css-modules

name	css-modules
description	CSS Modules and styling conventions — design tokens, naming, variants, responsive design, layout, dark mode/theming. Use when styling or visually adjusting components, or editing any *.module.css, tokens, or global styles. For animations/transitions/motion use the `motion` skill instead. 日本語の依頼例:「スタイルを当てて」「見た目を整えて」「デザイン調整」「ダークモード」「レスポンシブ対応」「CSS書いて」。

CSS Modules

Files & naming

• One module per component, colocated: UserCard.tsx + UserCard.module.css.
• Class names: camelCase (primaryButton, not primary-button) — enables styles.primaryButton without bracket access.
• Name by role, not appearance: .errorMessage not .redText. The module scope already namespaces — no BEM prefixes needed.
• Import as styles: import styles from './UserCard.module.css'.

Design tokens — single source of truth

• All colors, spacing, radii, shadows, typography, z-index live as CSS custom properties in src/styles/tokens.css, defined on :root.
• Component modules consume tokens only — the semantic/component tier where tiers exist (see design-system), never primitives like --blue-600. A raw hex/px value for a themable property in a component module is a bug.
• Spacing on a scale (--space-1 … --space-8); z-index from a fixed ladder (--z-dropdown, --z-modal, --z-toast) — never z-index: 9999.
• Theming (dark mode) by redefining tokens — components never branch on theme themselves. Default to prefers-color-scheme; add [data-theme] only when a user-facing toggle is required, set pre-paint by a tiny inline script (CSP nonce applies — see frontend-security) to avoid a wrong-theme flash.

Variants & state

• Variants via data attributes, not class string concatenation:

<button className={styles.button} data-variant={variant} data-size={size}>

.button[data-variant='danger'] { background: var(--color-danger); }

• Boolean UI state the same way (data-open, aria-expanded) — style off ARIA attributes when one exists: .menu[aria-expanded='true'].
• clsx has exactly one job here: merging a consumer-passed className prop with the base class. Enumerated variants and boolean state always go through data attributes, never conditional class lists; a component wanting 3+ conditional classes should be restructured into variants.

Layout & responsive

• Mobile-first: base styles, then @media (min-width: …) upward. Breakpoints documented once in tokens.css comments (custom properties don't work in media queries — keep the canonical list there).
• Prefer modern layout primitives: flex gap / grid over margin hacks; aspect-ratio; container queries for components that adapt to their container, media queries only for page-level layout.
• Aspect-preserving media: give the box a defined aspect-ratio and use object-fit: contain/cover. Never size an image with max-width/max-height inside a flex item — the main-axis shrink sizes width independently of the height cap and stretches the image (a mixed-aspect-ratio thumbnail grid is where this bites).
• Logical properties (margin-inline, padding-block, inset-inline-start) over physical ones — free RTL support (the foundation the i18n skill builds on).
• No fixed heights on text containers; min-height if needed.

Globals — the only allowed global CSS

src/styles/ contains exactly: tokens.css, reset.css, globals.css (base element styles: body, headings, links). Nothing else is global. No :global() in component modules except to target third-party library DOM you don't control — with a comment saying which library.

In Astro projects, the per-component styling primitive is the single-file scoped <style> block instead of *.module.css — see astro. The token/naming/variant rules here still apply.

Motion

• Animate transform and opacity only (compositor-friendly); never animate width/height/top/left for transitions.
• Every significant animation (anything beyond a subtle hover/focus transition) respects @media (prefers-reduced-motion: reduce) — same threshold as the a11y skill.
• Durations/easings as tokens (--duration-fast, --ease-out).
• These are the invariants only — escalation (keyframes / WAAPI / animation libraries), View Transitions, entrance/exit animations, and scroll effects live in the motion skill.

Enforcement

The token/var() and class-naming rules above are machine-enforced by Stylelint — see tooling for the exact rules and the shipped templates/stylelint.config.mjs.