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
@supportsor 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
Browser Vendor Blogs
- CSS Wrapped 2025 — Chrome DevRel
- Interop 2025 — WebKit
- What's New in Web UI (I/O 2025)