Tailwind CSS v4, CSS Modules, utility-first patterns, and styling strategies for React and Next.js — structured for AI-assisted development.

CSS Architecture

Tailwind CSS v4, CSS Modules, utility-first patterns, and styling strategies for React and Next.js — structured for AI-assisted development.

Principles

1. The Styling Landscape in 2025-2026

Tailwind CSS dominates. It's the default choice for new projects, especially AI-assisted ones — utilities map 1:1 to CSS properties, making LLM output predictable and consistent.

CSS Modules remain solid for scoped component styles when you prefer writing traditional CSS or need complex selectors (pseudo-elements, animations, @container queries with named containers).

Runtime CSS-in-JS is dead for Server Components. Libraries like styled-components and Emotion inject styles via JavaScript at runtime. Server Components don't run JavaScript on the client, so runtime CSS-in-JS libraries break. Use Tailwind or CSS Modules instead.

CSS-in-JS alternatives that work with RSC: StyleX (Meta's compile-time CSS-in-JS), Panda CSS, and vanilla-extract all compile to static CSS at build time.

2. Tailwind CSS: Philosophy and Configuration

Tailwind is a utility-first CSS framework. Instead of writing custom CSS, you compose small utility classes directly in markup: className="flex items-center gap-4 rounded-lg bg-white p-6 shadow".

Why it works for AI coding: Every utility maps to exactly one CSS property-value pair. There's no naming to invent, no cascade to debug, no specificity wars. The AI generates consistent output because the vocabulary is fixed.

Tailwind v4 uses CSS-first configuration with @theme:

/* app/globals.css */
@import "tailwindcss";

@theme {
  --color-brand-50: oklch(0.97 0.01 250);
  --color-brand-500: oklch(0.55 0.15 250);
  --color-brand-900: oklch(0.25 0.08 250);

  --font-sans: "Inter", system-ui, sans-serif;
  --font-mono: "JetBrains Mono", monospace;

  --radius-DEFAULT: 0.5rem;
  --radius-lg: 0.75rem;

  --breakpoint-xs: 30rem;
}

In Tailwind v4, the CSS file IS the config. No tailwind.config.ts needed for most projects.

3. CSS Modules: Scoped Styles

CSS Modules generate unique class names at build time, preventing style collisions:

/* Button.module.css */
.button {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  border-radius: 0.5rem;
  padding: 0.5rem 1rem;
  font-weight: 500;
  transition: background-color 150ms;
}

.primary {
  background-color: var(--color-brand-500);
  color: white;
}

.primary:hover {
  background-color: var(--color-brand-600);
}

import styles from "./Button.module.css";

export function Button({ variant = "primary", children }: ButtonProps) {
  return (
    <button className={`${styles.button} ${styles[variant]}`}>
      {children}
    </button>
  );
}

When to choose CSS Modules over Tailwind:

Complex pseudo-element styling (::before, ::after)
Named container queries with @container
Migrating an existing codebase that uses CSS Modules
Team preference for writing traditional CSS

4. Utility-First vs Semantic CSS

Utility-first scales better for AI-assisted development because:

No naming decisions (the AI doesn't have to invent .card-header-wrapper)
No dead CSS — unused utilities are purged automatically
Colocation — styles live in the component, not a separate file
Predictable output — p-4 always means padding: 1rem

Semantic CSS (class="card") reads more naturally but creates naming burden, dead-code risk, and requires maintaining a separate stylesheet.

The practical choice: Use Tailwind for components. Use CSS custom properties for design tokens. Use CSS Modules only when Tailwind can't express what you need.

5. Styling Server Components

Server Components render on the server and send HTML to the client. They support:

Tailwind utility classes (compiled at build time)
CSS Modules (compiled at build time)
Inline styles (style={{ color: "red" }})
Global CSS imported in layout.tsx

They do NOT support:

Runtime CSS-in-JS (styled-components, Emotion) — no client-side JavaScript to inject styles
useEffect-based style manipulation

6. Global Styles Strategy

Keep global CSS minimal. Use it for:

CSS reset / normalize
CSS custom properties (design tokens)
Base typography styles
Third-party library overrides

/* globals.css */
@import "tailwindcss";

@theme {
  /* Design tokens */
  --color-background: oklch(1 0 0);
  --color-foreground: oklch(0.15 0 0);
  --color-muted: oklch(0.55 0 0);
  --color-border: oklch(0.9 0 0);
}

@layer base {
  *,
  *::before,
  *::after {
    border-color: var(--color-border);
  }

  body {
    background-color: var(--color-background);
    color: var(--color-foreground);
  }
}

7. The cn() Utility Pattern

Every project using Tailwind needs cn() — a function that merges class names and handles Tailwind conflicts:

// lib/utils.ts
import { clsx, type ClassValue } from "clsx";
import { twMerge } from "tailwind-merge";

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

Why: Without cn(), conflicting Tailwind classes don't resolve correctly:

// Without cn() — both padding classes apply, unpredictable result
<div className={`p-4 ${large ? "p-8" : ""}`} />

// With cn() — p-8 wins when large is true
<div className={cn("p-4", large && "p-8")} />

Use cn() in every component that accepts a className prop:

interface CardProps {
  className?: string;
  children: React.ReactNode;
}

export function Card({ className, children }: CardProps) {
  return (
    <div className={cn("rounded-lg border bg-white p-6 shadow-sm", className)}>
      {children}
    </div>
  );
}

8. Responsive Patterns in React

Tailwind uses mobile-first breakpoints: sm:, md:, lg:, xl:, 2xl:. Write base styles for mobile, then add breakpoints for larger screens:

<div className="grid grid-cols-1 gap-4 md:grid-cols-2 lg:grid-cols-3">
  {items.map(item => <Card key={item.id} item={item} />)}
</div>

Container queries scope responsive behavior to the container, not the viewport:

<div className="@container">
  <div className="grid grid-cols-1 @md:grid-cols-2 @lg:grid-cols-3">
    {/* Responds to container width, not viewport */}
  </div>
</div>

Fluid typography with clamp():

@theme {
  --font-size-heading: clamp(1.5rem, 1rem + 2vw, 3rem);
}

9. Dark Mode Implementation

Tailwind's dark: variant combined with next-themes:

// app/providers.tsx
"use client";

import { ThemeProvider } from "next-themes";

export function Providers({ children }: { children: React.ReactNode }) {
  return (
    <ThemeProvider attribute="class" defaultTheme="system" enableSystem>
      {children}
    </ThemeProvider>
  );
}

// app/layout.tsx
import { Providers } from "./providers";

export default function RootLayout({ children }: { children: React.ReactNode }) {
  return (
    <html lang="en" suppressHydrationMismatch>
      <body>
        <Providers>{children}</Providers>
      </body>
    </html>
  );
}

Semantic tokens that adapt to dark mode:

@theme {
  --color-surface: oklch(1 0 0);
  --color-surface-elevated: oklch(0.98 0 0);
  --color-text: oklch(0.15 0 0);
  --color-text-muted: oklch(0.45 0 0);
}

.dark {
  --color-surface: oklch(0.15 0 0);
  --color-surface-elevated: oklch(0.2 0 0);
  --color-text: oklch(0.95 0 0);
  --color-text-muted: oklch(0.65 0 0);
}

// Component uses semantic tokens — works in both modes
<div className="bg-surface text-text rounded-lg p-6">
  <p className="text-text-muted">Subtitle</p>
</div>

10. Animation with Tailwind

Simple transitions:

<button className="rounded bg-blue-600 px-4 py-2 text-white transition-colors hover:bg-blue-700">
  Click me
</button>

Tailwind's built-in animations:

<div className="animate-spin" />    {/* Spinner */}
<div className="animate-pulse" />   {/* Skeleton loading */}
<div className="animate-bounce" />  {/* Attention */}

Custom keyframes in Tailwind v4:

@theme {
  --animate-fade-in: fade-in 0.3s ease-out;
  --animate-slide-up: slide-up 0.3s ease-out;
}

@keyframes fade-in {
  from { opacity: 0; }
  to { opacity: 1; }
}

@keyframes slide-up {
  from { opacity: 0; transform: translateY(0.5rem); }
  to { opacity: 1; transform: translateY(0); }
}

<div className="animate-fade-in">Appears with fade</div>

For complex animations (spring physics, gestures, layout animations), use Motion (formerly Framer Motion). See the UIUX Animation & Motion guide.

Always respect reduced motion:

<div className="animate-slide-up motion-reduce:animate-none">
  Content
</div>

LLM Instructions

Setting Up Tailwind

When setting up a new project with Tailwind:

Install Tailwind v4: npm install tailwindcss @tailwindcss/postcss
Configure PostCSS with @tailwindcss/postcss
Create globals.css with @import "tailwindcss" and @theme block
Install clsx and tailwind-merge, create the cn() utility
Set up dark mode with next-themes if needed
Use CSS custom properties in @theme for all design tokens (colors, fonts, spacing, radii)

Writing Styles

When generating component styles:

Use Tailwind utility classes as the default styling method
Accept className?: string as a prop on all reusable components
Use cn() to merge base styles with the className prop
Use cva (class-variance-authority) for components with multiple variants
Write mobile-first: base classes for mobile, md: for tablet, lg: for desktop
Use dark: variant for dark mode overrides, or semantic tokens that auto-switch

Dark Mode

When implementing dark mode:

Install next-themes and create a Providers component
Wrap the app in ThemeProvider with attribute="class"
Add suppressHydrationMismatch to <html>
Define semantic color tokens that change between light/dark
Test both themes — especially contrast ratios
Provide a three-way toggle: System / Light / Dark

Responsive Layouts

When building responsive layouts:

Always start mobile-first — base styles are the mobile layout
Use Tailwind's responsive prefixes: sm:, md:, lg:, xl:
Use CSS Grid for 2D layouts, Flexbox for 1D layouts
Use container queries (@container + @md:, @lg:) for reusable components
Use clamp() for fluid typography and spacing
Test at all breakpoints — don't just check desktop and mobile

Choosing a Styling Approach

When deciding how to style a component:

Default: Tailwind — for all standard component styling
CSS Modules — when you need complex selectors, pseudo-elements, or @container with named containers
Inline styles — only for truly dynamic values (calculated positions, user-selected colors)
Never runtime CSS-in-JS — not compatible with Server Components

Examples

1. Tailwind v4 Theme Configuration

A complete theme setup using Tailwind v4's CSS-first configuration:

/* globals.css */
@import "tailwindcss";

@theme {
  /* Colors — oklch for perceptual uniformity */
  --color-brand-50: oklch(0.97 0.01 250);
  --color-brand-100: oklch(0.93 0.03 250);
  --color-brand-200: oklch(0.87 0.06 250);
  --color-brand-300: oklch(0.77 0.10 250);
  --color-brand-400: oklch(0.65 0.14 250);
  --color-brand-500: oklch(0.55 0.15 250);
  --color-brand-600: oklch(0.47 0.14 250);
  --color-brand-700: oklch(0.39 0.12 250);
  --color-brand-800: oklch(0.31 0.09 250);
  --color-brand-900: oklch(0.25 0.08 250);

  --color-gray-50: oklch(0.98 0 0);
  --color-gray-100: oklch(0.96 0 0);
  --color-gray-200: oklch(0.90 0 0);
  --color-gray-300: oklch(0.83 0 0);
  --color-gray-400: oklch(0.65 0 0);
  --color-gray-500: oklch(0.55 0 0);
  --color-gray-600: oklch(0.44 0 0);
  --color-gray-700: oklch(0.37 0 0);
  --color-gray-800: oklch(0.27 0 0);
  --color-gray-900: oklch(0.18 0 0);

  /* Semantic tokens */
  --color-background: var(--color-gray-50);
  --color-foreground: var(--color-gray-900);
  --color-muted: var(--color-gray-500);
  --color-border: var(--color-gray-200);
  --color-ring: var(--color-brand-500);

  /* Typography */
  --font-sans: "Inter Variable", system-ui, sans-serif;
  --font-mono: "JetBrains Mono Variable", ui-monospace, monospace;

  /* Spacing / sizing */
  --radius-sm: 0.25rem;
  --radius-DEFAULT: 0.5rem;
  --radius-lg: 0.75rem;
  --radius-xl: 1rem;

  /* Animation */
  --animate-fade-in: fade-in 0.2s ease-out;
  --animate-slide-down: slide-down 0.2s ease-out;
}

/* Dark mode overrides */
.dark {
  --color-background: var(--color-gray-900);
  --color-foreground: var(--color-gray-50);
  --color-muted: var(--color-gray-400);
  --color-border: var(--color-gray-800);
}

@keyframes fade-in {
  from { opacity: 0; }
  to { opacity: 1; }
}

@keyframes slide-down {
  from { opacity: 0; transform: translateY(-0.5rem); }
  to { opacity: 1; transform: translateY(0); }
}

@layer base {
  body {
    background-color: var(--color-background);
    color: var(--color-foreground);
  }
}

2. cn() + cva Variant Pattern

Component variants using class-variance-authority (cva) with cn():

// components/ui/button.tsx
import { cva, type VariantProps } from "class-variance-authority";
import { cn } from "@/lib/utils";

const buttonVariants = cva(
  // Base styles
  "inline-flex items-center justify-center gap-2 rounded font-medium transition-colors focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 disabled:pointer-events-none disabled:opacity-50",
  {
    variants: {
      variant: {
        default: "bg-brand-500 text-white hover:bg-brand-600",
        secondary: "bg-gray-100 text-gray-900 hover:bg-gray-200 dark:bg-gray-800 dark:text-gray-50",
        outline: "border border-border bg-transparent hover:bg-gray-100 dark:hover:bg-gray-800",
        ghost: "hover:bg-gray-100 dark:hover:bg-gray-800",
        destructive: "bg-red-600 text-white hover:bg-red-700",
        link: "text-brand-500 underline-offset-4 hover:underline",
      },
      size: {
        sm: "h-8 px-3 text-sm",
        default: "h-10 px-4 text-sm",
        lg: "h-12 px-6 text-base",
        icon: "h-10 w-10",
      },
    },
    defaultVariants: {
      variant: "default",
      size: "default",
    },
  },
);

interface ButtonProps
  extends React.ButtonHTMLAttributes<HTMLButtonElement>,
    VariantProps<typeof buttonVariants> {}

export function Button({ className, variant, size, ...props }: ButtonProps) {
  return (
    <button className={cn(buttonVariants({ variant, size }), className)} {...props} />
  );
}

// Usage
<Button variant="default" size="lg">Primary</Button>
<Button variant="outline">Outline</Button>
<Button variant="ghost" size="icon"><SearchIcon /></Button>
<Button variant="destructive" className="w-full">Delete Account</Button>

3. CSS Modules Component

A notification component using CSS Modules for complex pseudo-element styling:

/* Notification.module.css */
.notification {
  position: relative;
  display: flex;
  gap: 0.75rem;
  padding: 1rem;
  border-radius: 0.5rem;
  border-left: 4px solid var(--accent-color, var(--color-brand-500));
  background-color: var(--bg-color, var(--color-brand-50));
}

.notification::before {
  content: "";
  position: absolute;
  inset: 0;
  border-radius: inherit;
  background: linear-gradient(
    135deg,
    oklch(1 0 0 / 0.1),
    transparent 50%
  );
  pointer-events: none;
}

.info {
  --accent-color: var(--color-brand-500);
  --bg-color: var(--color-brand-50);
}

.warning {
  --accent-color: oklch(0.75 0.15 85);
  --bg-color: oklch(0.97 0.02 85);
}

.error {
  --accent-color: oklch(0.55 0.2 25);
  --bg-color: oklch(0.97 0.02 25);
}

.success {
  --accent-color: oklch(0.55 0.15 145);
  --bg-color: oklch(0.97 0.02 145);
}

.icon {
  flex-shrink: 0;
  width: 1.25rem;
  height: 1.25rem;
  color: var(--accent-color);
}

.content {
  flex: 1;
  min-width: 0;
}

.title {
  font-weight: 600;
  font-size: 0.875rem;
}

.message {
  margin-top: 0.25rem;
  font-size: 0.875rem;
  color: var(--color-muted);
}

// Notification.tsx
import styles from "./Notification.module.css";

type NotificationType = "info" | "warning" | "error" | "success";

interface NotificationProps {
  type: NotificationType;
  title: string;
  message?: string;
}

export function Notification({ type, title, message }: NotificationProps) {
  return (
    <div className={`${styles.notification} ${styles[type]}`} role="alert">
      <NotificationIcon type={type} className={styles.icon} />
      <div className={styles.content}>
        <p className={styles.title}>{title}</p>
        {message && <p className={styles.message}>{message}</p>}
      </div>
    </div>
  );
}

4. Dark Mode with next-themes

Complete dark mode setup with a three-way toggle:

// components/theme-toggle.tsx
"use client";

import { useTheme } from "next-themes";
import { useEffect, useState } from "react";

export function ThemeToggle() {
  const { theme, setTheme } = useTheme();
  const [mounted, setMounted] = useState(false);

  // Avoid hydration mismatch — render nothing until mounted
  useEffect(() => setMounted(true), []);
  if (!mounted) return <div className="h-10 w-10" />;

  return (
    <button
      onClick={() => {
        if (theme === "light") setTheme("dark");
        else if (theme === "dark") setTheme("system");
        else setTheme("light");
      }}
      className="rounded-lg p-2 hover:bg-gray-100 dark:hover:bg-gray-800"
      aria-label={`Current theme: ${theme}. Click to switch.`}
    >
      {theme === "light" && <SunIcon className="h-5 w-5" />}
      {theme === "dark" && <MoonIcon className="h-5 w-5" />}
      {theme === "system" && <MonitorIcon className="h-5 w-5" />}
    </button>
  );
}

/* globals.css — Token-based dark mode */
@theme {
  --color-surface: oklch(1 0 0);
  --color-surface-elevated: oklch(0.98 0 0);
  --color-surface-sunken: oklch(0.96 0 0);
  --color-text-primary: oklch(0.13 0 0);
  --color-text-secondary: oklch(0.45 0 0);
  --color-border: oklch(0.90 0 0);
}

.dark {
  --color-surface: oklch(0.13 0 0);
  --color-surface-elevated: oklch(0.18 0 0);
  --color-surface-sunken: oklch(0.10 0 0);
  --color-text-primary: oklch(0.95 0 0);
  --color-text-secondary: oklch(0.65 0 0);
  --color-border: oklch(0.25 0 0);
}

5. Responsive Dashboard Layout

A responsive layout that adapts from stacked mobile to sidebar desktop:

export function DashboardLayout({ children }: { children: React.ReactNode }) {
  return (
    <div className="flex min-h-screen flex-col lg:flex-row">
      {/* Sidebar — hidden on mobile, fixed on desktop */}
      <aside className="hidden w-64 shrink-0 border-r border-border bg-surface-elevated lg:block">
        <nav className="sticky top-0 space-y-1 p-4">
          <NavLink href="/dashboard" icon={<HomeIcon />}>Home</NavLink>
          <NavLink href="/dashboard/analytics" icon={<ChartIcon />}>Analytics</NavLink>
          <NavLink href="/dashboard/settings" icon={<GearIcon />}>Settings</NavLink>
        </nav>
      </aside>

      {/* Mobile bottom nav */}
      <nav className="fixed inset-x-0 bottom-0 z-50 border-t border-border bg-surface lg:hidden">
        <div className="flex justify-around py-2">
          <MobileNavLink href="/dashboard" icon={<HomeIcon />} label="Home" />
          <MobileNavLink href="/dashboard/analytics" icon={<ChartIcon />} label="Analytics" />
          <MobileNavLink href="/dashboard/settings" icon={<GearIcon />} label="Settings" />
        </div>
      </nav>

      {/* Main content */}
      <main className="flex-1 pb-16 lg:pb-0">
        <div className="mx-auto max-w-5xl p-4 md:p-6 lg:p-8">
          {children}
        </div>
      </main>
    </div>
  );
}

// Responsive card grid
function DashboardContent() {
  return (
    <div className="space-y-6">
      {/* Stats — 1 col mobile, 2 col tablet, 4 col desktop */}
      <div className="grid grid-cols-1 gap-4 sm:grid-cols-2 lg:grid-cols-4">
        <StatCard label="Revenue" value="$12,345" />
        <StatCard label="Users" value="1,234" />
        <StatCard label="Orders" value="567" />
        <StatCard label="Conversion" value="3.2%" />
      </div>

      {/* Chart + sidebar — stacked on mobile, side-by-side on desktop */}
      <div className="grid grid-cols-1 gap-6 lg:grid-cols-3">
        <div className="lg:col-span-2">
          <ChartCard />
        </div>
        <div>
          <RecentActivity />
        </div>
      </div>
    </div>
  );
}

Common Mistakes

1. Using Runtime CSS-in-JS with Server Components

Wrong:

// This fails in Server Components — no client JS to inject styles
import styled from "styled-components";
const Title = styled.h1`font-size: 2rem;`;

Fix: Use Tailwind or CSS Modules:

<h1 className="text-3xl font-bold">Title</h1>

2. Not Using cn() for Class Merging

Wrong:

export function Card({ className }: { className?: string }) {
  return <div className={`rounded-lg p-4 bg-white ${className}`} />;
  // If className contains "p-8", both p-4 and p-8 apply — unpredictable
}

Fix:

import { cn } from "@/lib/utils";

export function Card({ className }: { className?: string }) {
  return <div className={cn("rounded-lg p-4 bg-white", className)} />;
  // cn() resolves conflicts — p-8 overrides p-4
}

3. Arbitrary Values Everywhere

Wrong:

<div className="w-[347px] h-[89px] mt-[13px] text-[15px] text-[#1a73e8]" />

Fix: Use Tailwind's scale or define tokens:

// Use existing scale values
<div className="w-80 h-20 mt-3 text-sm text-brand-500" />

// Or add tokens if you need custom values
// globals.css: @theme { --spacing-card: 22rem; }

4. Desktop-First Responsive Design

Wrong:

// Start with desktop, hide things for mobile
<div className="grid-cols-4 lg:grid-cols-4 md:grid-cols-2 sm:grid-cols-1" />

Fix: Mobile-first — base classes for mobile, add breakpoints for larger:

<div className="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-4" />

5. Misconfigured Dark Mode Colors

Wrong:

// Hardcoded colors that don't adapt to dark mode
<div className="bg-white text-black border-gray-200" />

Fix: Use semantic tokens:

<div className="bg-surface text-text-primary border-border" />

Or use explicit dark variants:

<div className="bg-white text-gray-900 dark:bg-gray-900 dark:text-gray-50" />

6. Duplicating Tokens Between CSS and Config

Wrong:

/* globals.css */
:root { --brand-color: #3b82f6; }

// tailwind.config.ts
module.exports = { theme: { extend: { colors: { brand: "#3b82f6" } } } }

Fix: In Tailwind v4, define everything once in CSS:

@theme {
  --color-brand: oklch(0.55 0.15 250);
}

7. Overusing @apply

Wrong:

/* Defeats the purpose of utility-first */
.btn {
  @apply inline-flex items-center justify-center gap-2 rounded-md bg-blue-600 px-4 py-2 text-sm font-medium text-white transition-colors hover:bg-blue-700 focus-visible:outline-none focus-visible:ring-2;
}

Fix: Keep utilities in markup. Use cva() for variant logic:

const button = cva("inline-flex items-center justify-center gap-2 rounded-md ...", {
  variants: { variant: { primary: "bg-blue-600 ...", secondary: "bg-gray-100 ..." } },
});

8. Custom Media Queries Alongside Tailwind Breakpoints

Wrong:

/* This doesn't align with Tailwind's breakpoints */
@media (min-width: 768px) { .sidebar { width: 250px; } }
/* But the component uses Tailwind's md: which is also 768px — confusing */

Fix: Use Tailwind's breakpoint utilities consistently. If you need CSS Modules with responsive behavior, use the same breakpoint values as Tailwind.

9. Mixing Multiple Styling Approaches in One Component

Wrong:

<div
  className="flex items-center" // Tailwind
  style={{ backgroundColor: token.bg }} // Inline
>
  <span className={styles.label}>Text</span> {/* CSS Modules */}
</div>

Fix: Pick one approach per component. Tailwind is the default:

<div className="flex items-center" style={{ backgroundColor: token.bg }}>
  <span className="text-sm font-medium text-gray-600">Text</span>
</div>

Inline styles are acceptable only for truly dynamic values (user-configurable colors, calculated positions).

See also: Design Systems | Dark Mode | Typography & Color | Responsive Design | Animation & Motion | Next.js Patterns | Component Patterns

Last reviewed: 2026-02

By Ryan Lind, Assisted by Claude Code and Google Gemini.

CSS Architecture

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