print) | print-styles | Print-specific layout, page breaks, hiding nav |
| Icon styling | icons | Use <x-icon> component, not inline SVG |
| Dark/light themes | data-attributes | State via data-theme, not classes |
| Responsive images | responsive-images | Image sizing, aspect ratios, art direction |
When styling buttons, toggles, or UI elements that need icons, ensure the HTML uses <x-icon>:
/* Styling icons is simple when using x-icon */
button x-icon {
color: currentColor;
}
button:hover x-icon {
color: var(--primary);
}
See the icons skill before adding any visual indicators to HTML.
<x-icon> Web Component