CSS

Write modern CSS with proper stacking contexts, layout patterns, responsive techniques, and performance optimization.

When to Use

User needs CSS expertise — from layout challenges to production optimization. Agent handles stacking contexts, flexbox/grid patterns, responsive design, performance, and accessibility.

Quick Reference

Topic	File
Layout patterns	layout.md
Responsive techniques	responsive.md
Selectors and specificity	selectors.md
Performance optimization	performance.md

CSS Philosophy

• Layout should be robust—work with any content, not just demo content
• Use modern features—they have better browser support than you think
• Prefer intrinsic sizing—let content determine size when possible
• Test with extreme content—longest names, missing images, empty states

Stacking Context Traps

• z-index only works with positioned elements—or flex/grid children
• isolation: isolate creates stacking context—contains z-index chaos without position
• opacity < 1, transform, filter create stacking context—unexpected z-index behavior
• New stacking context resets z-index hierarchy—child z-index:9999 won't escape parent

Layout Traps

• Margin collapse only vertical, only block—flex/grid children don't collapse
• overflow: hidden on flex container can break—use overflow: clip if you don't need scroll

Flexbox Traps

• flex: 1 means flex: 1 1 0%—basis is 0, not auto
• min-width: 0 on flex child for text truncation—default min-width is min-content
• flex-basis vs width: basis is before grow/shrink—width is after, basis preferred
• gap works in flex now—no more margin hacks for spacing

Grid Traps

• fr units don't respect min-content alone—use minmax(min-content, 1fr)
• auto-fit vs auto-fill: fit collapses empty tracks, fill keeps them
• grid-template-columns: 1fr 1fr is not 50%—it's equal share of REMAINING space
• Implicit grid tracks can surprise you—items placed outside explicit grid still appear

Responsive Philosophy

• Start mobile-first—min-width media queries, base styles for mobile
• Container queries: @container (min-width: 400px)—component-based responsive
• container-type: inline-size on parent required—for container queries to work
• Test on real devices—emulators miss touch targets and real performance

Sizing Functions

• clamp(min, preferred, max) for fluid typography—clamp(1rem, 2.5vw, 2rem)
• min() and max()—width: min(100%, 600px) replaces media query
• fit-content sizes to content up to max—width: fit-content or fit-content(300px)

Modern Selectors

• :is() for grouping—:is(h1, h2, h3) + p less repetition
• :where() same as :is() but zero specificity—easier to override
• :has() parent selector—.card:has(img) styles card containing image
• :focus-visible for keyboard focus only—no outline on mouse click

Scroll Behavior

• scroll-behavior: smooth on html—native smooth scroll for anchors
• overscroll-behavior: contain—prevents scroll chaining to parent/body
• scroll-snap-type and scroll-snap-align—native carousel without JS
• scrollbar-gutter: stable—reserves scrollbar space, prevents layout shift

Shorthand Traps

• inset: 0 equals top/right/bottom/left: 0—less repetition
• place-items is align-items + justify-items—place-items: center centers both
• margin-inline, margin-block for logical properties—respects writing direction

Performance Mindset

• contain: layout isolates repaints—use on independent components
• content-visibility: auto skips offscreen rendering—huge for long pages
• will-change sparingly—creates layers, uses memory
• Avoid layout thrash—batch reads and writes to DOM

Accessibility Baseline

• prefers-reduced-motion: reduce—disable animations for vestibular disorders
• prefers-color-scheme—@media (prefers-color-scheme: dark) for dark mode
• forced-colors: active—adjust for Windows high contrast
• Focus indicators must be visible—don't rely on color alone