Tailwind CSS

Tailwind CSS v4

Expert guidance for Tailwind CSS v4, CSS-first configuration, modern utility patterns, and type-safe component styling with tailwind-variants.

CSS-First Configuration

Tailwind CSS v4 eliminates tailwind.config.ts in favor of CSS-only configuration. All configuration lives in CSS files using special directives.

Core Directives:

• @import "tailwindcss" - Entry point that loads Tailwind
• @theme { } - Define or extend design tokens
• @theme static { } - Define tokens that should not generate utilities
• @utility - Create custom utilities
• @custom-variant - Define custom variants

Minimal Example:

@import "tailwindcss";

@theme {
  --color-brand: oklch(0.72 0.11 178);
  --font-display: "Inter", sans-serif;
  --spacing-edge: 1.5rem;
}

All theme tokens defined with @theme automatically become available as utility classes. For example, --color-brand can be used as bg-brand, text-brand, border-brand, etc.

ESLint Integration

Use eslint-plugin-better-tailwindcss for Tailwind CSS v4 class validation and style enforcement.

Correctness Rules (errors):

• no-conflicting-classes - Detect classes that override each other
• no-unknown-classes - Flag classes not registered with Tailwind

Stylistic Rules (warnings):

• enforce-canonical-classes - Use standard v4 class names
• enforce-shorthand-classes - Use abbreviated class versions
• no-deprecated-classes - Remove outdated class names
• no-duplicate-classes - Eliminate redundant declarations
• no-unnecessary-whitespace - Clean up extra spacing

Examples:

// ❌ Bad: separate padding
<div className="px-6 py-6">

// ✅ Good: shorthand
<div className="p-6">

// ❌ Bad: separate width/height
<div className="w-6 h-6">

// ✅ Good: size utility
<div className="size-6">

Run the project's ESLint check after modifying Tailwind classes to validate all changes across the codebase.

Coding Preferences

Layout and Spacing

Use gap for flex/grid spacing, not space-x/space-y:

The gap utilities handle wrapping correctly, while space-* utilities break when flex/grid items wrap to multiple lines.

// ✅ Good: gap handles wrapping
<div className="flex gap-4">

// ❌ Bad: breaks when items wrap
<div className="flex space-x-4">

Prefer size-* over separate w-*/h-* for equal dimensions:

// ✅ Good: concise
<div className="size-16">

// ❌ Bad: redundant
<div className="w-16 h-16">

Always use min-h-dvh instead of min-h-screen:

Dynamic viewport height (dvh) accounts for mobile browser chrome, while vh units ignore it and cause layout issues on mobile Safari.

// ✅ Good: works on mobile Safari
<main className="min-h-dvh">

// ❌ Bad: buggy on mobile Safari
<main className="min-h-screen">

Prefer top/left margins over bottom/right:

Consistent directionality improves layout predictability.

// ✅ Good: top margin
<div className="mt-4">

// ❌ Avoid: bottom margin (unless needed)
<div className="mb-4">

Use padding on parent containers instead of bottom margins on last child:

Padding provides consistent spacing without needing :last-child selectors.

// ✅ Good: padding on parent
<section className="pb-8">
  <div>Item 1</div>
  <div>Item 2</div>
  <div>Item 3</div>
</section>

// ❌ Avoid: margin on children
<section>
  <div className="mb-4">Item 1</div>
  <div className="mb-4">Item 2</div>
  <div>Item 3</div>
</section>

For max-widths, prefer container scale over pixel values:

// ✅ Good: semantic container size
<div className="max-w-2xl">

// ❌ Avoid: arbitrary pixel value
<div className="max-w-[672px]">

Typography

Avoid leading-* classes; use line height modifiers:

Tailwind v4 supports inline line height modifiers with the text-{size}/{leading} syntax.

// ✅ Good: combined size and line height
<p className="text-base/7">

// ❌ Bad: separate utilities
<p className="text-base leading-7">

Font Size Reference:

Colors and Opacity

Use opacity modifier syntax, not separate opacity utilities:

All *-opacity-* utilities were removed in Tailwind v4. Use the modifier syntax instead.

// ✅ Good: opacity modifier
<div className="bg-red-500/60">

// ❌ Bad: removed in v4
<div className="bg-red-500 bg-opacity-60">

Prefer design tokens over arbitrary hex values:

Check the project's @theme configuration before using arbitrary color values.

// ✅ Good: theme token
<div className="bg-brand">

// ❌ Avoid: arbitrary hex (check theme first)
<div className="bg-[#4f46e5]">

Border Radius

Tailwind v4 renamed border radius utilities:

Use the v4 names when writing new code.

Gradients

Tailwind v4 renamed gradient utilities and added new gradient types.

Use bg-linear-*, not bg-gradient-*:

// ✅ Good: v4 syntax
<div className="bg-linear-to-r from-blue-500 to-purple-500">

// ❌ Bad: removed in v4
<div className="bg-gradient-to-r from-blue-500 to-purple-500">

New gradient types:

• bg-radial - Radial gradients
• bg-conic - Conic gradients

Example:

<div className="bg-radial from-blue-500 to-purple-500">
<div className="bg-conic from-red-500 via-yellow-500 to-green-500">

Arbitrary Values

Always prefer Tailwind's predefined scale:

Check the project's @theme configuration for available tokens before using arbitrary values.

// ✅ Good: predefined scale
<div className="ml-4">

// ❌ Avoid: arbitrary pixel value
<div className="ml-[16px]">

General rule: Prefer sizing scale over pixel values. Three similar lines of code is better than a premature abstraction.

Class Merging

The common pattern is a cn utility combining clsx + tailwind-merge.

Use cn for:

• Static constants: const CARD_BASE = cn("fixed classes")
• Conditional classes: cn("base", condition && "conditional")
• Dynamic merging: cn(baseClasses, className)
• Conflict resolution: cn("p-4", "p-6") → "p-6"

Do NOT use cn for:

• Static strings in className attributes: className="fixed classes"

Examples:

// ✅ Good: static string in className
<button className="rounded-lg px-4 py-2 font-medium bg-blue-600">

// ✅ Good: static constant with cn
const CARD_BASE = cn("rounded-lg border border-gray-300 p-4");
<div className={CARD_BASE} />

// ✅ Good: conditional with cn
<button className={cn(
  "rounded-lg px-4 py-2 font-medium",
  isActive ? "bg-blue-600" : "bg-gray-700",
  disabled && "opacity-50"
)} />

// ❌ Bad: unnecessary cn for static className attribute
<button className={cn("rounded-lg px-4 py-2 font-medium")} />

Image Sizing

Use Tailwind size classes instead of pixel values for Image components.

// ✅ Good: Tailwind units
<Image src={src} alt={alt} className="size-16" />
<Image src={src} alt={alt} className="w-24 h-auto" />

// ❌ Bad: pixel values
<Image src={src} alt={alt} width={64} height={64} />

Z-Index

Define z-index values as CSS custom properties in @theme, then reference them with the z-(--z-*) syntax.

Never use arbitrary z-index numbers:

// ✅ Good: theme z-index value
<div className="z-(--z-modal)">
<div className="z-(--z-sticky)">

// ❌ Bad: arbitrary z-index numbers
<div className="z-[100]">
<div className="z-[9999]">

Define z-index tokens in CSS:

@theme {
  --z-base: 0;
  --z-sticky: 10;
  --z-modal: 100;
  --z-tooltip: 1000;
}

Dark Mode

Use the plain dark: variant for dark mode styles.

Pattern:

Write light mode styles first, then add dark mode overrides.

// ✅ Good: light mode first, then dark override
<div className="bg-white text-gray-900 dark:bg-gray-900 dark:text-white">

// ❌ Avoid: dark mode first (less readable)
<div className="dark:bg-gray-900 dark:text-white bg-white text-gray-900">

CSS Modules

Use CSS Modules only as a last resort for complex CSS that cannot be easily written with Tailwind classes.

All .module.css files must include @reference "#tailwind"; at the top to enable Tailwind utilities and theme tokens inside the module.

Example:

/* component.module.css */
@reference "#tailwind";

.component {
  /* Complex CSS that can't be expressed with Tailwind utilities */
  /* Can still use Tailwind utilities and theme tokens */
}

Common Tasks

Adding a Component with Variants

1. Read references/tailwind-variants.md for patterns
2. Check the project's @theme configuration for available tokens
3. Use tv() from tailwind-variants for type-safe variants

Example:

import { tv } from "tailwind-variants";

const button = tv({
  base: "rounded-lg px-4 py-2 font-medium",
  variants: {
    color: {
      primary: "bg-blue-600 text-white",
      secondary: "bg-gray-600 text-white",
    },
    size: {
      sm: "text-sm",
      md: "text-base",
      lg: "text-lg",
    },
  },
});

Debugging Styles

1. Check references/tailwind-v4-rules.md for breaking changes
2. Verify gradient syntax (bg-linear-*, not bg-gradient-*)
3. Verify CSS variable syntax (bg-my-color, not bg-[--var-my-color])
4. Check if arbitrary value exists in the project's @theme configuration

Working with Colors

1. Check the project's @theme configuration first to see available colors
2. Use semantic color names when available
3. Use opacity modifiers for transparency (/20, /50, etc.)
4. Avoid arbitrary colors unless absolutely necessary

Example:

// ✅ Good: theme token with opacity
<div className="bg-brand/20 text-brand">

// ❌ Avoid: arbitrary hex
<div className="bg-[#4f46e5]/20 text-[#4f46e5]">

Adding Animations

1. Read references/tw-animate-css.md for available animations
2. Combine a base class (animate-in or animate-out) with effect classes
3. Note decimal spacing gotcha: use [0.625rem] syntax, not 2.5

Example:

// Enter: fade + slide up
<div className="fade-in slide-in-from-bottom-4 duration-300 animate-in">

// Exit: fade + slide down
<div className="fade-out slide-out-to-bottom-4 duration-200 animate-out">

Quick Reference Table

Reference Documentation

• Tailwind v4 Rules & Best Practices: references/tailwind-v4-rules.md — Breaking changes, removed/renamed utilities, layout rules, typography, gradients, CSS variables, new v4 features, common pitfalls
• tailwind-variants Patterns: references/tailwind-variants.md — Component variants, slots API, composition, TypeScript integration, responsive variants
• tw-animate-css Reference: references/tw-animate-css.md — Enter/exit animations, slide/fade/zoom utilities, spacing gotchas

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