/* ==========================================================================
   Sample CSS Stylesheet
   A realistic stylesheet demonstrating modern CSS patterns and best practices.
   ========================================================================== */

/* --- Custom Properties (Design Tokens) --- */
:root {
    --color-primary: #3b82f6;
    --color-primary-dark: #1d4ed8;
    --color-secondary: #8b5cf6;
    --color-success: #10b981;
    --color-warning: #f59e0b;
    --color-danger: #ef4444;
    --color-text: #1f2937;
    --color-text-muted: #6b7280;
    --color-bg: #ffffff;
    --color-bg-alt: #f9fafb;
    --color-border: #e5e7eb;

    --font-sans: 'Inter', system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
    --font-mono: 'Fira Code', 'Cascadia Code', 'JetBrains Mono', monospace;

    --spacing-xs: 0.25rem;
    --spacing-sm: 0.5rem;
    --spacing-md: 1rem;
    --spacing-lg: 1.5rem;
    --spacing-xl: 2rem;
    --spacing-2xl: 3rem;

    --radius-sm: 0.25rem;
    --radius-md: 0.5rem;
    --radius-lg: 1rem;
    --radius-full: 9999px;

    --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.05);
    --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -2px rgba(0, 0, 0, 0.1);
    --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -4px rgba(0, 0, 0, 0.1);

    --transition-fast: 150ms ease;
    --transition-base: 250ms ease;
}

/* --- CSS Reset --- */
*,
*::before,
*::after {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

html {
    font-size: 16px;
    scroll-behavior: smooth;
    -webkit-text-size-adjust: 100%;
}

body {
    font-family: var(--font-sans);
    color: var(--color-text);
    background-color: var(--color-bg);
    line-height: 1.6;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

img, picture, video, canvas, svg {
    display: block;
    max-width: 100%;
    height: auto;
}

input, button, textarea, select {
    font: inherit;
    color: inherit;
}

a {
    color: var(--color-primary);
    text-decoration: none;
    transition: color var(--transition-fast);
}

a:hover { color: var(--color-primary-dark); }

/* --- Typography --- */
h1, h2, h3, h4, h5, h6 {
    font-weight: 700;
    line-height: 1.25;
    margin-bottom: var(--spacing-md);
}

h1 { font-size: 2.25rem; letter-spacing: -0.025em; }
h2 { font-size: 1.875rem; letter-spacing: -0.02em; }
h3 { font-size: 1.5rem; }
h4 { font-size: 1.25rem; }
p { margin-bottom: var(--spacing-md); }
code { font-family: var(--font-mono); font-size: 0.875em; background: var(--color-bg-alt); padding: 0.2em 0.4em; border-radius: var(--radius-sm); }

/* --- Layout --- */
.container {
    width: 100%;
    max-width: 1200px;
    margin-inline: auto;
    padding-inline: var(--spacing-xl);
}

.content-grid {
    display: grid;
    grid-template-columns: 1fr 300px;
    gap: var(--spacing-xl);
    align-items: start;
}

.footer-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: var(--spacing-xl);
    padding: var(--spacing-2xl) 0;
}

/* --- Components: Card --- */
.card {
    background: var(--color-bg);
    border: 1px solid var(--color-border);
    border-radius: var(--radius-lg);
    padding: var(--spacing-lg);
    box-shadow: var(--shadow-sm);
    transition: box-shadow var(--transition-base), transform var(--transition-base);
}

.card:hover {
    box-shadow: var(--shadow-md);
    transform: translateY(-2px);
}

.card__title { font-size: 1.25rem; margin-bottom: var(--spacing-sm); }
.card__body { color: var(--color-text-muted); font-size: 0.95rem; }
.card__footer { margin-top: var(--spacing-md); padding-top: var(--spacing-md); border-top: 1px solid var(--color-border); display: flex; justify-content: space-between; align-items: center; }

/* --- Components: Button --- */
.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: var(--spacing-sm);
    padding: 0.625rem 1.25rem;
    font-weight: 600;
    font-size: 0.875rem;
    border: none;
    border-radius: var(--radius-md);
    cursor: pointer;
    transition: background-color var(--transition-fast), transform var(--transition-fast);
    text-decoration: none;
}

.btn:active { transform: scale(0.98); }
.btn--primary { background-color: var(--color-primary); color: #fff; }
.btn--primary:hover { background-color: var(--color-primary-dark); color: #fff; }
.btn--secondary { background-color: var(--color-bg-alt); color: var(--color-text); border: 1px solid var(--color-border); }
.btn--secondary:hover { background-color: var(--color-border); }
.btn--danger { background-color: var(--color-danger); color: #fff; }
.btn--sm { padding: 0.375rem 0.75rem; font-size: 0.8125rem; }
.btn--lg { padding: 0.875rem 1.75rem; font-size: 1rem; }

/* --- Components: Navigation --- */
nav {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: var(--spacing-md) var(--spacing-xl);
    background: var(--color-bg);
    border-bottom: 1px solid var(--color-border);
    position: sticky;
    top: 0;
    z-index: 100;
}

.nav-brand a {
    font-size: 1.25rem;
    font-weight: 700;
    color: var(--color-text);
}

.nav-links {
    display: flex;
    list-style: none;
    gap: var(--spacing-lg);
}

.nav-links a {
    color: var(--color-text-muted);
    font-weight: 500;
    padding: var(--spacing-xs) 0;
    border-bottom: 2px solid transparent;
    transition: color var(--transition-fast), border-color var(--transition-fast);
}

.nav-links a:hover,
.nav-links a.active {
    color: var(--color-primary);
    border-bottom-color: var(--color-primary);
}

.nav-toggle { display: none; }

/* --- Media Queries --- */
@media (max-width: 768px) {
    .content-grid {
        grid-template-columns: 1fr;
    }

    .footer-grid {
        grid-template-columns: 1fr;
        gap: var(--spacing-lg);
    }

    .nav-links { display: none; }
    .nav-toggle { display: block; background: none; border: none; cursor: pointer; }

    h1 { font-size: 1.75rem; }
    h2 { font-size: 1.5rem; }
}

@media (prefers-color-scheme: dark) {
    :root {
        --color-text: #f3f4f6;
        --color-text-muted: #9ca3af;
        --color-bg: #111827;
        --color-bg-alt: #1f2937;
        --color-border: #374151;
    }
}

@media (prefers-reduced-motion: reduce) {
    * { transition-duration: 0.01ms !important; animation-duration: 0.01ms !important; scroll-behavior: auto; }
}