tailwindcss

What this skill does

# Tailwind CSS v4

**Utility classes are the default. Custom CSS is the escape hatch.**

<prerequisite>
**Tailwind builds on CSS fundamentals.** Before writing or reviewing
Tailwind code, invoke the `css` skill to load specificity, box model,
and layout knowledge.

```
Skill(frontend:css)
```

Skip only for trivial class additions where no CSS reasoning is needed.

</prerequisite>

Tailwind CSS uses CSS-first configuration: design tokens live in `@theme`, custom utilities use `@utility`, and there is
no JavaScript configuration file. Constrain yourself to the design system; break out only with intention.

## References

- **Theme** — [`${CLAUDE_SKILL_DIR}/references/theme-configuration.md`]: Theme tokens, `@theme` options, namespace
  mapping, color system
- **Class authoring** — [`${CLAUDE_SKILL_DIR}/references/class-authoring.md`]: Class composition, variants, dark mode,
  breakpoints
- **Custom utilities** — [`${CLAUDE_SKILL_DIR}/references/custom-utilities-and-variants.md`]: `@utility`,
  `@custom-variant`, directives, `@source`
- **Layout** — [`${CLAUDE_SKILL_DIR}/references/layout.md`]: Display, position, flexbox, grid, alignment, order
  utilities
- **Sizing** — [`${CLAUDE_SKILL_DIR}/references/sizing-and-spacing.md`]: Spacing scale, width/height, padding/margin,
  borders, box model
- **Typography** — [`${CLAUDE_SKILL_DIR}/references/typography.md`]: Font properties, text spacing, styling, decoration,
  layout
- **Backgrounds** — [`${CLAUDE_SKILL_DIR}/references/backgrounds-and-effects.md`]: Gradients, shadows, rings, opacity,
  SVG, filters
- **Transforms** — [`${CLAUDE_SKILL_DIR}/references/transforms-and-animations.md`]: Transitions, animations, 2D/3D
  transforms, masks
- **Framework** — [`${CLAUDE_SKILL_DIR}/references/framework-integration.md`]: Preflight, CSS Modules, class binding
  (React, Vue, Svelte)

## Entry Point and Installation

- Single import: `@import "tailwindcss";` — provides preflight reset, theme variables, and all utilities. No
  `@tailwind base/components/utilities` (v3 syntax)
- Vite: install `@tailwindcss/vite` plugin. PostCSS: install `@tailwindcss/postcss`. CLI:
  `npx @tailwindcss/cli -i input.css -o output.css`
- No `tailwind.config.js` in v4 — all configuration lives in CSS via `@theme`
- Remove `postcss-import` and `autoprefixer` — v4 handles both internally
- Do not use Sass, Less, or Stylus with Tailwind v4 — Tailwind is the preprocessor (handles `@import`, nesting,
  variables, vendor prefixes)

## Theme Configuration (`@theme`)

### Core Rules

- `@theme` defines design tokens that generate utility classes — not equivalent to `:root`. Use `@theme` for values
  needing utilities; use `:root` for CSS variables that only need `var()` access
- `@theme` must be top-level (not nested under selectors or media queries)
- All `@theme` values compile to `:root { }` CSS vars in output
- Only used CSS vars are emitted by default
- Semantic token names: `--color-primary`, `--color-surface` — not `--color-blue-500` or `--color-gray-100`
- OKLCH for custom colors: `oklch(0.72 0.11 178)` — perceptually uniform, works with CSS `color-mix()`

### `@theme` Options

- **`@theme { }`** — Default: only emit used vars
- **`@theme static { }`** — Always emit all vars
- **`@theme inline { }`** — Inline `var()` references into utility output

Use `@theme inline` when a token references another variable — prevents CSS variable resolution failures in the cascade.

### Namespace → Utility Mapping

- **`--color-*`** → `bg-*`, `text-*`, `border-*`, `ring-*`, `fill-*`, `stroke-*`, etc.
- **`--font-*`** → `font-*` (family)
- **`--text-*`** → `text-*` (size)
- **`--font-weight-*`** → `font-*` (weight)
- **`--tracking-*`** → `tracking-*`
- **`--leading-*`** → `leading-*`
- **`--breakpoint-*`** → Responsive variants: `sm:*`, `md:*`
- **`--container-*`** → Container query variants: `@sm:*`, and `max-w-*`
- **`--spacing-*` or `--spacing`** → `px-*`, `py-*`, `m-*`, `w-*`, `h-*`, etc.
- **`--radius-*`** → `rounded-*`
- **`--shadow-*` / `--inset-shadow-*`** → `shadow-*` / `inset-shadow-*`
- **`--blur-*`** → `blur-*`
- **`--ease-*`** → `ease-*`
- **`--animate-*`** → `animate-*`

Breakpoints generate variants, not utilities. Colors generate multiple utility families from a single namespace.

### Extending, Replacing, Resetting

- **Extend:** Add new tokens alongside defaults — just declare new vars in `@theme`
- **Override:** Redeclare a default var to change its value
- **Reset namespace:** `--color-*: initial` removes all defaults in that namespace
- **Reset everything:** `--*: initial` for fully custom theme
- **Disable specific colors:** `--color-lime-*: initial`

### Colors

- 22 color families x 11 steps (50-950) plus `black` and `white`
- Every `--color-*` token generates utilities across `bg-*`, `text-*`, `border-*`, `ring-*`, `fill-*`, `stroke-*`, etc.
- Opacity modifier: `bg-sky-500/50` — per-property, not whole-element
- `--alpha()` for CSS opacity: compiles to `color-mix(in oklab, ...)`
- Never use `bg-opacity-*` (removed in v4) — always `bg-color/opacity`

### Sharing Themes

Put `@theme` in a standalone CSS file and `@import` it after `@import "tailwindcss"`.

## Class Authoring

### Fundamental Rules

- **Complete class names only.** Never concatenate or interpolate — `text-red-600` yes, `` `text-${color}-600` `` never.
  Tailwind scans source files as plain text
- Map dynamic values to static class string lookups
- **Prettier plugin for ordering.** Install `prettier-plugin-tailwindcss` — do not manually sort classes
- **CSS variable shorthand:** `bg-(--brand-color)` — parenthesis syntax auto-wraps in `var()`. Do not use
  `bg-[var(--brand)]` (v3 verbose form)
- **Modifiers stack left-to-right** (v4): `dark:lg:hover:bg-indigo-600`. v3 was right-to-left — reverse stacking order
  when migrating
- **Arbitrary values for one-offs only.** Repeated values belong in `@theme`
- **Important suffix:** `bg-red-500!` — the `!` goes at end, after all modifiers
- **Conflict resolution:** Last class in the generated stylesheet wins, not last in the HTML attribute. Don't rely on
  attribute order — use conditional rendering
- **Underscores = spaces** in arbitrary values: `grid-cols-[1fr_500px_2fr]`. Escape for literal underscore:
  `content-['hello\_world']`
- **Type hints** for ambiguous CSS vars: `text-(length:--my-var)` for font-size, `text-(color:--my-var)` for text color

### Responsive Breakpoints (Mobile-First)

Unprefixed = all sizes. Prefix = that breakpoint **and up**.

- **`sm:`** — 40rem (640px)
- **`md:`** — 48rem (768px)
- **`lg:`** — 64rem (1024px)
- **`xl:`** — 80rem (1280px)
- **`2xl:`** — 96rem (1536px)

- Don't use `sm:` to mean "mobile only" — it means 640px and up
- Unprefixed for mobile base, override at breakpoints
- Range targeting: `md:max-xl:flex` (only between md and xl)
- Arbitrary breakpoints: `min-[900px]:grid-cols-3`
- Custom breakpoints: define in `@theme { --breakpoint-xs: 30rem; }`

### Container Queries

- `@container` on parent, `@md:flex-row` on children
- Named containers: `@container/main` + `@sm/main:flex-col`
- Sizes range `@3xs` (16rem) through `@7xl` (80rem)
- Arbitrary: `@min-[475px]:flex-row`
- Customize via `--container-*` in `@theme`

### State Variants

- **Pseudo-classes:** `hover:`, `focus:`, `active:`, `visited:`, `focus-visible:`, `focus-within:`, `disabled:`,
  `required:`, `invalid:`, `checked:`, `read-only:`, `indeterminate:`, `first:`, `last:`, `odd:`, `even:`, `empty:`
- **Conditional:** `has-checked:` (element has checked descendant), `not-focus:` (element is NOT focused)
- **Group** (style children based on parent): `group` on parent, `group-hover:text-white` on child. Named groups:
  `group/item` + `group-hover/item:visible` for nested disambiguation
- **In-\*:** Like group but without marking the parent: `in-focus:opacity-100`
- **Peer** (style based on preceding sibling): `peer` on sibling, `peer-invalid:visible` on target. Named peers for
  disambiguation
- **has-\* variant:** `has-c