ia-tailwind-css

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name: ia-tailwind-css class: language description: >- Tailwind CSS v4 patterns: CSS-first config, utility classes, component variants, v3 migration. Use when styling with Tailwind, configuring @theme tokens, using tailwind-variants/CVA, migrating v3 to v4, or fixing Tailwind styles and dark mode. paths: "/*.css,/tailwind.config.*"

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Tailwind CSS v4

Verify before implementing: For v4-specific syntax (@theme, @variant, CSS-first config), search current docs via search_docs before writing code. Tailwind v4 changed significantly from v3 and training data may be stale.

CSS-First Configuration

v4 eliminates tailwind.config.ts. All configuration lives in CSS.

| Directive | Purpose | |-----------|---------| | @import "tailwindcss" | Entry point (replaces @tailwind base/components/utilities) | | @theme { } | Define/extend design tokens -- auto-generates utility classes | | @theme inline { } | Map CSS variables to Tailwind utilities without generating new vars | | @theme static { } | Define tokens that don't generate utilities | | @utility name { } | Create custom utilities (replaces @layer components + @apply) | | @custom-variant name (selector) | Define custom variants |

@import "tailwindcss";
@theme {
  --color-brand: oklch(0.72 0.11 178);
  --font-display: "Inter", sans-serif;
  --animate-fade-in: fade-in 0.2s ease-out;
  @keyframes fade-in { from { opacity: 0; } to { opacity: 1; } }
}
@custom-variant dark (&:where(.dark, .dark *));

Tokens defined with @theme become utilities automatically: --color-brand produces bg-brand, text-brand, border-brand. Define z-index as tokens (--z-modal: 50) and reference via z-(--z-modal) instead of arbitrary z-50.

CSS Modules: when using .module.css with Tailwind v4, add @reference "#tailwind"; at the top of the module file to enable theme token access inside the module.

Animations (tw-animate-css): use animate-in/animate-out base classes combined with effect classes (fade-in, slide-in-from-top). Decimal spacing gotcha: use bracket notation [0.625rem] instead of fractional values like 2.5.

v3 to v4 Migration

For projects upgrading from v3 to v4, see v3-to-v4-migration.md for the full breaking-change table and codemod guidance. For greenfield v4 work, current patterns are above.

Coding Rules

gap over space-x/space-y -- gap handles wrapping; space-* breaks on wrap

size-* over w-* h-* -- for equal dimensions

min-h-dvh over min-h-screen -- dvh accounts for mobile browser chrome

Opacity modifier (bg-black/50) -- *-opacity-* utilities are removed in v4

Design tokens over arbitrary values -- check @theme before using [#hex]

Never construct classes dynamically -- text-${color}-500 won't be detected; use complete class names

@utility over @apply with @layer -- @apply on @layer classes fails in v4

Parent padding over last-child margin -- use padding on containers instead of bottom margins on the last child

ESLint Integration

Use eslint-plugin-better-tailwindcss for automated class validation:

no-conflicting-classes -- catches text-red-500 text-blue-500

no-unknown-classes -- flags typos

enforce-canonical-classes -- normalizes shorthand

no-duplicate-classes -- removes redundant entries

no-deprecated-classes -- catches v3 classes removed in v4

useSortedClasses -- enforces canonical class order; configure attributes: ["classList"] and functions: ["clsx", "cva", "cn", "tv", "tw"] to cover JSX utility functions

Class Merging

Use cn() combining clsx + tailwind-merge for conditional/dynamic classes. Use plain strings for static className attributes.

import { type ClassValue, clsx } from "clsx";
import { twMerge } from "tailwind-merge";
export function cn(...inputs: ClassValue[]) { return twMerge(clsx(inputs)); }

// Static: plain string
// Conditional: use cn()

Component Variants

Use tailwind-variants (tv()) for type-safe variant components. Alternative: class-variance-authority (cva()).

import { tv } from "tailwind-variants";
const button = tv({
  base: "rounded-lg px-4 py-2 font-medium transition-colors",
  variants: {
    color: { primary: "bg-blue-600 text-white", secondary: "bg-gray-200 text-gray-800" },
    size: { sm: "text-sm px-3 py-1", md: "text-base", lg: "text-lg px-6 py-3" },
  },
  defaultVariants: { color: "primary", size: "md" },
});

See tailwind-variants patterns for slots, composition, and responsive variants.

Common Errors

| Symptom | Fix | |---------|-----| | bg-primary doesn't work | Add @theme inline { --color-primary: var(--primary); } | | Colors all black/white | Double hsl() wrapping -- use var(--color) not hsl(var(--color)) | | @apply fails on custom class | Use @utility instead of @layer components | | Build fails after migration | Delete tailwind.config.ts | | Animations broken | Replace tailwindcss-animate with tw-animate-css | | .dark { @theme { } } fails | v4 does not support nested @theme -- use :root/.dark CSS vars mapped via @theme inline |

Dark Mode (v4 Pattern)

:root { --background: hsl(0 0% 100%); --foreground: hsl(222 84% 4.9%); }
.dark { --background: hsl(222 84% 4.9%); --foreground: hsl(210 40% 98%); }
@theme inline { --color-background: var(--background); --color-foreground: var(--foreground); }

Semantic classes (bg-background, text-foreground) auto-switch -- no dark: variants needed for themed colors.

Verify

Build passes with zero errors (npm run build or equivalent)

No v3 class names remain in changed files (check with @tailwindcss/upgrade --dry-run if available)

No conflicting classes on the same element

References

Component patterns -- tailwind-variants slots, CVA, compound components

Layout patterns -- grid areas, container queries, z-index management, fluid typography