tailwind-css-coding

Tailwind CSS Coding

Match the project's existing conventions. When uncertain, read 2-3 existing components to infer the local style. Check package.json for the tailwindcss version. v4 signals: @tailwindcss/postcss or @tailwindcss/vite in deps, @import "tailwindcss" in CSS, @theme {} blocks. v3 signals: tailwind.config.js with module.exports, @tailwind base; directives, autoprefixer as separate dep. These defaults apply only when the project has no established convention.

Never rules

These are unconditional. They prevent broken builds, invisible bugs, and inaccessible UI regardless of project style.

• Never construct class names dynamically -- Tailwind's compiler scans source files as plain text with regex. It never executes code. Template literals, string concatenation, and interpolation produce classes the compiler cannot find. Use lookup maps of complete static strings.

// Wrong: compiler cannot extract "bg-red-500" from this
const cls = `bg-${color}-500`;

// Correct: every class is a complete static string
const bgMap = {
  red: "bg-red-500",
  blue: "bg-blue-500",
} as const;
const cls = bgMap[color];

• Never use template literal concatenation for class composition -- CSS source order determines which class wins when two utilities target the same property, not HTML attribute order. p-4 p-6 is unpredictable. Use cn() (clsx + tailwind-merge) to merge classes safely.

// Wrong: conflicting padding, last-in-source wins (not last-in-string)
className={`p-4 ${isLarge ? "p-6" : ""}`}

// Correct: tailwind-merge resolves conflicts deterministically
import { cn } from "@/lib/utils";
className={cn("p-4", isLarge && "p-6")}

• Never use arbitrary values when a design token exists -- p-[16px] is p-4. bg-[#3b82f6] is bg-blue-500. w-[100%] is w-full. text-[14px] is text-sm. Arbitrary values bypass the design system and create inconsistency. Likewise, prefer semantic tokens over raw palette colors when the design system defines them (bg-primary, not bg-blue-500) -- see shadcn-ui-coding.

• Never omit interaction states on interactive elements -- every button and link needs hover:, focus-visible:, and disabled: states. Add transition-colors for smooth feedback. In v4: add cursor-pointer explicitly -- Preflight no longer sets it on buttons.

// Wrong: no interaction feedback
<button className="bg-blue-500 text-white px-4 py-2 rounded">Save</button>

// Correct: full interaction states (v4: include cursor-pointer)
<button className="bg-blue-500 text-white px-4 py-2 rounded hover:bg-blue-600 focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-blue-500 disabled:opacity-50 disabled:cursor-not-allowed transition-colors cursor-pointer">
  Save
</button>

• Never use @apply for patterns extractable to components -- extract a React/Vue/Svelte component instead. @apply is only for third-party library overrides, CMS/Markdown HTML, and non-component template languages.

• Never leave colors unthemed when the project supports dark mode -- prefer semantic tokens/CSS variables, which theme both modes in one declaration with no dark: variants needed (see Dark mode below); with raw palette utilities, every bg-, text-, and border- color needs a dark: counterpart. Projects without dark mode need neither.

• Never use sm: thinking it means "small screens" -- sm: means 640px AND ABOVE. Unprefixed utilities apply to all screens (mobile-first). Write base styles for mobile, then layer breakpoints upward.

<!-- Wrong mental model: "sm means small phones" -->
<div class="hidden sm:block">Only on small screens</div>

<!-- Correct: sm:block means "show at 640px and above" -->
<div class="block sm:hidden">Mobile only</div>
<div class="hidden sm:block">Desktop only</div>

• Never hallucinate class names -- common fakes: flex-center (use flex items-center justify-center), text-bold (use font-bold), bg-grey-500 (American spelling: bg-gray-500), d-flex (Bootstrap, not Tailwind).

• Never output conflicting utilities -- p-4 p-6 is unpredictable. One value per CSS property. Don't redundantly set defaults (flex flex-row, flex flex-nowrap).

• Never forget accessibility -- use sr-only for icon-only button labels, focus:not-sr-only for skip links. Every interactive element needs visible focus indication via focus-visible:.

Dynamic styling

For conditional classes, use cn() (clsx + tailwind-merge). For truly dynamic values that cannot be enumerated (user-set colors, computed positions), use inline style props -- these bypass the compiler entirely and always work.

// Conditional classes: cn()
<div className={cn("rounded p-4", isActive && "ring-2 ring-blue-500")} />

// Truly dynamic: inline style
<div className="rounded p-4" style={{ backgroundColor: user.brandColor }} />

For variant-driven styling, use a lookup map with complete static strings:

const sizeClasses = {
  sm: "px-2 py-1 text-sm",
  md: "px-4 py-2 text-base",
  lg: "px-6 py-3 text-lg",
} as const;

function Button({ size = "md", className, ...props }: ButtonProps) {
  return <button className={cn(sizeClasses[size], className)} {...props} />;
}

For components with 2+ variant dimensions, consider cva from class-variance-authority.

When the compiler must see classes that only appear in dynamic data (CMS content, database values), safelist them. In v4: @source inline("bg-{red,blue}-500") -- brace expansion generates each class; use one pattern per directive. In v3: safelist array in tailwind.config.js.

Class composition

The cn() helper combines clsx (conditional joining) with tailwind-merge (conflict resolution). Standard setup:

import { clsx, type ClassValue } from "clsx";
import { twMerge } from "tailwind-merge";

export function cn(...inputs: ClassValue[]) {
  return twMerge(clsx(inputs));
}

Use cn() whenever merging external className props with internal defaults -- raw concatenation silently breaks when both sides set the same property.

// Wrong: parent's p-6 may or may not override internal p-4
function Card({ className }: { className?: string }) {
  return <div className={`rounded bg-white p-4 ${className}`} />;
}

// Correct: tailwind-merge ensures parent overrides win
function Card({ className }: { className?: string }) {
  return <div className={cn("rounded bg-white p-4", className)} />;
}

Responsive design

Mobile-first: write base styles for the smallest screen, then add breakpoints upward. Write breakpoint variants in ascending order (sm: -> md: -> lg: -> xl: -> 2xl:) -- a readability convention (what the Prettier plugin enforces); order within the class string has zero CSS effect. Never skip to lg: without considering the gap.

<!-- Single column on mobile, two on tablet, three on desktop -->
<div class="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 gap-4">

In v4: container queries are built-in (no plugin). Use @container for component-scoped responsive design:

// Parent declares a container
<div className="@container">
  {/* Child responds to container width, not viewport */}
  <div className="flex flex-col @md:flex-row @lg:grid @lg:grid-cols-3 gap-4">
    {children}
  </div>
</div>

Dark mode

Cover every visible color. A component with bg-white text-gray-900 needs dark:bg-gray-900 dark:text-white. Missing a single dark: variant causes unreadable text or invisible elements.

Better approach -- CSS variable theming. Define colors once, switch palettes:

/* v4: @theme block */
@theme {
  --color-surface: #ffffff;
  --color-on-surface: #111827;
}

@custom-variant dark (&:where(.dark, .dark *));

.dark {
  --color-surface: #111827;
  --color-on-surface: #f9fafb;
}

Then use bg-surface text-on-surface everywhere -- no dark: variants needed per component.

Interaction states

Minimum states for buttons: hover:, focus-visible:, disabled:, transition-colors -- full button example under Never rules. Minimum for inputs: focus:, disabled:, placeholder:. Minimum for links: hover:, focus-visible:.

Complete input pattern (semantic tokens -- both modes themed with no dark: variants):

<input className={cn(
  "w-full rounded border px-3 py-2 transition-colors",
  "border-input bg-background text-foreground",
  "placeholder:text-muted-foreground",
  "focus:border-ring focus:outline-none focus:ring-1 focus:ring-ring",
  "disabled:cursor-not-allowed disabled:bg-muted disabled:opacity-50",
)} />

In v4: hover: only fires on hover-capable devices (@media (hover: hover)). Touch-only devices skip hover styles entirely -- don't rely on hover for critical information.

Accessibility

Icon-only buttons need screen reader text:

<button className="p-2 rounded hover:bg-gray-100">
  <SearchIcon className="h-5 w-5" aria-hidden="true" />
  <span className="sr-only">Search</span>
</button>

Skip links use sr-only that becomes visible on focus:

<a href="#main" className="sr-only focus:not-sr-only focus:absolute focus:top-4 focus:left-4 focus:z-50 focus:px-4 focus:py-2 focus:bg-white focus:text-black focus:rounded">
  Skip to main content
</a>

Every focusable element needs a visible focus indicator. focus-visible: is preferred over focus: -- it only shows for keyboard navigation, not mouse clicks.

Ensure sufficient color contrast on interactive states. A hover:bg-blue-700 on bg-blue-600 is barely visible -- test both light and dark modes.

v4 configuration

In v4, configuration lives in CSS, not JavaScript. Entry point is @import "tailwindcss". Autoprefixer is built-in (don't add it separately).

@import "tailwindcss";

@theme {
  --font-display: "Inter", sans-serif;
  --color-brand: #4f46e5;
  --breakpoint-3xl: 1920px;
}

@utility card {
  background: var(--color-sur

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