Robust-skills modern-css

Proactively apply when creating design systems, component libraries, or any frontend application. Triggers on CSS Grid, Subgrid, Flexbox, Container Queries, :has(), @layer, @scope, CSS nesting, @property, @function, if(), oklch, color-mix, light-dark, relative color, @starting-style, scroll-driven animations, view transitions, anchor positioning, popover, customizable select, content-visibility, logical properties, text-wrap, interpolate-size, clamp, field-sizing, modern CSS, CSS architecture, responsive design, dark mode, theming, design tokens, cascade layers. Use when writing CSS for any web project, choosing layout approaches, building responsive components, implementing dark mode or theming, creating animations or transitions, styling form elements, or modernizing legacy stylesheets. Modern CSS features and best practices for building interfaces with pure native CSS.

install

source · Clone the upstream repo

git clone https://github.com/ccheney/robust-skills

Claude Code · Install into ~/.claude/skills/

T=$(mktemp -d) && git clone --depth=1 https://github.com/ccheney/robust-skills "$T" && mkdir -p ~/.claude/skills && cp -r "$T/skills/modern-css" ~/.claude/skills/ccheney-robust-skills-modern-css && rm -rf "$T"

manifest: skills/modern-css/SKILL.md

source content

Modern CSS

Pure native CSS for building interfaces — no preprocessors, no frameworks.

When to Use (and When NOT to)

Use Freely (Baseline)	Feature-Detect First
CSS Grid, Subgrid, Flexbox	`@function` , `if()` (Chrome-only)
Container Queries (size + style)	Customizable `<select>` (Chrome-only)
`:has()` , `:is()` , `:where()`	Scroll-state queries (Chrome-only)
CSS Nesting, `@layer` , `@scope`	`sibling-index()` , `sibling-count()`
`@property` (typed custom props)	`::scroll-button()` , `::scroll-marker`
`oklch()` , `color-mix()` , `light-dark()`	Typed `attr()` beyond `content`
Relative color syntax	`field-sizing: content`
`@starting-style` , `transition-behavior`	`interpolate-size` (Chrome-only)
Scroll-driven animations	Grid Lanes / masonry (experimental)
Anchor positioning, Popover API	`random()` (Safari TP only)
`text-wrap: balance` , `linear()` easing	`@mixin` / `@apply` (no browser yet)
View Transitions, logical properties

CRITICAL: The Modern Cascade

Understanding how styles resolve is the single most important concept in CSS. The additions of

@layer

and

@scope

fundamentally changed the cascade algorithm.

Style Resolution Order (highest priority wins):
┌─────────────────────────────────────────────────┐
│ 1. Transitions (active transition wins)         │
│ 2. !important (user-agent > user > author)      │
│ 3. @layer order (later layer > earlier layer)   │
│ 4. Unlayered styles (beat ALL layers)           │
│ 5. Specificity (ID > class > element)           │
│ 6. @scope proximity (closer root wins)      NEW │
│ 7. Source order (later > earlier)               │
└─────────────────────────────────────────────────┘

Unlayered > Last layer > ... > First layer
           (utilities)        (reset)

Cascade layers (

@layer

) and scope proximity (

@scope

) are now more powerful than selector specificity. Define your layer order once (

@layer reset, base, components, utilities;

) and specificity wars disappear. Unlayered styles always beat layered styles — use this for overrides.

Quick Decision Trees

"How do I lay this out?"

Layout approach?
├─ 2D grid (rows + columns)         → CSS Grid
│  ├─ Children must align across    → Grid + Subgrid
│  └─ Waterfall / masonry           → grid-lanes (experimental)
├─ 1D row OR column                 → Flexbox
├─ Component adapts to container    → Container Query + Grid/Flex
├─ Viewport-based responsiveness    → @media range syntax
└─ Element sized to content         → fit-content / min-content / stretch

"How do I style this state?"

Style based on what?
├─ Child/descendant presence        → :has()
├─ Container size                   → @container (inline-size)
├─ Container custom property        → @container style()
├─ Scroll position (stuck/snapped)  → scroll-state() query
├─ Element's own custom property    → if(style(...))
├─ Browser feature support          → @supports
├─ User preference (motion/color)   → @media (prefers-*)
└─ Multiple selectors efficiently   → :is() / :where()

"How do I animate this?"

Animation type?
├─ Enter/appear on DOM              → @starting-style + transition
├─ Exit/disappear (display:none)    → transition-behavior: allow-discrete
├─ Animate to/from auto height      → interpolate-size: allow-keywords
├─ Scroll-linked (parallax/reveal)  → animation-timeline: scroll()/view()
├─ Page/view navigation             → View Transitions API
├─ Custom easing (bounce/spring)    → linear() function
└─ Always: respect user preference  → @media (prefers-reduced-motion)

What CSS Replaced JavaScript For

JavaScript Pattern	CSS Replacement
Scroll position listeners	Scroll-driven animations
IntersectionObserver for reveal	`animation-timeline: view()`
Sticky header shadow toggle	`scroll-state(stuck: top)`
Floating UI / Popper.js	Anchor positioning
Carousel prev/next/dots	`::scroll-button()` , `::scroll-marker`
Auto-expanding textarea	`field-sizing: content`
Staggered animation delays	`sibling-index()`
`max-height: 9999px` hack	`interpolate-size: allow-keywords`
Parent element selection	`:has()`
Theme toggle logic	`light-dark()` + `color-scheme`
Tooltip/popover show/hide	Popover API + invoker commands
Color manipulation functions	`color-mix()` , relative color syntax

For non-Baseline features, always feature-detect with
@supports
or use progressive enhancement. Check MDN or Baseline for current browser support.

Anti-Patterns (CRITICAL)

Anti-Pattern	Problem	Fix
Overusing `!important`	Specificity arms race	Use `@layer` for cascade control
Deep nesting ( `.a .b .c .d` )	Fragile, DOM-coupled	Flat selectors, `@scope`
IDs for styling ( `#header` )	Too specific to override	Classes ( `.header` )
`@media` for component layout	Viewport-coupled, not reusable	Container queries
JS scroll listeners for effects	Janky, expensive	Scroll-driven animations
JS for tooltip positioning	Floating UI dependency	Anchor positioning
JS for carousel controls	Fragile, a11y issues	`::scroll-button` , `::scroll-marker`
JS for auto-expanding textarea	Unnecessary complexity	`field-sizing: content`
`max-height: 9999px` for animation	Wrong duration, janky	`interpolate-size: allow-keywords`
`margin-left` / `padding-right`	Breaks in RTL/vertical	Logical properties ( `margin-inline-start` )
`rgba()` with commas	Legacy syntax	`rgb(r g b / a)` space-separated
`appearance: none` on selects	Removes ALL functionality	`appearance: base-select`
Preprocessor-only variables	Can't change at runtime	CSS custom properties
Preprocessor-only nesting	Extra build step dependency	Native CSS nesting
Preprocessor color functions	Can't respond to context	`color-mix()` , relative colors
`text-wrap: balance` on paragraphs	Performance-heavy	Only headings/short text
`content-visibility` above fold	Delays LCP rendering	Only off-screen sections
Overusing `will-change`	Wastes GPU memory	Apply only to animating elements

Reference Documentation

File	Purpose
references/CASCADE.md	Nesting, `@layer` , `@scope` , cascade control, and CSS architecture
references/LAYOUT.md	Grid, Subgrid, Flexbox, Container Queries, and intrinsic sizing
references/SELECTORS.md	`:has()` , `:is()` , `:where()` , pseudo-elements, and state-based selection
references/COLOR.md	OKLCH, `color-mix()` , relative colors, `light-dark()` , and theming
references/TOKENS.md	`@property` , `@function` , `if()` , math functions, and design tokens
references/ANIMATION.md	`@starting-style` , `interpolate-size` , `linear()` , view transitions
references/SCROLL.md	Scroll-driven animations, scroll-state queries, native carousels
references/COMPONENTS.md	Customizable `<select>` , popover, anchor positioning, `field-sizing`
references/PERFORMANCE.md	`content-visibility` , typography, logical properties, accessibility
references/CHEATSHEET.md	Quick reference: browser support, legacy→modern patterns, units

Sources

Official Specifications

CSS Snapshot 2025 — W3C
CSS Values and Units Level 5 —
```
if()
```
,
```
random()
```
,
```
sibling-index/count()
```
CSS Functions and Mixins Level 1 —
```
@function
```
,
```
@mixin
```
CSS Conditional Rules Level 5 — Scroll-state queries
CSS Anchor Positioning
CSS Overflow Level 5 — Scroll markers/buttons