:root {
    color-scheme: light;
    --primary: #2563eb;
    --primary-dark: #1d4ed8;
    --primary-light: #dbeafe;
    --secondary: #7c3aed;
    --accent: #059669;
    --accent-light: #d1fae5;
    --accent-dark: #047857;
    --warm: #f59e0b;
    --warm-light: #fef3c7;
    --rose: #f43f5e;
    --rose-light: #ffe4e6;
    --cyan: #06b6d4;
    --cyan-light: #cffafe;
    --violet: #8b5cf6;
    --violet-light: #ede9fe;
    --bg: #f0f4f8;
    --surface: #ffffff;
    --surface-alt: #f8fafc;
    --text: #0f172a;
    --text-secondary: #475569;
    --text-muted: #94a3b8;
    --border: #e2e8f0;
    --shadow-sm: 0 1px 2px rgba(0,0,0,0.05);
    --shadow: 0 1px 3px rgba(0,0,0,0.1), 0 1px 2px rgba(0,0,0,0.06);
    --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);
    --shadow-xl: 0 20px 25px -5px rgba(0,0,0,0.1), 0 8px 10px -6px rgba(0,0,0,0.1);
    --radius: 12px;
    --radius-sm: 8px;
    --radius-lg: 16px;
}

* { margin: 0; padding: 0; box-sizing: border-box; }

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
    line-height: 1.75;
    color: var(--text);
    background: var(--bg);
    -webkit-font-smoothing: antialiased;
}

.container { max-width: 1200px; margin: 0 auto; padding: 0 24px; }

a { color: var(--primary); text-decoration: none; transition: color 0.2s; }
a:hover { color: var(--primary-dark); text-decoration: underline; }

/* Header */
header {
    background: linear-gradient(135deg, #1e3a5f 0%, #0f172a 50%, #1e293b 100%);
    color: white;
    padding: 1rem 0;
    position: sticky;
    top: 0;
    z-index: 100;
    box-shadow: 0 4px 20px rgba(0,0,0,0.15);
    border-bottom: 3px solid;
    border-image: linear-gradient(90deg, var(--primary), var(--secondary), var(--accent), var(--warm), var(--rose)) 1;
}

header .container {
    display: flex;
    justify-content: space-between;
    align-items: center;
}

header h1 { font-size: 1.35rem; font-weight: 700; letter-spacing: -0.02em; }
header h1 a { color: white; text-decoration: none; display: flex; align-items: center; gap: 0.5rem; }
header h1 a:hover { text-decoration: none; opacity: 0.9; }
header a { color: rgba(255,255,255,0.8); text-decoration: none; font-size: 0.85rem; }
header a:hover { color: white; }

/* Navigation */
nav {
    background: linear-gradient(180deg, var(--surface) 0%, #f8fafc 100%);
    border-bottom: 2px solid var(--border);
    padding: 0.6rem 0;
    box-shadow: var(--shadow-sm);
    position: sticky;
    top: 60px;
    z-index: 99;
    border-top: 3px solid transparent;
    border-image: linear-gradient(90deg, var(--primary), var(--secondary), var(--accent), var(--warm)) 1;
}

nav .container {
    display: flex;
    flex-wrap: wrap;
    gap: 0.4rem;
    align-items: center;
}

nav a {
    color: var(--text-secondary);
    text-decoration: none;
    font-size: 0.78rem;
    font-weight: 500;
    padding: 0.3rem 0.7rem;
    border-radius: 20px;
    transition: all 0.2s;
    white-space: nowrap;
}

nav a:hover, nav a.active {
    color: var(--primary);
    background: var(--primary-light);
    text-decoration: none;
}

/* Main content */
main { padding: 2rem 0 4rem; min-height: 60vh; background: var(--bg); }

main::before {
    content: '';
    position: fixed;
    top: 0; left: 0; right: 0;
    height: 4px;
    background: linear-gradient(90deg, var(--primary), var(--secondary), var(--accent), var(--warm), var(--rose));
    z-index: 200;
}

/* Footer */
footer {
    background: linear-gradient(135deg, #1e293b 0%, #0f172a 100%);
    color: #94a3b8;
    padding: 2.5rem 0;
    text-align: center;
    font-size: 0.85rem;
    margin-top: 3rem;
    border-top: 3px solid;
    border-image: linear-gradient(90deg, var(--primary), var(--secondary), var(--accent), var(--warm), var(--rose)) 1;
}

footer a { color: #cbd5e1; text-decoration: none; }
footer a:hover { color: white; }

/* Chemical formula styling */
sub { font-size: 0.75em; vertical-align: sub; }
sup { font-size: 0.75em; vertical-align: super; }
u { text-decoration: underline; text-decoration-color: var(--accent); text-underline-offset: 0.2em; }

/* Hero section */
.hero {
    background: linear-gradient(135deg, #1e3a5f 0%, #0f172a 40%, #1e293b 100%);
    color: white;
    padding: 5rem 0 4rem;
    text-align: center;
    position: relative;
    overflow: hidden;
}

.hero::before {
    content: '';
    position: absolute;
    top: -50%; left: -50%; right: -50%; bottom: -50%;
    background: radial-gradient(ellipse at 30% 50%, rgba(37, 99, 235, 0.15) 0%, transparent 50%),
                radial-gradient(ellipse at 70% 50%, rgba(139, 92, 246, 0.1) 0%, transparent 50%);
    animation: heroPulse 8s ease-in-out infinite;
}

@keyframes heroPulse {
    0%, 100% { opacity: 0.5; transform: scale(1); }
    50% { opacity: 1; transform: scale(1.05); }
}

.hero * { position: relative; z-index: 1; }

.hero h1 {
    font-size: 3.2rem;
    font-weight: 800;
    margin-bottom: 1rem;
    letter-spacing: -0.03em;
    background: linear-gradient(135deg, #ffffff 0%, #93c5fd 100%);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.hero p {
    font-size: 1.2rem;
    opacity: 0.85;
    max-width: 640px;
    margin: 0 auto;
    line-height: 1.8;
}

/* Section cards grid */
.section-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
    gap: 1.25rem;
    margin: 2.5rem 0;
}

.section-card {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 1.5rem;
    transition: all 0.25s cubic-bezier(0.4, 0, 0.2, 1);
    text-decoration: none;
    color: inherit;
    position: relative;
    overflow: hidden;
}

.section-card::before {
    content: '';
    position: absolute;
    top: 0; left: 0; right: 0;
    height: 3px;
    background: linear-gradient(90deg, var(--primary), var(--secondary));
    opacity: 0;
    transition: opacity 0.25s;
}

.section-card:hover {
    box-shadow: var(--shadow-lg);
    transform: translateY(-3px);
    border-color: transparent;
    text-decoration: none;
}

.section-card:hover::before { opacity: 1; }
.section-card:hover { text-decoration: none; }

.section-card h3 {
    font-size: 1.15rem;
    margin-bottom: 0.5rem;
    color: var(--text);
    font-weight: 600;
}

.section-card p {
    color: var(--text-secondary);
    font-size: 0.9rem;
    line-height: 1.6;
}

/* Content cards */
.card-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
    gap: 1.25rem;
    margin: 2rem 0;
}

.card {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 1.25rem;
    transition: all 0.25s cubic-bezier(0.4, 0, 0.2, 1);
    text-decoration: none;
    color: inherit;
    position: relative;
}

.card:hover {
    box-shadow: var(--shadow-md);
    transform: translateY(-2px);
    border-color: var(--primary);
    background: linear-gradient(135deg, var(--surface) 0%, var(--primary-light) 100%);
}

.card h2, .card h3 {
    font-size: 1.05rem;
    margin-bottom: 0.5rem;
    color: var(--text);
}

.card h2 a, .card h3 a { color: var(--text); text-decoration: none; }
.card h2 a:hover, .card h3 a:hover { color: var(--primary); text-decoration: none; }

.card .tag {
    display: inline-block;
    padding: 0.2rem 0.6rem;
    border-radius: 9999px;
    font-size: 0.72rem;
    font-weight: 600;
    margin-top: 0.5rem;
    letter-spacing: 0.02em;
}

.tag-primary { background: var(--primary-light); color: var(--primary-dark); }
.tag-accent { background: var(--accent-light); color: var(--accent-dark); }
.tag-warm { background: var(--warm-light); color: #92400e; }
.tag-rose { background: var(--rose-light); color: #be123c; }
.tag-cyan { background: var(--cyan-light); color: #0e7490; }
.tag-violet { background: var(--violet-light); color: #6d28d9; }

/* Breadcrumbs */
.breadcrumb {
    padding: 0.75rem 0;
    font-size: 0.85rem;
    color: var(--text-muted);
    border-bottom: 1px solid var(--border);
    margin-bottom: 1.5rem;
}

.breadcrumb a {
    color: var(--primary);
    text-decoration: none;
    font-weight: 500;
}

.breadcrumb a:hover { text-decoration: underline; color: var(--primary-dark); }
.breadcrumb span { margin: 0 0.4rem; color: var(--text-muted); }

/* Article content */
.article-content { max-width: 800px; }

.article-content h1 {
    font-size: 2.2rem;
    font-weight: 800;
    margin: 0 0 1rem 0;
    letter-spacing: -0.02em;
    color: var(--text);
    line-height: 1.3;
}

.article-content h2 {
    font-size: 1.5rem;
    margin: 2.5rem 0 1rem;
    padding: 0.75rem 0 0.75rem 1rem;
    border-top: none;
    border-left: 4px solid var(--primary);
    background: linear-gradient(90deg, var(--primary-light) 0%, transparent 100%);
    border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
    color: var(--text);
    font-weight: 700;
    letter-spacing: -0.01em;
}

.article-content h2:first-of-type {
    border-top: none;
    padding-top: 0.75rem;
    margin-top: 0;
}

.article-content h3 {
    font-size: 1.2rem;
    margin: 2rem 0 0.75rem;
    color: var(--primary-dark);
    font-weight: 600;
    padding-left: 0.75rem;
    border-left: 3px solid var(--accent);
}

.article-content h4 {
    font-size: 1.05rem;
    margin: 1.5rem 0 0.5rem;
    color: var(--secondary);
    font-weight: 600;
}

.article-content p {
    margin-bottom: 1rem;
    color: var(--text-secondary);
    line-height: 1.8;
}

.article-content ul, .article-content ol {
    margin: 1rem 0;
    padding-left: 2rem;
}

.article-content li {
    margin-bottom: 0.5rem;
    color: var(--text-secondary);
    padding-left: 0.25rem;
}

.article-content li::marker {
    color: var(--accent);
    font-weight: 700;
    font-size: 1.1em;
}

.article-content strong { color: var(--text); font-weight: 600; }

.article-content a { color: var(--primary); text-decoration: none; border-bottom: 1px solid transparent; transition: border-color 0.2s; }
.article-content a:hover { border-bottom-color: var(--primary); }

.article-content blockquote {
    border-left: 4px solid var(--primary);
    padding: 1rem 1.25rem;
    margin: 1.5rem 0;
    background: linear-gradient(135deg, var(--primary-light) 0%, #eff6ff 100%);
    border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
    color: var(--text-secondary);
    font-style: italic;
}

.article-content code {
    background: linear-gradient(135deg, #fef2f2, #fff1f2);
    padding: 0.15rem 0.4rem;
    border-radius: 4px;
    font-size: 0.9em;
    font-family: 'JetBrains Mono', 'Fira Code', monospace;
    color: var(--rose);
    border: 1px solid #fecdd3;
}

.article-content pre {
    background: linear-gradient(135deg, #0f172a, #1e293b);
    color: #e2e8f0;
    padding: 1.25rem;
    border-radius: var(--radius);
    overflow-x: auto;
    margin: 1.5rem 0;
    font-family: 'JetBrains Mono', 'Fira Code', monospace;
    font-size: 0.875rem;
    line-height: 1.6;
    border-left: 4px solid var(--secondary);
    box-shadow: var(--shadow-md);
}

.article-content pre code {
    background: transparent;
    color: inherit;
    padding: 0;
}

.article-content hr {
    border: none;
    border-top: 2px solid;
    border-image: linear-gradient(90deg, var(--primary), var(--accent), transparent) 1;
    margin: 2.5rem 0;
}

/* Info box */
.info-box {
    background: linear-gradient(135deg, #eff6ff 0%, #dbeafe 100%);
    border-left: 4px solid var(--primary);
    padding: 1.25rem 1.5rem;
    border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
    margin: 1.5rem 0;
}

.info-box h4 {
    color: var(--primary-dark);
    margin-bottom: 0.5rem;
    font-size: 1rem;
}

.info-box p { color: var(--text-secondary); margin: 0.25rem 0; }

.info-box.warning {
    background: linear-gradient(135deg, #fffbeb 0%, #fef3c7 100%);
    border-left-color: var(--warm);
}

.info-box.warning h4 { color: #92400e; }

.info-box.danger {
    background: linear-gradient(135deg, #fff1f2 0%, #ffe4e6 100%);
    border-left-color: var(--rose);
}

.info-box.danger h4 { color: #be123c; }

.info-box.success {
    background: linear-gradient(135deg, #f0fdf4 0%, #dcfce7 100%);
    border-left-color: var(--accent);
}

.info-box.success h4 { color: var(--accent-dark); }

/* Table styling */
table {
    width: 100%;
    border-collapse: collapse;
    margin: 1.5rem 0;
    font-size: 0.9rem;
    border-radius: var(--radius);
    overflow: hidden;
    box-shadow: var(--shadow-sm);
}

th, td {
    padding: 0.7rem 1rem;
    text-align: left;
    border-bottom: 1px solid var(--border);
}

th {
    background: linear-gradient(135deg, var(--primary) 0%, var(--secondary) 100%);
    font-weight: 600;
    color: white;
    font-size: 0.8rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

tr:hover { background: #f8fafc; }

tr:nth-child(even) { background: #fafbfc; }
tr:nth-child(even):hover { background: #f1f5f9; }

/* FAQ section */
.faq-section { margin: 3rem 0; }

.faq-item {
    border: 1px solid var(--border);
    border-radius: var(--radius);
    margin-bottom: 0.75rem;
    overflow: hidden;
    transition: box-shadow 0.2s;
}

.faq-item:hover { box-shadow: var(--shadow-sm); }

.faq-question {
    padding: 1rem 1.25rem;
    background: linear-gradient(135deg, var(--surface) 0%, #f8fafc 100%);
    font-weight: 600;
    cursor: pointer;
    display: flex;
    justify-content: space-between;
    align-items: center;
    color: var(--text);
    border-left: 3px solid var(--primary);
}

.faq-question::after {
    content: '+';
    font-size: 1.2rem;
    color: var(--primary);
    font-weight: 300;
}

.faq-answer {
    padding: 1rem 1.25rem;
    background: var(--surface-alt);
    border-top: 1px solid var(--border);
}

/* Pagination */
.pagination {
    display: flex;
    justify-content: center;
    gap: 0.4rem;
    margin: 2.5rem 0;
    flex-wrap: wrap;
}

.pagination a, .pagination span {
    padding: 0.5rem 0.85rem;
    border: 1px solid var(--border);
    border-radius: var(--radius-sm);
    text-decoration: none;
    color: var(--text-secondary);
    font-size: 0.85rem;
    font-weight: 500;
    transition: all 0.2s;
    background: var(--surface);
}

.pagination a:hover {
    background: var(--primary-light);
    border-color: var(--primary);
    color: var(--primary);
    text-decoration: none;
    transform: translateY(-1px);
    box-shadow: var(--shadow-sm);
}

.pagination .active {
    background: linear-gradient(135deg, var(--primary), var(--secondary));
    color: white;
    border-color: var(--primary);
    box-shadow: var(--shadow-md);
}

/* Cards page listing */
.card-listing {
    margin: 1rem 0;
}

.card-listing .card {
    display: flex;
    justify-content: space-between;
    align-items: flex-start;
    gap: 1rem;
}

.card-listing .card .card-meta {
    font-size: 0.8rem;
    color: var(--text-muted);
    margin-top: 0.25rem;
}

/* Stats bar */
.stats-bar {
    display: flex;
    gap: 1.5rem;
    flex-wrap: wrap;
    margin: 2rem 0;
}

.stat-item {
    flex: 1;
    min-width: 150px;
    text-align: center;
    padding: 1.25rem;
    background: var(--surface);
    border-radius: var(--radius);
    border: 1px solid var(--border);
}

.stat-item .stat-number {
    font-size: 2rem;
    font-weight: 800;
    background: linear-gradient(135deg, var(--primary), var(--secondary));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    line-height: 1;
}

.stat-item .stat-label {
    font-size: 0.8rem;
    color: var(--text-muted);
    margin-top: 0.25rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

/* Badge */
.badge {
    display: inline-flex;
    align-items: center;
    padding: 0.2rem 0.6rem;
    border-radius: 9999px;
    font-size: 0.72rem;
    font-weight: 600;
    letter-spacing: 0.02em;
}

.badge-primary { background: var(--primary-light); color: var(--primary-dark); }
.badge-accent { background: var(--accent-light); color: var(--accent-dark); }
.badge-warm { background: var(--warm-light); color: #92400e; }
.badge-rose { background: var(--rose-light); color: #be123c; }

/* Info box cite */
.cite-box {
    margin-top: 2rem;
    padding: 1.5rem;
    background: linear-gradient(135deg, var(--violet-light) 0%, var(--primary-light) 100%);
    border-radius: var(--radius);
    border: 1px solid #c4b5fd;
    border-left: 4px solid var(--violet);
}

.cite-box h3 {
    font-size: 0.9rem;
    color: var(--violet);
    margin-bottom: 0.5rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

.cite-box p {
    font-size: 0.85rem;
    color: var(--text-secondary);
    margin: 0.2rem 0;
}

.cite-box a { color: var(--violet); font-weight: 500; }

/* Internal links section */
.internal-links-section {
    margin-top: 2rem;
    padding: 1.5rem;
    background: linear-gradient(135deg, #f0fdf4 0%, #ecfdf5 100%);
    border-radius: var(--radius);
    border: 1px solid #bbf7d0;
}

.internal-links-section h2 {
    font-size: 1.1rem;
    margin-bottom: 1rem;
    padding-bottom: 0.5rem;
    border-bottom: 2px solid var(--accent-light);
    color: var(--accent-dark);
}

.internal-links-section h2 a { text-decoration: none; color: var(--text); }

/* Related topics */
.related-topics {
    margin-top: 3rem;
    padding-top: 1.5rem;
    border-top: 3px solid;
    border-image: linear-gradient(90deg, var(--primary), var(--accent), var(--warm)) 1;
}

.related-topics h2 {
    font-size: 1.1rem;
    margin-bottom: 1rem;
    color: var(--text-secondary);
}

.related-topics ul {
    list-style: none;
    padding: 0;
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
    gap: 0.5rem;
}

.related-topics li a {
    display: block;
    padding: 0.5rem 0.75rem;
    border-radius: var(--radius-sm);
    background: var(--surface-alt);
    color: var(--text-secondary);
    font-size: 0.85rem;
    text-decoration: none;
    transition: all 0.2s;
}

.related-topics li a:hover {
    background: var(--primary-light);
    color: var(--primary);
    text-decoration: none;
}

/* Section header with gradient */
.section-header {
    background: linear-gradient(135deg, var(--primary-light) 0%, var(--violet-light) 100%);
    padding: 1.5rem;
    border-radius: var(--radius);
    margin-bottom: 1.5rem;
}

.section-header h2 {
    font-size: 1.3rem;
    margin: 0;
}

/* Key takeaway box */
.key-takeaway {
    background: linear-gradient(135deg, #f0fdf4 0%, #dcfce7 100%);
    border: 1px solid #bbf7d0;
    border-radius: var(--radius);
    padding: 1.25rem 1.5rem;
    margin: 1.5rem 0;
}

.key-takeaway h4 {
    color: var(--accent-dark);
    margin-bottom: 0.5rem;
}

.key-takeaway p {
    color: var(--text-secondary);
    font-size: 0.95rem;
}

/* TOC (Table of Contents) */
.toc {
    background: linear-gradient(135deg, var(--surface) 0%, #f8fafc 100%);
    border: 1px solid var(--border);
    border-left: 4px solid var(--secondary);
    border-radius: var(--radius);
    padding: 1.25rem 1.5rem;
    margin: 1.5rem 0;
}

.toc h3 {
    font-size: 0.9rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    color: var(--secondary);
    margin-bottom: 0.75rem;
}

.toc ul {
    list-style: none;
    padding: 0;
}

.toc li { margin-bottom: 0.35rem; }

.toc a {
    color: var(--text-secondary);
    font-size: 0.9rem;
    text-decoration: none;
    transition: color 0.2s;
}

.toc a:hover { color: var(--primary); text-decoration: none; }

/* Responsive */
@media (max-width: 768px) {
    .hero { padding: 3rem 0; }
    .hero h1 { font-size: 2rem; }
    .hero p { font-size: 1rem; }
    .section-grid { grid-template-columns: 1fr; }
    .card-grid { grid-template-columns: 1fr; }
    nav .container { gap: 0.3rem; }
    nav a { font-size: 0.7rem; padding: 0.25rem 0.5rem; }
    .article-content h1 { font-size: 1.7rem; }
    .article-content h2 { font-size: 1.3rem; }
    .stats-bar { flex-direction: column; }
    .related-topics ul { grid-template-columns: 1fr; }
}