CSS

Writes and debugs CSS: flexbox and grid layout, z-index and stacking, responsive breakpoints, theming, and render performance. Use when a layout breaks or overflows, z-index does nothing, the page scrolls sideways, position: sticky won't stick, text won't truncate, styles lose the cascade or need !important, content jumps while loading (CLS), transitions stutter, fonts flash, a modal or dropdown sits behind other content, form controls need restyling, a table needs sticky headers, printing comes out wrong, or a style works in Chrome but not Safari. Also for starting a stylesheet from scratch (reset and base layer), centering, container queries, fluid type with clamp(), design tokens and dark mode, RTL and logical properties, and modernizing old stylesheets with :has(), @layer, and nesting. Not for visual design taste — palettes, spacing scales, typeface choice.

User preferences and memory live in ~/Clawic/data/css/ (see setup.md on first use, memory-template.md for the file format). If you have data at an old location (~/css/ or ~/clawic/css/), move it to ~/Clawic/data/css/.

Configuration

User-dependent variables. Defaults apply until the user states a preference; store them in ~/Clawic/data/css/config.yaml.

Variable	Type	Default	Effect
authoring_mode	plain-css | sass | tailwind | css-in-js	plain-css	Syntax of every emitted example, and which advice in architecture.md applies (layers and nesting vs utility ordering vs zero-runtime extraction)
browser_support	evergreen | widely-available | legacy	evergreen	Gates which features ship bare vs behind @supports: legacy also adds vendor-prefixed fallbacks and blocks the Chromium-first features flagged in compatibility.md
naming_convention	bem | utility | css-modules | none	none	Class names in examples and the specificity ceiling enforced in reviews (architecture.md — Keeping It Clean)
rem_base	number px (10-16)	16	Every px→rem conversion, including the fluid-type derivation in responsive.md (rem term = px ÷ rem_base)
a11y_target	aa | aaa	aa	Which row of Accessibility Floor gates output: AA = 4.5:1 body / 24×24 targets, AAA = 7:1 body / 44×44 targets
explanation_depth	mechanism | fix-only	mechanism	How much of the why ships with each answer: mechanism names the cause (stacking context, flex sizing) before the declaration; fix-only emits the declaration plus one line
output_shape	diff | full-sheet	diff	Emitted code: diff = only the changed declarations in context; full-sheet = the complete rewritten stylesheet or component block

Preference areas to record as the user reveals them:

• tooling — build chain (PostCSS, Lightning CSS, bundler), formatter and lint rules, whether native nesting is allowed
• conventions — spacing and type scale in use, token naming, file-per-component vs grouped sheets
• output — beyond explanation_depth and output_shape: comment density in emitted CSS, longhand vs shorthand, whether fallbacks and @supports branches are shown or assumed, how much of the trade-off to state before choosing
• platform — target surfaces (app, marketing site, HTML email, print/PDF), device mix, whether RTL or CJK is in scope
• risk posture — appetite for Chromium-first features, tolerance for !important in third-party overrides, how loudly to flag accessibility regressions
• constraints — banned techniques (CSS-in-JS, utility classes, @import), inherited legacy stylesheets that must keep working

When To Use

• Debugging layout: z-index that won't apply, overflow, dead height: 100%, broken position: sticky, mystery horizontal scroll
• Building components and starting stylesheets: reset and base layers, flexbox/grid patterns, centering, forms, tables, overlays, responsive behavior without media-query sprawl
• Production hardening: layout shift, animation jank, font loading, print output, the accessibility floor
• Replacing JS or preprocessor hacks with native CSS (:has(), @layer, @scope, container queries, scroll snap, anchor positioning)
• Cross-engine work: a rule that lands in Chrome and not Safari, RTL mirroring, HTML email constraints
• Not for visual design decisions (palettes, spacing scales, typography choice) — this skill covers mechanics, not taste

Quick Reference

Situation	Play
z-index ignored despite a huge value	Stacking Contexts below — find the context root; never just bump the number
Flex item overflows / text won't truncate	min-width: 0 on the flex child (default min-width is min-content)
The rule is written but nothing changes	debugging.md — symptom→cause chains, starting with "is it even matching?"
Breaks with real content, sticky dead, footer floats, margin leaks	layout.md
Component must adapt to its container; fluid type; mobile viewport bugs	responsive.md
Specificity fight, @layer, :has(), custom-property gotchas	selectors.md
Jank, layout shift, slow paint, font flash	performance.md
Transition never fires, enter/exit animation, view transitions, reduced motion	animations.md
Line-height inheritance, measure, variable fonts, hyphenation, tabular numbers	typography.md
oklch vs hsl, color-mix(), gradient banding, wide gamut	color.md
Dark mode, design tokens, multi-brand, theme flash on load	theming.md
Inputs, selects, checkboxes, validation states, autofill styling	forms.md
Modal behind the header, dropdown clipped, tooltip placement	overlays.md
Scroll snap, custom scrollbars, anchor link lands under the sticky header	scrolling.md
Data table: sticky header, responsive behavior, column sizing	tables.md
Shadows, filters, masks, clip-path, blend modes, 3D transforms	effects.md
Starting a stylesheet: what goes in the reset and base layers, in what order	reset.md
Sheet organization, layer strategy, nesting, Sass migration, shadow DOM	architecture.md
Works in Chrome, breaks in Safari or Firefox; HTML email	compatibility.md
Print or PDF output wrong	print.md
RTL mirroring, CJK line breaking, logical properties, text expansion	internationalization.md
Anything else CSS	Core Rules below, then debugging.md to name the mechanism

Core Rules

1. Diagnose before adding CSS: reproduce, isolate in DevTools, name the mechanism (stacking context, flex sizing algorithm, margin collapse). A property added without a named mechanism is the next bug.
2. Animate only transform and opacity — the only common properties that skip layout and paint. Frame budget = 1000ms / 60fps ≈ 16.7ms for style, paint, and your JS combined; one layout-triggering animation spends it alone.
3. One centering default: parent display: grid; place-content: center. Escape hatch: position: absolute; inset: 0; margin: auto when the child must overlay (needs a resolvable size, e.g. width: fit-content).
4. Never bare viewport units for text. font-size: clamp(1rem, 0.77rem + 0.91vw, 1.5rem) — the rem term is what keeps browser zoom and user font-size working; pure-vw text fails WCAG 1.4.4 (resize to 200%). Derivation of the numbers: responsive.md.
5. Size intrinsically first (min(), clamp(), fit-content, auto-fit grids), media queries second, container queries when one component lives at different widths.
6. !important in component code is a debt marker. Order wars belong in @layer — unlayered author styles beat all layered ones regardless of specificity (selectors.md).
7. Overlays belong in the top layer, not high in the z-index scale. <dialog>.showModal() and popover escape every stacking context and every overflow: hidden ancestor; a z-index arms race means the wrong mechanism is in use (overlays.md).
8. A component styles its inside, never its outside: no margin, no width, no position on the component root — the parent layout owns placement. Components that set their own outer geometry break on the second reuse and get "fixed" with !important.

Stacking Contexts

The single most common CSS debugging failure: raising z-index on an element trapped inside a context.

• Context creators (memorize): positioned element with z-index, flex/grid child with z-index, opacity < 1, transform, filter, backdrop-filter, will-change, contain: layout or paint, position: fixed/sticky, isolation: isolate.
• Inside a context, z-index competes only among siblings of that context. A child's z-index: 9999 never escapes its parent's z-index: 1.
• Debug procedure, in order: (1) walk up from the losing element to its first context-creating ancestor; (2) same for the winning element; (3) compare those two ancestors — that comparison decides the paint order; (4) fix z-index there, or delete the accidental trigger (usually a leftover transform or opacity from an animation).
• isolation: isolate creates a context with zero visual side effects — use it to cap a component's internal z-index so it can't leak out.
• transform, filter, and will-change also make the element the containing block for position: fixed descendants — the fixed element behaves as absolute with no warning. Same walk-up diagnosis.
• Elements promoted to the top layer (modal <dialog>, popover) ignore all of the above: they paint above the page and above each other in open order (rule 7).

Flexbox and Grid Mental Model

• flex: 1 = 1 1 0%: ALL space divided equally. flex: auto = 1 1 auto: only leftover space divided, so larger content keeps a larger track. Choose per intent; equal columns need basis 0.
• Flex children default to min-width: min-content — the root cause of both overflow and un-truncatable text. Release with min-width: 0 (or overflow: hidden). Column direction: same story with min-height.
• 1fr means minmax(auto, 1fr): the track refuses to shrink below its content. grid-template-columns: 1fr 1fr is NOT 50/50 with unequal content — write minmax(0, 1fr) for true halves.
• auto-fit collapses empty tracks (remaining cards stretch); auto-fill keeps them (cards hold max width). Card grid default: repeat(auto-fit, minmax(min(250px, 100%), 1fr)) — the inner min() prevents overflow on viewports under 250px.
• gap never collapses; margins collapse (vertical, block layout only, including parent-child bleed-through). Prefer gap and treat margin collapse as legacy behavior to route around (layout.md).
• margin: auto on a flex/grid child absorbs free space: margin-inline-start: auto on the last nav item is the entire "push right" pattern.
• Grid when the parent decides both axes (page scaffolding, card grids, overlapping layers via named areas); flex when the children decide and simply wrap (toolbars, tag lists, button rows).

Modern CSS Worth Using

Compatibility floor: everything here is in all three engines unless marked; version-sensitive items are dated in compatibility.md.

• :has() — parent and previous-sibling selection; kills a whole class of state-mirroring JS (selectors.md for patterns and cost).
• @starting-style + transition-behavior: allow-discrete — transition from display: none; replaces enter-animation JS (all engines since mid-2024).
• light-dark() + color-scheme — one declaration per token instead of a duplicated dark block (all engines since 2024; theming.md).
• text-wrap: balance on headings — engines skip long blocks (Chromium caps at 6 lines), so it is safe to apply to all headings.
• scrollbar-gutter: stable on scroll containers — reserves the gutter, no shift when the scrollbar appears.
• overscroll-behavior: contain on modals and drawers — stops scroll chaining into the page.
• scroll-snap-type + scroll-snap-align — carousels without JS (scrolling.md).
• aspect-ratio — reserve media space before load (layout-shift numbers: performance.md).
• accent-color — form controls on brand without rebuilding them (forms.md).
• @scope and native nesting — component boundaries without naming conventions; specificity traps in architecture.md.
• Individual transforms (translate, rotate, scale) — compose in a fixed order and animate independently, no more one-property transform collisions.
• Anchor positioning (anchor-name, position-area) — tethered popovers without a positioning library; still needs a fallback, see overlays.md.

Accessibility Floor

Canonical home for these numbers — other files point here.

• Contrast (WCAG 2.2 AA): 4.5:1 body text; 3:1 for large text (≥24px, or ≥18.66px bold) and for UI components and focus indicators (1.4.3, 1.4.11). AAA raises body text to 7:1 and large text to 4.5:1 (1.4.6) — applies when a11y_target: aaa.
• Touch targets: ≥24×24 CSS px is the AA minimum (2.5.8); 44×44 matches Apple HIG and WCAG AAA (2.5.5) — use 44 for primary mobile actions and whenever a11y_target: aaa.
• Text survives 200% zoom (1.4.4): rem-based sizes plus the clamp rule (Core Rule 4).
• Motion is opt-in: wrap animation in @media (prefers-reduced-motion: no-preference) rather than overriding after the fact.
• Style :focus-visible; never outline: none without a replacement in the same rule.
• @media (forced-colors: active): system colors replace yours — check borders and focus still exist there.
• Dark mode: @media (prefers-color-scheme: dark) plus color-scheme: light dark so form controls and scrollbars follow.
• Content reflows to a 320px-wide viewport without two-axis scrolling (1.4.10) — the practical floor for "does it work zoomed on a phone".
• Three different hides, chosen deliberately: display: none / visibility: hidden remove content from the accessibility tree; aria-hidden hides from assistive tech while staying visible; screen-reader-only text needs the clip pattern — position: absolute; width: 1px; height: 1px; overflow: hidden; clip-path: inset(50%); white-space: nowrap.

Output Gates

Before shipping a stylesheet or component styles, verify:

• Hostile content survives: longest word (URL, German compound), empty state, missing image, 3-line title where you designed one line?
• 200% zoom and a 320px viewport reflow without horizontal scroll?
• Every interactive element has a visible :focus-visible style and meets the contrast and target-size rows above?
• Animation wrapped in prefers-reduced-motion: no-preference, and only transform/opacity in the frame loop?
• Media and embeds have reserved space (aspect-ratio or width/height attributes)?
• No !important outside a third-party override, and no new selector above single-class specificity?
• Dark mode checked if the project themes, and RTL checked if internationalization.md is in scope?

Traps

Trap	Why it fails	Do instead
Bumping z-index to 9999	Element is inside a stacking context; only the context root competes outside	Walk-up procedure (→ Stacking Contexts)
Animating height/top/left/margin	Layout runs every frame and blows the 16.7ms budget (Core Rule 2)	transform; for height-to-auto, the grid-rows trick (→ layout.md)
overflow: hidden to kill a stray scrollbar	Hides the symptom and creates a scroll container: breaks sticky descendants, clips shadows and focus rings	Find the overflowing element first; when it cannot be removed, html { overflow-x: clip } — clip creates no scroll container, so sticky survives (→ layout.md)
var(--x, fallback) as a safety net	A declared-but-invalid value skips the fallback ("invalid at computed-value time")	@property with initial-value (→ selectors.md)
Global will-change or translateZ(0) "GPU hints"	Every layer holds GPU memory; hundreds of layers slow compositing	will-change only on elements actually animating, only while animating (→ performance.md)
100vh full-screen sections	Mobile browser UI overlaps the bottom of the section	100svh; dvh only when live resize is acceptable (→ responsive.md)
!important to win a specificity fight	Escalation is one-way; the next override needs another !important	@layer ordering (→ selectors.md)
:empty for empty states	Whitespace text nodes count as content in most engines	Control the markup, or a class set by the renderer
transition: all	Animates properties added later — including layout ones — and turns theme swaps into visible sweeps	Enumerate the properties you mean (→ animations.md)
:invalid for error styling	Matches an untouched empty required field on first paint: the form is red before typing	:user-invalid (→ forms.md)
line-height: 150%	Percentages inherit the COMPUTED value, so a big heading inherits the body's pixel leading	Unitless line-height: 1.5 (→ typography.md)
display: none for screen-reader-only text	Removes it from the accessibility tree — nobody hears it	The clip pattern (→ Accessibility Floor)
text-align: left and margin-left in a themeable product	The first RTL locale mirrors everything except your CSS	Logical properties (→ internationalization.md)

Where Experts Disagree

• Selector performance: the old guard writes selectors for right-to-left matching cost; modern engines bucket by rightmost simple selector, making it negligible. Boundary: only act on a DevTools trace showing Style/Recalculate cost — usually :has() or universal selectors on large, frequently-mutating DOMs (performance.md).
• Utility-first vs handwritten CSS: utilities win on team consistency and dead-code elimination; handwritten wins for animation-heavy and design-led work. Boundary: follow whichever the codebase already uses; never mix systems inside one component.
• CSS-in-JS: colocation and typed themes vs runtime cost. Boundary: server-rendered, performance-critical pages want zero-runtime extraction (or plain CSS + @layer); internal dashboards can afford runtime styling.
• Contrast math: WCAG 2.x ratios are the legally referenced standard but misjudge dark backgrounds and thin type; APCA (the WCAG 3 candidate) models them better and is not yet normative. Boundary: ship to WCAG 2.2 numbers for compliance, use APCA as a tiebreaker when two palettes both pass.

Related Skills

More Clawic skills, get them at https://clawic.com/skills/css (install if the user confirms):

• html - semantic structure and document-level concerns the CSS hooks into
• frontend - component architecture, frameworks, and build tooling around the styles
• animations - motion design and choreography beyond single-property transitions
• accessibility-audit - full WCAG review beyond the CSS floor here
• design-system - tokens, theming, and scaling styles across a product

Feedback

• If useful, star it: https://clawic.com/skills/css
• Latest version: https://clawic.com/skills/css

Part of Clawic, the verified skill library. Get this skill: https://clawic.com/skills/css.