Modern Css

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

Browser Vendor Blogs

• CSS Wrapped 2025 — Chrome DevRel
• Interop 2025 — WebKit
• What's New in Web UI (I/O 2025)

Reference

• MDN Web Docs: CSS
• State of CSS 2025
• What You Need to Know About Modern CSS (2025)
• CSS in 2026

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: ccheney
• Source: ccheney/robust-skills
• License: MIT

Install and usage instructions live in the source repository linked above.