:root {
  --bg: #f8fafc;
  --card: #ffffff;
  --text: #0f172a;
  --muted: #64748b;
  --primary: #1d4ed8;
  --primary-soft: #dbeafe;
  --accent: #0d9488;
  --border: #e2e8f0;
  --success: #059669;
  --font: "Segoe UI", system-ui, -apple-system, sans-serif;
  --font-heading: Georgia, "Times New Roman", serif;
}

*, *::before, *::after { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  font-family: var(--font);
  font-size: 1.05rem;
  line-height: 1.7;
  color: var(--text);
  background: var(--bg);
}

a { color: var(--primary); }
a:hover { text-decoration: underline; }

.wrap { max-width: 720px; margin: 0 auto; padding: 0 1.25rem; }
.wrap-wide { max-width: 900px; margin: 0 auto; padding: 0 1.25rem; }

/* Top bar */
.course-bar {
  background: var(--card);
  border-bottom: 1px solid var(--border);
  padding: 0.75rem 1.25rem;
  position: sticky;
  top: 0;
  z-index: 100;
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: space-between;
  gap: 0.75rem;
}
.course-bar a { font-weight: 600; color: var(--text); text-decoration: none; }
.course-bar a:hover { color: var(--primary); }
.course-bar nav { display: flex; flex-wrap: wrap; gap: 1rem; font-size: 0.9rem; }

/* Hero */
.hero {
  padding: 2.5rem 1.25rem 2rem;
  background: linear-gradient(135deg, #1e3a5f 0%, #0f172a 100%);
  color: #f1f5f9;
}
.hero h1 {
  font-family: var(--font-heading);
  font-size: clamp(1.75rem, 4vw, 2.35rem);
  margin: 0 0 0.75rem;
  line-height: 1.2;
}
.hero .tagline { margin: 0; opacity: 0.92; font-size: 1.05rem; }
.hero .chapter-label {
  margin: 0 0 0.35rem;
  font-size: 0.8rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  opacity: 0.85;
  font-weight: 600;
}
.hero .teacher-note {
  margin-top: 1.25rem;
  padding: 1rem 1.25rem;
  background: rgba(255,255,255,0.1);
  border-radius: 8px;
  border-left: 4px solid var(--accent);
  font-size: 0.98rem;
}

/* Sections */
.section {
  padding: 2.5rem 1.25rem;
}
.section-alt { background: var(--card); border-top: 1px solid var(--border); border-bottom: 1px solid var(--border); }

h2 {
  font-family: var(--font-heading);
  font-size: 1.5rem;
  margin: 0 0 1rem;
  color: #0f172a;
}
h3 { font-size: 1.15rem; margin: 1.75rem 0 0.5rem; color: #1e293b; }
h4 { font-size: 1rem; margin: 1.25rem 0 0.4rem; color: #334155; }

.lead { color: var(--muted); margin: 0 0 1rem; }

/* Teacher boxes */
.teacher {
  background: #fffbeb;
  border: 1px solid #fde68a;
  border-radius: 8px;
  padding: 1rem 1.25rem;
  margin: 1rem 0;
}
.teacher strong { color: #92400e; }
.tip {
  background: var(--primary-soft);
  border-left: 4px solid var(--primary);
  padding: 0.85rem 1.1rem;
  margin: 1rem 0;
  border-radius: 0 6px 6px 0;
  font-size: 0.98rem;
}

/* Article lists */
.article-grid {
  display: grid;
  gap: 0.5rem;
  margin: 0.75rem 0 1rem;
  font-size: 0.95rem;
}
.article-grid dt {
  font-weight: 600;
  color: var(--primary);
  margin-top: 0.5rem;
}
.article-grid dd {
  margin: 0 0 0.75rem 1rem;
  color: #334155;
  display: flex;
  flex-direction: column;
  align-items: stretch;
  gap: 0.5rem;
}
.article-grid-more-wrap {
  line-height: 1.55;
}
.article-grid-more {
  align-self: flex-end;
  margin-top: 0.15rem;
  padding: 0.35rem 0.75rem;
  font: inherit;
  font-size: 0.88rem;
  font-weight: 600;
  color: var(--primary);
  background: var(--primary-soft);
  border: 1px solid rgba(29, 78, 216, 0.25);
  border-radius: 6px;
  cursor: pointer;
  transition: background 0.15s, border-color 0.15s;
}
.article-grid-more:hover {
  background: #dbeafe;
  border-color: var(--primary);
}
.article-grid-more:focus-visible {
  outline: 2px solid var(--primary);
  outline-offset: 2px;
}

ul.check { margin: 0.5rem 0 1rem; padding-left: 1.35rem; }
ul.check li { margin-bottom: 0.35rem; }

/* Quiz */
.quiz {
  margin-top: 1.75rem;
  padding: 1.35rem 1.5rem;
  background: #f1f5f9;
  border-radius: 10px;
  border: 1px solid var(--border);
}
.quiz h3 {
  margin: 0 0 1rem;
  font-size: 1.1rem;
  color: #0f172a;
}
.quiz details {
  margin-bottom: 0.85rem;
  background: var(--card);
  border-radius: 6px;
  border: 1px solid var(--border);
  overflow: hidden;
}
.quiz summary {
  padding: 0.85rem 1rem;
  cursor: pointer;
  font-weight: 500;
  list-style: none;
}
.quiz summary::-webkit-details-marker { display: none; }
.quiz summary::before { content: "❓ "; }
.quiz details[open] summary { border-bottom: 1px solid var(--border); }
.quiz .answer {
  padding: 0.75rem 1rem 1rem;
  font-size: 0.95rem;
  color: var(--muted);
  border-top: 1px dashed var(--border);
}
.quiz .answer strong { color: var(--success); }

/* TOC */
.toc {
  display: grid;
  gap: 0.5rem;
  margin: 1rem 0 0;
  padding: 0;
  list-style: none;
}
.toc a {
  display: block;
  padding: 0.65rem 1rem;
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 6px;
  text-decoration: none;
  color: var(--text);
  transition: border-color 0.2s, box-shadow 0.2s;
}
.toc a:hover {
  border-color: var(--primary);
  box-shadow: 0 2px 8px rgba(29, 78, 216, 0.12);
}

.toc-chapters {
  display: grid;
  gap: 0.45rem;
}
@media (min-width: 640px) {
  .toc-chapters {
    grid-template-columns: repeat(2, 1fr);
  }
}

/* Multiple choice (chapter pages) */
.mcq-section h2 { margin-top: 0; }
.mcq-block {
  margin-bottom: 1.35rem;
  padding: 1rem 1.15rem;
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 8px;
}
.mcq-q { margin: 0 0 0.65rem; }
.mcq-options {
  display: flex;
  flex-direction: column;
  gap: 0.4rem;
  margin-bottom: 0.65rem;
}
.mcq-opt {
  display: flex;
  align-items: flex-start;
  gap: 0.35rem;
  cursor: default;
  font-size: 0.98rem;
  color: #334155;
}
.mcq-opt input { margin-top: 0.35rem; accent-color: var(--primary); }
.mcq-reveal {
  border-top: 1px dashed var(--border);
  padding-top: 0.5rem;
}
.mcq-reveal summary {
  cursor: pointer;
  font-weight: 600;
  color: var(--primary);
  list-style: none;
  font-size: 0.95rem;
}
.mcq-reveal summary::-webkit-details-marker { display: none; }
.mcq-reveal[open] summary { margin-bottom: 0.5rem; }
.mcq-answer {
  padding: 0.65rem 0.85rem;
  background: #ecfdf5;
  border-radius: 6px;
  border-left: 3px solid var(--success);
  font-size: 0.95rem;
}
.mcq-answer p { margin: 0; }

/* Article-by-article list (opens overlay) */
.article-section h2 { margin-top: 0; }
.article-entry-list {
  list-style: none;
  margin: 0;
  padding: 0;
  display: grid;
  gap: 0.5rem;
}

.article-entry-item { margin: 0; }
.article-entry-trigger {
  width: 100%;
  text-align: left;
  cursor: pointer;
  font: inherit;
  font-size: 0.98rem;
  padding: 0.85rem 1rem;
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 8px;
  color: #1e293b;
  transition: border-color 0.2s, box-shadow 0.2s;
  display: flex;
  flex-direction: column;
  align-items: flex-start;
  gap: 0.35rem;
}
.article-entry-trigger:hover {
  border-color: var(--primary);
  box-shadow: 0 2px 8px rgba(29, 78, 216, 0.1);
}
.article-entry-trigger strong { color: var(--primary); }
.article-entry-title-row {
  display: block;
  width: 100%;
  line-height: 1.35;
}
.article-entry-name {
  font-weight: 400;
  color: #334155;
}
.article-entry-name::before {
  content: "\2009\2014\2009";
  font-weight: 400;
  color: #94a3b8;
}
.article-entry-intro {
  display: block;
  width: 100%;
  font-size: 0.88rem;
  line-height: 1.45;
  color: #64748b;
}

/* Article explanation overlay */
.article-overlay {
  position: fixed;
  inset: 0;
  z-index: 1000;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 1rem;
  visibility: hidden;
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.2s ease, visibility 0.2s ease;
}
.article-overlay.is-open {
  visibility: visible;
  opacity: 1;
  pointer-events: auto;
}
.article-overlay-backdrop {
  position: absolute;
  inset: 0;
  background: rgba(15, 23, 42, 0.55);
  cursor: pointer;
}
.article-overlay-panel {
  position: relative;
  z-index: 1;
  width: 100%;
  max-width: 40rem;
  max-height: min(85vh, 32rem);
  display: flex;
  flex-direction: column;
  background: var(--card);
  border-radius: 12px;
  box-shadow: 0 20px 50px rgba(15, 23, 42, 0.25);
  border: 1px solid var(--border);
  overflow: hidden;
}
.article-overlay-close {
  position: absolute;
  top: 0.65rem;
  right: 0.65rem;
  width: 2.25rem;
  height: 2.25rem;
  border: none;
  border-radius: 8px;
  background: #f1f5f9;
  color: #475569;
  font-size: 1.35rem;
  line-height: 1;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
}
.article-overlay-close:hover {
  background: #e2e8f0;
  color: #0f172a;
}
.article-overlay-heading {
  font-family: var(--font-heading);
  font-size: 1.15rem;
  margin: 0;
  padding: 1.1rem 3rem 0.75rem 1.25rem;
  color: #0f172a;
  border-bottom: 1px solid var(--border);
  line-height: 1.4;
  letter-spacing: -0.01em;
}
.article-overlay-body {
  padding: 1.1rem 1.25rem 1.35rem;
  overflow-y: auto;
  color: #334155;
  font-size: 0.98rem;
  line-height: 1.65;
}
.article-overlay-body .article-excerpt-plain {
  margin: 0;
}
.article-overlay-body .article-excerpt-clauses {
  list-style: none;
  margin: 0;
  padding: 0;
}
.article-overlay-body .article-excerpt-clauses > li {
  display: flex;
  gap: 0.65rem;
  align-items: flex-start;
  margin-bottom: 1rem;
}
.article-overlay-body .article-excerpt-clauses > li:last-child {
  margin-bottom: 0;
}
.article-overlay-body .article-clause-label {
  flex: 0 0 auto;
  min-width: 1.75em;
  font-variant-numeric: tabular-nums;
  font-weight: 600;
  color: var(--primary);
  line-height: 1.65;
}
.article-overlay-body .article-clause-text {
  flex: 1;
  min-width: 0;
}
.article-overlay-body .article-clause-text > p {
  margin: 0 0 0.5rem 0;
}
.article-overlay-body .article-clause-text > p:last-child {
  margin-bottom: 0;
}
.article-overlay-body .article-excerpt-lead {
  margin: 0 0 0.6rem 0;
}
.article-overlay-body .article-excerpt-subclauses {
  list-style: none;
  margin: 0.35rem 0 0 0;
  padding: 0 0 0 0.65rem;
  border-left: 2px solid #e2e8f0;
}
.article-overlay-body .article-excerpt-subclauses li {
  margin-bottom: 0.45rem;
  display: flex;
  gap: 0.4rem;
  align-items: flex-start;
  line-height: 1.55;
}
.article-overlay-body .article-excerpt-subclauses li:last-child {
  margin-bottom: 0;
}
.article-overlay-body .article-sub-label {
  flex: 0 0 auto;
  font-weight: 600;
  color: #475569;
  font-size: 0.95em;
}
.article-overlay-body .article-clause-unnumbered .article-clause-text {
  flex: 1;
}
.article-overlay-body .article-overlay-range-section + .article-overlay-range-section {
  margin-top: 1.15rem;
  padding-top: 1.15rem;
  border-top: 1px solid var(--border);
}
.article-overlay-subheading {
  font-family: var(--font-heading);
  font-size: 0.95rem;
  font-weight: 600;
  margin: 0 0 0.5rem 0;
  color: #0f172a;
}

/* Footer */
footer {
  padding: 2rem 1.25rem;
  text-align: center;
  font-size: 0.85rem;
  color: var(--muted);
  border-top: 1px solid var(--border);
}

.page-nav {
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
  gap: 1rem;
  margin: 2rem 0 0;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border);
}
.page-nav a {
  display: inline-block;
  padding: 0.6rem 1.2rem;
  background: var(--primary);
  color: #fff !important;
  text-decoration: none;
  border-radius: 6px;
  font-weight: 600;
  font-size: 0.95rem;
}
.page-nav a:hover { opacity: 0.92; text-decoration: none; }
.page-nav a.secondary {
  background: var(--card);
  color: var(--primary) !important;
  border: 1px solid var(--border);
}

kbd {
  font-size: 0.85em;
  padding: 0.15rem 0.4rem;
  background: #e2e8f0;
  border-radius: 4px;
}