Tailwind CSS (v4)

Overview

Tailwind CSS is a utility-first CSS framework. Style by combining single-purpose classes in markup.

We use Tailwind v4 only. There is no tailwind.config.js, tailwind.config.ts, or tailwind.config.mjs. Configuration is done in CSS via @import "tailwindcss" and the @theme directive, plus the Vite or PostCSS plugin.

• Entry: One main CSS file with @import "tailwindcss";
• Build: @tailwindcss/vite (Vite) or @tailwindcss/postcss (Next.js)
• Theme: @theme { ... } in that same CSS file (or imported CSS). No JS config.

Core principle: Prefer utility classes in markup; extend with @theme and custom CSS when needed. Never suggest or create tailwind.config.* or content/theme in a config file.

Installation

Next.js (App Router, 15+)

1. Create project (if needed): npx create-next-app@latest my-project --typescript --eslint --app
2. Install: npm install tailwindcss @tailwindcss/postcss postcss
3. Create postcss.config.mjs in project root:

const config = {
  plugins: {
    "@tailwindcss/postcss": {},
  },
};
export default config;

4. In ./app/globals.css:

@import "tailwindcss";

5. Run: npm run dev

Use className in React/TSX (e.g. in app/page.tsx):

export default function Home() {
  return (
    <h1 className="text-3xl font-bold underline">
      Hello world!
    </h1>
  );
}

Ensure app/globals.css is imported in app/layout.tsx (Create Next App does this by default).

Vite (React, Vue, Qwik, etc.)

1. Install: npm install tailwindcss @tailwindcss/vite
2. In vite.config.ts:

import tailwindcss from '@tailwindcss/vite'

export default defineConfig({
  plugins: [tailwindcss(), /* ... */],
})

3. Main CSS (e.g. src/index.css): @import "tailwindcss";

Tailwind CLI (standalone)

1. Install: npm install tailwindcss @tailwindcss/cli
2. Main CSS: @import "tailwindcss";
3. Build: npx @tailwindcss/cli -i ./src/input.css -o ./src/output.css --watch

Using Tailwind

• Use utility classes in markup: className in React/Next.js, class in Qwik/Vue.
• Prefer design tokens (theme) over arbitrary values when possible.
• Group related utilities for readability (layout → spacing → typography → colors → effects).

Examples: text-3xl font-bold underline, flex items-center gap-4, p-6 rounded-xl bg-white shadow-lg, max-w-sm mx-auto.

Theme: @theme only (no config file)

Only in CSS. Use @theme in the same file as @import "tailwindcss" (or in CSS imported by it).

Extending the default theme

@import "tailwindcss";

@theme {
  --font-display: "Satoshi", "sans-serif";
  --breakpoint-3xl: 120rem;
  --color-avocado-500: oklch(0.84 0.18 117.33);
  --ease-fluid: cubic-bezier(0.3, 0, 0, 1);
}

Use: font-display, 3xl:, bg-avocado-500, ease-fluid.

Overriding defaults

Redefine a variable inside @theme to override:

@theme {
  --breakpoint-sm: 30rem;
}

Theme variable namespaces (quick reference)

Referencing other variables

Use @theme inline when a token should reference another variable:

@theme inline {
  --font-sans: var(--font-inter);
}

Custom keyframes

Define keyframes inside @theme so they are used by --animate-*:

@theme {
  --animate-fade-in-scale: fade-in-scale 0.3s ease-out;
  @keyframes fade-in-scale {
    0% { opacity: 0; transform: scale(0.95); }
    100% { opacity: 1; transform: scale(1); }
  }
}

For more theme options (overriding full namespaces, custom theme, sharing across projects), see reference.md.

Custom styles

Plain CSS

Add any custom CSS in the same file as @import "tailwindcss" or in imported files:

@import "tailwindcss";

.my-custom-style {
  /* ... */
}

Base layer

Use @layer base for element defaults:

@layer base {
  h1 { font-size: var(--text-2xl); }
  h2 { font-size: var(--text-xl); }
}

Component layer

Use @layer components for reusable component-like classes (override with utilities when needed):

@layer components {
  .card {
    background-color: var(--color-white);
    border-radius: var(--radius-lg);
    padding: var(--spacing-6);
    box-shadow: var(--shadow-xl);
  }
}

Custom utilities: @utility

Simple utility:

@utility content-auto {
  content-visibility: auto;
}

Complex (e.g. scrollbar hidden):

@utility scrollbar-hidden {
  &::-webkit-scrollbar {
    display: none;
  }
}

Custom variants: @variant

Use @variant inside custom CSS:

.my-element {
  background: white;
  @variant dark {
    background: black;
  }
}

Or register a named variant with @custom-variant:

@custom-variant theme-midnight (&:where([data-theme="midnight"] *));

Then use: theme-midnight:bg-black.

Arbitrary values and properties

• Arbitrary value: top-[117px], bg-[#bada55], text-[22px], lg:top-[344px].
• CSS variable in arbitrary value: fill-(--my-brand-color) (shorthand for fill-[var(--my-brand-color)]).
• Arbitrary property: [mask-type:luminance], [--scroll-offset:56px].
• Whitespace in value: Use underscore _ for space, e.g. grid-cols-[1fr_500px_2fr].
• Ambiguity: Use a type hint: text-(length:--my-var) vs text-(color:--my-var).

Compatibility

• Browsers: Tailwind v4 targets modern browsers (Chrome 111+, Safari 16.4+, Firefox 128+). Some utilities use newer features; avoid those if you need older support.
• Preprocessors: Do not use Tailwind with Sass/Less/Stylus in the same pipeline; use Tailwind as the main styling layer.

Quick reference

Common mistakes

• Wrong attribute: React/Next use className; Qwik/Vue use class.
• Missing import: The file with @import "tailwindcss" must be the one loaded by the app (e.g. in root layout).
• Config file: Do not create or suggest tailwind.config.js, tailwind.config.ts, or any tailwind.config.*. Do not suggest content: [], theme: {}, or plugins in a JS config. Tailwind v4 uses only CSS (@import "tailwindcss" + @theme) and the Vite/PostCSS plugin.

Additional resources

• reference.md — Theme options, official doc links, default theme summary.
• Official: Tailwind CSS v4 Docs — Installation, theme, adding custom styles, compatibility.