/* ═════════════════════════════════════════
   RL Learning Lab — Design System
   Dark theme, consistent typography, reusable components
   ═════════════════════════════════════════ */

:root {
  /* Colors */
  --bg:          #0d1117;
  --bg-card:     #161b22;
  --bg-card-hover: #1c2333;
  --bg-input:    #0d1117;
  --border:      #30363d;
  --border-focus:#58a6ff;
  --text:        #e6edf3;
  --text-muted:  #8b949e;
  --text-dim:    #6e7681;
  --text-desc:   #9CA3AF;

  --accent:      #58a6ff;
  --accent-hover:#79c0ff;
  --green:       #3fb950;
  --green-dim:   #238636;
  --amber:       #d29922;
  --red:         #f85149;
  --purple:      #bc8cff;
  --teal:        #2dd4bf;
  --pink:        #f778ba;

  /* Typography */
  --font-sans:  'Inter', -apple-system, 'Segoe UI', system-ui, sans-serif;
  --font-mono:  'SF Mono', 'Cascadia Code', 'DejaVu Sans Mono', monospace;

  /* Spacing */
  --radius:     8px;
  --radius-sm:  4px;

  /* Shadows & Effects */
  --shadow-card: 0 4px 16px rgba(0,0,0,0.2);
  --shadow-lg:   0 8px 32px rgba(0,0,0,0.3);
  --transition-card: transform 0.2s ease, border-color 0.2s ease, box-shadow 0.2s ease;

  /* Homepage-specific tokens */
  --accent-home:  #22D3EE;
  --shadow-glow:  0 0 24px rgba(34, 211, 238, 0.25);
  --bg-home-card: #131922;
}

/* ═════════════════════════════════════════
   Reset & Base
   ═════════════════════════════════════════ */

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

html {
  font-size: 15px;
  -webkit-font-smoothing: antialiased;
}

body {
  font-family: var(--font-sans);
  font-size: 0.95rem;
  line-height: 1.6;
  color: var(--text);
  background: var(--bg);
  min-height: 100vh;
}

a {
  color: var(--accent);
  text-decoration: none;
}
a:hover {
  color: var(--accent-hover);
  text-decoration: underline;
}

/* ═════════════════════════════════════════
   Layout
   ═════════════════════════════════════════ */

.container {
  max-width: 1100px;
  margin: 0 auto;
  padding: 24px 20px 60px;
}

.header {
  padding: 20px 0 16px;
  border-bottom: 1px solid var(--border);
  margin-bottom: 28px;
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: 12px;
}

.header h1 {
  font-size: 1.6rem;
  font-weight: 700;
  color: var(--text);
}

.header .subtitle {
  color: var(--text-muted);
  font-size: 0.9rem;
}

.header nav a {
  margin-left: 16px;
  font-size: 0.9rem;
}

/* ═════════════════════════════════════════
   Cards
   ═════════════════════════════════════════ */

.card {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 20px;
  margin-bottom: 16px;
}

.card:hover {
  border-color: var(--text-dim);
}

.card h2 {
  font-size: 1.1rem;
  margin-bottom: 6px;
}

.card p {
  color: var(--text-muted);
  font-size: 0.9rem;
}

.card-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
  gap: 16px;
}

/* ═════════════════════════════════════════
   Controls
   ═════════════════════════════════════════ */

.controls {
  display: flex;
  flex-wrap: wrap;
  gap: 12px 20px;
  align-items: center;
}

.control-group {
  display: flex;
  flex-direction: column;
  gap: 4px;
}

.control-group label {
  font-size: 0.8rem;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

select, input[type="number"], input[type="text"] {
  background: var(--bg-input);
  color: var(--text);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  padding: 5px 10px;
  font-size: 1rem;
  font-family: var(--font-sans);
  min-width: 80px;
}

select:focus, input:focus {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
  border-color: var(--border-focus);
  box-shadow: 0 0 0 2px rgba(88, 166, 255, 0.15);
}

button {
  background: var(--green-dim);
  color: #fff;
  border: none;
  border-radius: var(--radius-sm);
  padding: 8px 18px;
  font-size: 0.9rem;
  font-weight: 500;
  cursor: pointer;
  transition: background 0.15s;
}

button:hover {
  background: var(--green);
}

button.secondary {
  background: var(--bg-card);
  border: 1px solid var(--border);
  color: var(--text);
}

button.secondary:hover {
  border-color: var(--text-muted);
}

button.danger {
  background: var(--red);
}

button:disabled {
  opacity: 0.4;
  cursor: not-allowed;
}

/* Toggle / mode selector button (ch10/11/12, RL mode switches) */
.mode-btn {
  flex: 1;
  min-width: 55px;
  font-size: 0.78rem;
  padding: 10px 8px;
  min-height: 36px;
  background: var(--bg-card);
  border: 1px solid var(--border);
  color: var(--text-muted);
  cursor: pointer;
  border-radius: var(--radius-sm);
  transition: color 0.15s, border-color 0.15s, background 0.15s;
}
.mode-btn:hover {
  border-color: var(--text-muted);
  background: var(--bg-card-hover);
  color: var(--text);
}
.mode-btn.active {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
}

/* ═════════════════════════════════════════
   Slider (range input)
   ═════════════════════════════════════════ */

input[type="range"] {
  -webkit-appearance: none;
  appearance: none;
  background: transparent;
  cursor: pointer;
  width: 140px;
  padding: 10px 0; /* expand touch target while keeping 4px visual track */
}

input[type="range"]::-webkit-slider-runnable-track {
  height: 4px;
  background: var(--border);
  border-radius: 2px;
}

input[type="range"]::-webkit-slider-thumb {
  -webkit-appearance: none;
  height: 16px;
  width: 16px;
  border-radius: 50%;
  background: var(--accent);
  margin-top: -6px;
}

input[type="range"]:focus::-webkit-slider-thumb {
  box-shadow: 0 0 0 3px rgba(88, 166, 255, 0.3);
}

/* Firefox */
input[type="range"]::-moz-range-track {
  height: 4px;
  background: var(--border);
  border-radius: 2px;
}

input[type="range"]::-moz-range-thumb {
  height: 16px;
  width: 16px;
  border: none;
  border-radius: 50%;
  background: var(--accent);
}

input[type="range"]:focus::-moz-range-thumb {
  box-shadow: 0 0 0 3px rgba(88, 166, 255, 0.3);
}

/* ═════════════════════════════════════════
   Chart containers
   ═════════════════════════════════════════ */

.chart-row {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 16px;
  margin: 16px 0;
}

.chart-box {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 12px;
  position: relative;
  height: 360px;
}

/* Height tiers — use these instead of per-page overrides */
.chart-box.tall  { height: 420px; }
.chart-box.short { height: 200px; }

.chart-box.full-width {
  grid-column: 1 / -1;
}

/* Canvas must be the only flow child of .chart-box
   (labels/参数行等请移出或用绝对定位), otherwise Chart.js
   responsive sizing will overlap the extra content. */
.chart-box canvas {
  width: 100% !important;
  height: 100% !important;
}

/* ═════════════════════════════════════════
   Chapter page layouts (shared across all chapters)
   ═════════════════════════════════════════ */


/* Section title label */
.section-title {
  font-size: 0.78rem;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  margin-bottom: 8px;
  padding-bottom: 6px;
  border-bottom: 1px solid var(--border);
}

/* Stat display line */
.stat {
  font-family: var(--font-mono);
  font-size: 0.85rem;
  line-height: 1.7;
}
.stat span {
  color: var(--accent);
  font-weight: 600;
}


/* Button row */
.btn-row {
  display: flex;
  gap: 8px;
  flex-wrap: wrap;
}
.btn-row button {
  flex: 1;
  min-width: 60px;
}

/* Explanation / hint box */
.explain-box {
  background: rgba(88,166,255,0.05);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 12px 14px;
  font-size: 0.8rem;
  line-height: 1.55;
  color: var(--text-muted);
}
.explain-box strong {
  color: var(--text);
}

/* Inline formula snippet */
.formula-inline {
  font-family: var(--font-mono);
  background: rgba(88,166,255,0.06);
  padding: 2px 6px;
  border-radius: 3px;
  font-size: 0.82rem;
  white-space: nowrap;
}

/* Step / page indicator */
.step-indicator {
  font-family: var(--font-mono);
  font-size: 0.82rem;
  color: var(--text-muted);
  text-align: center;
  padding: 6px 0;
}

/* ═════════════════════════════════════════
   Formula blocks
   ═════════════════════════════════════════ */

.formula {
  font-family: var(--font-mono);
  background: rgba(88, 166, 255, 0.06);
  border-left: 3px solid var(--accent);
  padding: 10px 14px;
  margin: 12px 0;
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  font-size: 0.9rem;
  overflow-x: auto;
}

/* ═════════════════════════════════════════
   Badges
   ═════════════════════════════════════════ */

.badge {
  display: inline-block;
  padding: 2px 8px;
  border-radius: 10px;
  font-size: 0.75rem;
  font-weight: 600;
}

.badge-ready    { background: rgba(63, 185, 80, 0.15); color: var(--green); }

/* ═════════════════════════════════════════
   Summary table
   ═════════════════════════════════════════ */

table.summary {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.88rem;
  margin-top: 8px;
}

table.summary th {
  text-align: left;
  color: var(--text-muted);
  font-weight: 500;
  padding: 6px 10px;
  border-bottom: 1px solid var(--border);
  font-size: 0.8rem;
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

table.summary td {
  padding: 6px 10px;
  border-bottom: 1px solid rgba(48, 54, 61, 0.5);
}

table.summary tr:hover td {
  background: rgba(88, 166, 255, 0.03);
}

/* ═════════════════════════════════════════
   Track theming — 主页三板块（ML / DL / RL）
   ═════════════════════════════════════════ */

.track-ml { --track-color: var(--accent); }
.track-dl { --track-color: var(--purple); }
.track-rl { --track-color: var(--green); }
.track-math { --track-color: var(--teal); }


/* 板块级迷你进度条 + 计数 */
.track-progress {
  display: flex;
  align-items: center;
  gap: 8px;
  margin-left: auto;
  font-size: 0.75rem;
  color: var(--text-desc);
}
.track-progress .tp-bar-outer {
  width: 90px;
  height: 5px;
  background: var(--border);
  border-radius: 3px;
  overflow: hidden;
}
.track-progress .tp-bar-inner {
  height: 100%;
  background: var(--track-color, var(--accent));
  border-radius: 3px;
  transition: width 0.5s ease;
}
.track-progress .tp-count {
  font-family: var(--font-mono);
  color: var(--text-muted);
  min-width: 36px;
  text-align: right;
}

/* 先修衔接卡（DL 板块开头 & 章节页先修链接） */
.prereq-card {
  grid-column: 1 / -1;
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: 8px 14px;
  padding: 12px 16px;
  background: rgba(188, 140, 255, 0.04);
  border: 1px dashed var(--border);
  border-radius: var(--radius);
  font-size: 0.82rem;
  color: var(--text-muted);
}
.prereq-card strong {
  color: var(--text);
  font-size: 0.8rem;
}
.prereq-card a {
  display: inline-flex;
  align-items: center;
  gap: 5px;
  padding: 3px 10px;
  border: 1px solid var(--border);
  border-radius: 12px;
  color: var(--accent);
  font-size: 0.78rem;
  transition: border-color 0.15s, color 0.15s;
}
.prereq-card a:hover {
  border-color: var(--accent);
  text-decoration: none;
}

/* ═════════════════════════════════════════
   Scrollbar
   ═════════════════════════════════════════ */

::-webkit-scrollbar {
  width: 8px;
  height: 8px;
}
::-webkit-scrollbar-track {
  background: var(--bg);
}
::-webkit-scrollbar-thumb {
  background: var(--border);
  border-radius: 4px;
}
::-webkit-scrollbar-thumb:hover {
  background: var(--text-dim);
}

/* ═════════════════════════════════════════
   Toast / notifications
   ═════════════════════════════════════════ */

.toast {
  position: fixed;
  bottom: 20px;
  right: 20px;
  background: var(--bg-card);
  border: 1px solid var(--border);
  padding: 10px 18px;
  border-radius: var(--radius);
  font-size: 0.85rem;
  z-index: 1000;
  animation: toastIn 0.3s ease;
}

@keyframes toastIn {
  from { opacity: 0; transform: translateY(10px); }
  to   { opacity: 1; transform: translateY(0); }
}

/* ═════════════════════════════════════════
   Responsive — Mobile (< 768px)
   ═════════════════════════════════════════ */

@media (max-width: 768px) {
  .container {
    padding: 16px 12px 40px;
  }

  .header {
    flex-direction: column;
    align-items: flex-start;
    gap: 6px;
    padding: 12px 0 12px;
    margin-bottom: 20px;
  }
  .header h1 {
    font-size: 1.3rem;
  }
  .header nav a {
    margin-left: 0;
    margin-right: 12px;
  }

  /* Stack chart + sidebar vertically */
  .chart-row {
    grid-template-columns: 1fr;
    gap: 12px;
  }

  .chart-box {
    height: 300px;
  }
  .chart-box.tall {
    height: 340px;
  }
  .chart-box.short {
    height: 180px;
  }

  /* Cards */
  .card-grid {
    grid-template-columns: 1fr;
    gap: 12px;
  }

  .card {
    padding: 14px;
  }

  /* Controls */
  .controls {
    gap: 8px 12px;
  }

  select, input[type="number"], input[type="text"] {
    font-size: 16px; /* prevent iOS auto-zoom on focus */
    padding: 4px 8px;
  }

  button {
    padding: 8px 14px;
    font-size: 0.85rem;
  }

  /* Bandit page (rl/ch02) tab bar & arm cards — page-specific rules kept here
     TODO: fold into a shared .tab-bar component when unifying tab styles */
  .tab-bar {
    flex-wrap: wrap;
    gap: 4px;
  }
  .tab {
    padding: 6px 12px;
    font-size: .82rem;
  }
  .arm-cards {
    gap: 4px;
  }
  .arm-card {
    width: 70px;
    padding: 6px 3px 8px;
  }
  .arm-card .arm-emoji {
    font-size: 1.1rem;
  }
  .arm-card .q-label {
    font-size: .7rem;
  }

  /* Config rows */
  .config-row {
    flex-direction: column;
    align-items: stretch;
    gap: 8px;
  }
}

/* ═════════════════════════════════════════
   Language toggle button
   ═════════════════════════════════════════ */

.lang-toggle {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 20px;
  color: var(--text-muted);
  font-size: 0.75rem;
  font-weight: 600;
  padding: 3px 3px;
  cursor: pointer;
  transition: border-color 0.15s;
  font-family: var(--font-mono);
  letter-spacing: 0.02em;
  vertical-align: middle;
  line-height: 1;
}
.lang-toggle:hover { border-color: var(--accent); }
.lang-toggle .lang-zh,
.lang-toggle .lang-en {
  display: inline-block;
  padding: 3px 10px;
  border-radius: 18px;
  transition: background 0.2s, color 0.2s;
}
.lang-toggle .lang-zh.active,
.lang-toggle .lang-en.active {
  background: var(--accent);
  color: #0d1117;
  font-weight: 700;
}
.lang-toggle .lang-divider { color: var(--text-dim); margin: 0 2px; }

/* ═════════════════════════════════════════
   Homepage track cards (collapsible)
   ═════════════════════════════════════════ */

.track-card {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  margin-bottom: 20px;
  overflow: hidden;
  transition: border-color 0.15s;
}
.track-card:hover { border-color: var(--text-dim); }
.track-card.expanded { border-color: var(--track-color, var(--accent)); }

.track-card-header {
  display: flex;
  align-items: center;
  gap: 14px;
  padding: 22px 24px;
  cursor: pointer;
  user-select: none;
  transition: background 0.15s;
}
.track-card-header:hover { background: rgba(88,166,255,0.03); }

.track-card-icon { font-size: 2rem; flex-shrink: 0; line-height: 1; }

.track-card-info { flex: 1; min-width: 0; }
.track-card-info h2 { font-size: 1.1rem; margin: 0 0 4px 0; color: var(--text); }
.track-card-info p { color: var(--text-muted); font-size: 0.85rem; margin: 0; line-height: 1.5; }

.track-card-stats {
  display: flex;
  align-items: center;
  gap: 10px;
  flex-shrink: 0;
}

.track-card-toggle {
  font-size: 0.78rem;
  color: var(--text-muted);
  padding: 4px 14px;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  white-space: nowrap;
  transition: border-color 0.15s, color 0.15s;
}
.track-card-header:hover .track-card-toggle {
  border-color: var(--text-muted);
  color: var(--text);
}

.track-card-body { display: none; padding: 0 24px 20px; }
.track-card.expanded .track-card-body { display: block; }

@media (max-width: 768px) {
  .track-card-header { padding: 16px; gap: 10px; }
  .track-card-icon { font-size: 1.5rem; }
  .track-card-info h2 { font-size: 0.95rem; }
  .track-card-info p { font-size: 0.78rem; display: none; }
  .track-card-body { padding: 0 12px 14px; }
  .track-card-stats { flex-direction: column; gap: 4px; align-items: flex-end; }

}


/* ═════════════════════════════════════════
   Enhanced Track Card — smooth expand animation
   ═════════════════════════════════════════ */

.track-card-header {
  background: linear-gradient(135deg, transparent, rgba(88,166,255,0.01));
}

.track-card.expanded .track-card-header {
  background: linear-gradient(135deg, rgba(88,166,255,0.02), transparent);
}

.track-card-body {
  display: grid;
  grid-template-rows: 0fr;
  padding-bottom: 0;
  transition: grid-template-rows 0.35s ease, padding-bottom 0.35s ease;
}

.track-card-body > * {
  overflow: hidden;
}

.track-card.expanded .track-card-body {
  grid-template-rows: 1fr;
  padding-bottom: 20px;
}

@media (max-width: 768px) {
  .track-card.expanded .track-card-body { padding-bottom: 14px; }
}

/* Chapter card hover enhancement */
.chapter-card {
  transition: var(--transition-card);
}
.chapter-card:hover {
  border-color: var(--track-color, var(--accent));
  box-shadow: 0 2px 8px rgba(0,0,0,0.15);
}

/* Track card header gradient per track */
.track-ml .track-card-header {
  background: linear-gradient(135deg, rgba(88,166,255,0.02), transparent);
}
.track-dl .track-card-header {
  background: linear-gradient(135deg, rgba(188,140,255,0.02), transparent);
}
.track-rl .track-card-header {
  background: linear-gradient(135deg, rgba(63,185,80,0.02), transparent);
}
.track-math .track-card-header {
  background: linear-gradient(135deg, rgba(45,212,191,0.03), transparent);
}

/* ═════════════════════════════════════════
   Keyframe animations
   ═════════════════════════════════════════ */

@keyframes nodePulse {
  0%, 100% { opacity: 0.5; }
  50%      { opacity: 1; }
}

@keyframes fadeInUp {
  from { opacity: 0; transform: translateY(24px); }
  to   { opacity: 1; transform: translateY(0); }
}

@keyframes shimmer {
  0%   { background-position: -200% 0; }
  100% { background-position: 200% 0; }
}


/* ═════════════════════════════════════════
   Reduced motion — honor OS accessibility setting
   ═════════════════════════════════════════ */

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
  }
}

/* ═════════════════════════════════════════
   Skip Link — keyboard accessibility
   ═════════════════════════════════════════ */

.skip-link {
  position: absolute;
  top: -100px;
  left: 16px;
  background: var(--accent);
  color: #000;
  padding: 8px 16px;
  border-radius: var(--radius-sm);
  z-index: 1000;
  font-weight: 600;
  font-size: 0.9rem;
  text-decoration: none;
  transition: top 0.2s;
}
.skip-link:focus {
  top: 16px;
  outline: 2px solid #fff;
  outline-offset: 2px;
}

/* ═════════════════════════════════════════
   Focus-visible — keyboard-only focus rings
   ═════════════════════════════════════════ */

a:focus-visible,
button:focus-visible,
.track-card-header:focus-visible,
.tab:focus-visible,
select:focus-visible,
input:focus-visible,
details > summary:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

/* Suppress focus ring for mouse clicks while keeping it for keyboard */
a:focus:not(:focus-visible),
button:focus:not(:focus-visible) {
  outline: none;
}

/* ═════════════════════════════════════════
   Enhanced Prereq Card — clear visual distinction
   ═════════════════════════════════════════ */

.prereq-section {
  margin-bottom: 16px;
}

.prereq-card {
  grid-column: 1 / -1;
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: 8px 14px;
  padding: 14px 18px;
  background: rgba(188, 140, 255, 0.08);
  border: 1px dashed var(--border);
  border-left: 3px solid var(--purple);
  border-radius: var(--radius);
  font-size: 0.82rem;
  color: var(--text-muted);
}
.prereq-card strong {
  color: var(--text);
  font-size: 0.8rem;
}
.prereq-card a {
  display: inline-flex;
  align-items: center;
  gap: 5px;
  padding: 3px 10px;
  border: 1px solid var(--border);
  border-radius: 12px;
  color: var(--accent);
  font-size: 0.78rem;
  transition: border-color 0.15s, color 0.15s;
}
.prereq-card a:hover {
  border-color: var(--accent);
  text-decoration: none;
}

/* ═════════════════════════════════════════
   Top Navigation — site-wide nav bar
   ═════════════════════════════════════════ */

.top-nav {
  position: sticky;
  top: 0;
  z-index: 100;
  background: rgba(13, 17, 23, 0.85);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--border);
}

.top-nav-inner {
  max-width: 1100px;
  margin: 0 auto;
  padding: 0 20px;
  height: 52px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
}

.top-nav-brand {
  font-size: 1rem;
  font-weight: 700;
  color: var(--text);
  text-decoration: none;
  letter-spacing: -0.01em;
  white-space: nowrap;
}
.top-nav-brand:hover {
  color: var(--accent);
  text-decoration: none;
}

.top-nav-links {
  display: flex;
  gap: 4px;
  flex: 1;
  justify-content: center;
}

.top-nav-links a {
  font-size: 0.85rem;
  color: var(--text-muted);
  text-decoration: none;
  padding: 6px 14px;
  border-radius: var(--radius-sm);
  transition: color 0.15s, background 0.15s;
}
.top-nav-links a:hover {
  color: var(--text);
  background: rgba(255, 255, 255, 0.04);
  text-decoration: none;
}
.top-nav-links a.active {
  color: var(--accent);
  background: rgba(88, 166, 255, 0.08);
}

@media (max-width: 768px) {
  .top-nav-inner { padding: 0 12px; height: 46px; }
  .top-nav-brand { font-size: 0.9rem; }
  .top-nav-links a { padding: 4px 10px; font-size: 0.8rem; }
}


/* ═════════════════════════════════════════
   Site Footer
   ═════════════════════════════════════════ */

.site-footer {
  display: flex;
  justify-content: space-between;
  align-items: flex-start;
  gap: 32px;
  margin-top: 48px;
  padding: 28px 0 0;
  border-top: 1px solid var(--border);
  flex-wrap: wrap;
}

.site-footer > div {
  flex: 1;
  min-width: 160px;
}

.footer-brand {
  font-size: 1rem;
  font-weight: 700;
  color: var(--text);
  margin-bottom: 4px;
}

.site-footer h4 {
  font-size: 0.82rem;
  color: var(--text);
  margin-bottom: 10px;
  font-weight: 600;
}

.site-footer ul {
  list-style: none;
  padding: 0;
  margin: 0;
}

.site-footer li {
  margin-bottom: 6px;
}

.site-footer a {
  font-size: 0.82rem;
  color: var(--text-muted);
  text-decoration: none;
  transition: color 0.15s;
}
.site-footer a:hover {
  color: var(--accent);
  text-decoration: none;
}

@media (max-width: 768px) {
  .site-footer { flex-direction: column; gap: 20px; }
  .site-footer > div { min-width: auto; }
}


/* ═══════════════════════════════════════════
   Homepage v0.4.2 Light Theme
   Spec: design-handoff/design-specs.md + interaction-notes.md
   Scoped to body.hs-page — does not affect chapter pages (which stay dark).
   All selectors use .hs- prefix to avoid clashing with global dark tokens.
   ═══════════════════════════════════════════ */

body.hs-page {
  /* Light theme tokens (scoped, override dark globals) */
  --hs-bg:           #FFFFFF;
  --hs-bg-alt:       #F9F9FB;
  --hs-text:         #0F1419;
  --hs-text-2:       #6B7280;
  --hs-text-3:       #9CA3AF;
  --hs-text-desc:    #4B5563;
  --hs-border:       #E7E7EA;
  --hs-purple:       #5046E5;
  --hs-purple-deep:  #4338CA;
  --hs-green:        #10B981;
  --hs-amber:        #F59E0B;
  --hs-teal:         #14B8A6;
  --hs-teal-deep:    #0D9488;
  --hs-badge-bg:     #F3F2FF;
  --hs-divider:      #D1D5DB;

  --hs-font-display: 'Inter Tight', 'PingFang SC', 'Noto Sans SC', sans-serif;
  --hs-font-text:    'DM Sans', 'PingFang SC', 'Noto Sans SC', sans-serif;
  --hs-easing:       cubic-bezier(0.25, 0.1, 0.25, 1);

  background: var(--hs-bg);
  color: var(--hs-text);
  font-family: var(--hs-font-text);
  font-size: 15px;
  line-height: 1.5;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

body.hs-page .skip-link {
  position: absolute;
  top: -100px;
  left: 16px;
  background: var(--hs-purple);
  color: #fff;
  padding: 8px 16px;
  border-radius: 4px;
  z-index: 1000;
  font-weight: 600;
  font-size: 0.9rem;
  text-decoration: none;
  transition: top 0.2s;
}
body.hs-page .skip-link:focus {
  top: 16px;
  outline: 2px solid var(--hs-purple);
  outline-offset: 2px;
}

/* ── Header ── */
.hs-header {
  position: sticky;
  top: 0;
  z-index: 100;
  background: rgba(255, 255, 255, 0.9);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--hs-border);
}

.hs-header-inner {
  max-width: 1440px;
  margin: 0 auto;
  height: 80px;
  padding: 0 64px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 24px;
}

.hs-logo {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  text-decoration: none;
  color: var(--hs-text);
}
.hs-logo:hover { text-decoration: none; color: var(--hs-text); }
.hs-logo-icon { display: block; width: 32px; height: 32px; }
.hs-logo-text {
  font-family: var(--hs-font-display);
  font-size: 20px;
  font-weight: 600;
  letter-spacing: -0.02em;
  color: var(--hs-text);
}

.hs-nav {
  display: flex;
  align-items: center;
  gap: 32px;
  flex: 1;
  justify-content: center;
}
.hs-nav a {
  font-family: var(--hs-font-text);
  font-size: 15px;
  font-weight: 500;
  color: var(--hs-text-2);
  text-decoration: none;
  transition: color 200ms var(--hs-easing);
}
.hs-nav a:hover { color: var(--hs-text); text-decoration: none; }
.hs-nav a.active { color: var(--hs-text); font-weight: 600; }

.hs-header-right {
  display: flex;
  align-items: center;
  gap: 24px;
  flex-shrink: 0;
}

.hs-lang {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  font-family: var(--hs-font-text);
}
.hs-lang-btn {
  background: none;
  border: none;
  padding: 0;
  font-size: 14px;
  font-weight: 500;
  color: var(--text-desc);
  cursor: pointer;
  font-family: var(--hs-font-text);
  transition: color 200ms var(--hs-easing);
}
.hs-lang-btn:hover { color: var(--hs-text-2); }
.hs-lang-btn.active {
  color: var(--hs-purple);
  font-weight: 600;
}
.hs-lang-divider {
  color: var(--hs-divider);
  font-size: 14px;
  user-select: none;
}

.hs-cta {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  background: var(--hs-purple);
  color: #fff;
  border: none;
  border-radius: 8px;
  padding: 12px 24px;
  font-family: var(--hs-font-text);
  font-size: 14px;
  font-weight: 600;
  text-decoration: none;
  transition: background 200ms var(--hs-easing), transform 200ms var(--hs-easing), box-shadow 200ms var(--hs-easing);
}
.hs-cta:hover {
  background: var(--hs-purple-deep);
  color: #fff;
  text-decoration: none;
  transform: translateY(-1px);
  box-shadow: 0 4px 12px rgba(80, 70, 229, 0.25);
}

/* ── Main / Hero ── */
.hs-main { flex: 1 0 auto; }

.hs-hero {
  max-width: 1440px;
  margin: 0 auto;
  padding: 80px 56px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 48px;
}

.hs-hero-left {
  width: 540px;
  flex-shrink: 0;
  display: flex;
  flex-direction: column;
  gap: 32px;
}

.hs-hero-right {
  width: 680px;
  height: 460px;
  flex-shrink: 0;
  border-radius: 16px;
  background: var(--hs-bg-alt);
  box-shadow: 0 4px 12px rgba(15, 20, 25, 0.06);
  overflow: hidden;
  display: flex;
  align-items: center;
  justify-content: center;
}
.hs-hero-right img {
  width: 100%;
  height: 100%;
  object-fit: contain;
  display: block;
}

/* ── Badge ── */
.hs-badge {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  padding: 8px 16px;
  border-radius: 100px;
  background: var(--hs-badge-bg);
  font-family: var(--hs-font-text);
  font-size: 13px;
  font-weight: 500;
  color: var(--hs-purple);
  width: fit-content;
}
.hs-badge-dot {
  width: 8px;
  height: 8px;
  border-radius: 50%;
  background: var(--hs-purple);
  flex-shrink: 0;
}

/* ── H1 + Subtitle ── */
.hs-h1 {
  font-family: var(--hs-font-display);
  font-size: 56px;
  font-weight: 600;
  line-height: 64px;
  letter-spacing: -0.02em;
  color: var(--hs-text);
  margin: 0;
}
.hs-subtitle {
  font-family: var(--hs-font-text);
  font-size: 18px;
  font-weight: 400;
  line-height: 30px;
  color: var(--hs-text-2);
  margin: 0;
  max-width: 540px;
}

/* ── Path cards ── */
.hs-paths {
  display: flex;
  flex-wrap: wrap;
  gap: 16px;
}

.hs-path-card {
  /* v1.17.0: 轨道从 3 条增至 4 条。固定 168px 会在 540px 容器里溢出，
     而单行 4 张（各约 123px）文字折行严重 —— 改为 2×2（各约 262px）。 */
  flex: 1 1 calc(50% - 8px);
  min-width: 0;
  background: var(--hs-bg);
  border: none;
  border-radius: 12px;
  padding: 20px;
  display: flex;
  flex-direction: column;
  gap: 16px;
  text-decoration: none;
  color: inherit;
  box-shadow: 0 1px 2px rgba(15, 20, 25, 0.04);
  transition: transform 200ms var(--hs-easing), box-shadow 200ms var(--hs-easing);
  position: relative;
  overflow: hidden;
}
.hs-path-card:hover {
  text-decoration: none;
  color: inherit;
  transform: translateY(-4px);
  box-shadow: 0 8px 24px rgba(15, 20, 25, 0.08);
}

.hs-path-top {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 3px;
  border-radius: 2px;
}
.hs-path-ml .hs-path-top { background: var(--hs-purple); }
.hs-path-dl .hs-path-top { background: var(--hs-green); }
.hs-path-rl .hs-path-top { background: var(--hs-amber); }
.hs-path-math .hs-path-top { background: var(--hs-teal); }

.hs-path-card:hover .hs-path-top { height: 4px; }

.hs-path-title {
  font-family: var(--hs-font-display);
  font-size: 18px;
  font-weight: 600;
  color: var(--hs-text);
  line-height: 1.2;
}
.hs-path-sub {
  font-family: var(--hs-font-text);
  font-size: 12px;
  font-weight: 400;
  color: var(--text-desc);
  margin-top: -10px;
}
.hs-path-desc {
  font-family: var(--hs-font-text);
  font-size: 13px;
  font-weight: 400;
  color: var(--hs-text-2);
  line-height: 1.5;
}
.hs-path-link {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  font-family: var(--hs-font-text);
  font-size: 14px;
  font-weight: 500;
}
.hs-path-arrow {
  display: inline-block;
  transition: transform 200ms var(--hs-easing);
}
.hs-path-card:hover .hs-path-arrow { transform: translateX(4px); }

.hs-path-ml .hs-path-link { color: var(--hs-purple); }
.hs-path-dl .hs-path-link { color: var(--hs-green); }
.hs-path-rl .hs-path-link { color: var(--hs-amber); }
.hs-path-math .hs-path-link { color: var(--hs-teal); }

/* ── Footer ── */
.hs-footer {
  background: var(--hs-bg-alt);
  flex-shrink: 0;
}
.hs-footer-inner {
  max-width: 1440px;
  margin: 0 auto;
  padding: 80px 64px;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 32px;
  text-align: center;
}
.hs-footer-brand {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  font-family: var(--hs-font-display);
  font-size: 18px;
  font-weight: 600;
  color: var(--hs-text);
}
.hs-footer-brand img { display: block; width: 24px; height: 24px; }
.hs-footer-copy {
  font-family: var(--hs-font-text);
  font-size: 13px;
  font-weight: 400;
  color: var(--text-desc);
  margin: 0;
}
.hs-footer-links {
  display: inline-flex;
  align-items: center;
  gap: 24px;
}
.hs-footer-links a {
  font-family: var(--hs-font-text);
  font-size: 14px;
  font-weight: 500;
  color: var(--hs-text-2);
  text-decoration: none;
  transition: color 200ms var(--hs-easing);
}
.hs-footer-links a:hover { color: var(--hs-text); text-decoration: none; }

.hs-footer-dots {
  display: inline-flex;
  align-items: center;
  gap: 10px;
}
.hs-dot {
  width: 8px;
  height: 8px;
  border-radius: 50%;
  display: inline-block;
}
.hs-dot-purple { background: rgba(80, 70, 229, 0.3); }
.hs-dot-green  { background: rgba(16, 185, 129, 0.3); }
.hs-dot-amber  { background: rgba(245, 158, 11, 0.3); }
.hs-dot-teal   { background: rgba(20, 184, 166, 0.3); }

/* ── Celebration overlay (light-theme restyle) ── */
.hs-celebrate.celebrate-overlay {
  display: none;
  position: fixed;
  inset: 0;
  background: rgba(15, 20, 25, 0.55);
  z-index: 999;
  align-items: center;
  justify-content: center;
}
.hs-celebrate.celebrate-overlay.show { display: flex; }

.hs-celebrate-card {
  position: relative;
  background: var(--hs-bg);
  border: 1px solid var(--hs-purple);
  border-radius: 16px;
  padding: 36px 40px;
  text-align: center;
  max-width: 440px;
  box-shadow: 0 12px 40px rgba(15, 20, 25, 0.15);
  animation: hsCelebrateIn 0.5s var(--hs-easing);
}
@keyframes hsCelebrateIn {
  from { opacity: 0; transform: scale(0.9) translateY(20px); }
  to   { opacity: 1; transform: scale(1) translateY(0); }
}
.hs-celebrate-dismiss {
  position: absolute;
  top: 14px;
  right: 16px;
  background: none;
  border: none;
  color: var(--hs-text-2);
  font-size: 1.3rem;
  cursor: pointer;
  padding: 4px 8px;
  line-height: 1;
  transition: color 200ms var(--hs-easing);
}
.hs-celebrate-dismiss:hover { color: var(--hs-text); }
.hs-celebrate-emoji {
  font-size: 3rem;
  display: block;
  margin-bottom: 10px;
  animation: hsBounce 0.6s ease infinite alternate;
}
@keyframes hsBounce {
  from { transform: translateY(0); }
  to   { transform: translateY(-8px); }
}
.hs-celebrate-card h2 {
  font-family: var(--hs-font-display);
  font-size: 1.5rem;
  font-weight: 600;
  color: var(--hs-text);
  margin-bottom: 8px;
}
.hs-celebrate-card p {
  color: var(--hs-text-2);
  font-size: 0.9rem;
  margin-bottom: 18px;
}
.hs-celebrate-actions {
  display: flex;
  gap: 10px;
  justify-content: center;
  flex-wrap: wrap;
}
.hs-celebrate-home {
  display: inline-flex;
  align-items: center;
  background: var(--hs-purple);
  color: #fff;
  border: none;
  border-radius: 8px;
  padding: 10px 20px;
  font-size: 0.9rem;
  font-weight: 600;
  text-decoration: none;
  transition: background 200ms var(--hs-easing);
}
.hs-celebrate-home:hover { background: var(--hs-purple-deep); color: #fff; text-decoration: none; }
.hs-celebrate-restart {
  display: inline-flex;
  align-items: center;
  background: var(--hs-bg);
  color: var(--hs-text);
  border: 1px solid var(--hs-border);
  border-radius: 8px;
  padding: 10px 20px;
  font-size: 0.9rem;
  font-weight: 500;
  cursor: pointer;
  transition: border-color 200ms var(--hs-easing), background 200ms var(--hs-easing);
}
.hs-celebrate-restart:hover { border-color: var(--hs-text-3); background: var(--hs-bg-alt); }

/* ── Entrance animations (per interaction-notes §3) ── */
@keyframes hsFadeUp {
  from { opacity: 0; transform: translateY(12px); }
  to   { opacity: 1; transform: translateY(0); }
}
@keyframes hsFadeUp16 {
  from { opacity: 0; transform: translateY(16px); }
  to   { opacity: 1; transform: translateY(0); }
}
@keyframes hsScaleIn {
  from { opacity: 0; transform: scale(0.98); }
  to   { opacity: 1; transform: scale(1); }
}

body.hs-page .hs-badge       { animation: hsFadeUp   400ms var(--hs-easing) 0ms   both; }
body.hs-page .hs-h1          { animation: hsFadeUp16 500ms var(--hs-easing) 100ms both; }
body.hs-page .hs-subtitle    { animation: hsFadeUp   400ms var(--hs-easing) 250ms both; }
body.hs-page .hs-path-card:nth-child(1) { animation: hsFadeUp 400ms var(--hs-easing) 350ms both; }
body.hs-page .hs-path-card:nth-child(2) { animation: hsFadeUp 400ms var(--hs-easing) 430ms both; }
body.hs-page .hs-path-card:nth-child(3) { animation: hsFadeUp 400ms var(--hs-easing) 510ms both; }
body.hs-page .hs-path-card:nth-child(4) { animation: hsFadeUp 400ms var(--hs-easing) 590ms both; }
body.hs-page .hs-hero-right  { animation: hsScaleIn  600ms var(--hs-easing) 200ms both; }

@media (prefers-reduced-motion: reduce) {
  body.hs-page *, body.hs-page *::before, body.hs-page *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
  }
}

/* ── Focus ring (a11y) ── */
body.hs-page a:focus-visible,
body.hs-page button:focus-visible {
  outline: 2px solid var(--hs-purple);
  outline-offset: 2px;
}

/* ── Responsive: tablet ≤1024px ── */
@media (max-width: 1024px) {
  .hs-header-inner { padding: 0 32px; }
  .hs-hero {
    flex-direction: column;
    padding: 56px 32px;
    align-items: center;
  }
  .hs-hero-left {
    width: 100%;
    max-width: 680px;
  }
  .hs-hero-right {
    width: 100%;
    max-width: 700px;
    height: auto;
    aspect-ratio: 680 / 460;
  }
  .hs-h1 { font-size: 44px; line-height: 52px; }
  .hs-footer-inner { padding: 56px 32px; }
}

/* ── Responsive: mobile ≤768px ── */
@media (max-width: 768px) {
  .hs-header-inner {
    padding: 0 20px;
    height: 64px;
  }
  .hs-nav { display: none; }
  .hs-cta { display: none; }
  .hs-logo-text { font-size: 18px; }

  .hs-hero {
    padding: 40px 20px;
    align-items: center;
    text-align: center;
  }
  .hs-hero-left {
    align-items: center;
  }
  .hs-h1 { font-size: 32px; line-height: 40px; }
  .hs-subtitle { font-size: 16px; line-height: 24px; }

  .hs-paths {
    flex-direction: column;
    gap: 12px;
    width: 100%;
  }
  .hs-path-card { width: 100%; }

  .hs-footer-inner { padding: 48px 20px; }
  .hs-footer-links { gap: 16px; flex-wrap: wrap; justify-content: center; }

  .hs-celebrate-card { margin: 0 16px; padding: 28px 24px; }
}

/* ═════════════════════════════════════════
   Sub-pages v0.4.3 — ML/DL/RL track landing pages
   Reuses homepage header/footer/badge; adds hero stats,
   chapter grid, track-colored CTA and number badges.
   ═════════════════════════════════════════ */

/* ── Track color scopes (set --hs-accent per track) ── */
.hs-track-ml { --hs-accent: var(--hs-purple); --hs-accent-deep: var(--hs-purple-deep); --hs-accent-soft: var(--hs-badge-bg); }
.hs-track-dl { --hs-accent: var(--hs-green);  --hs-accent-deep: #059669;              --hs-accent-soft: rgba(16,185,129,0.10); }
.hs-track-rl { --hs-accent: var(--hs-amber);  --hs-accent-deep: #D97706;              --hs-accent-soft: rgba(245,158,11,0.10); }
.hs-track-math { --hs-accent: var(--hs-teal); --hs-accent-deep: var(--hs-teal-deep);  --hs-accent-soft: rgba(45,212,191,0.10); }

/* ── Sub-page Hero ── */
.hs-sub-hero {
  padding: 80px 0;
  background: var(--hs-bg);
}
.hs-sub-hero-inner {
  max-width: 1440px;
  margin: 0 auto;
  padding: 0 64px;
  display: flex;
  gap: 48px;
  align-items: center;
}
.hs-sub-hero-left {
  flex: 0 0 540px;
  display: flex;
  flex-direction: column;
  gap: 28px;
}
.hs-sub-hero-right {
  flex: 1;
  display: flex;
  justify-content: center;
  align-items: center;
}
.hs-sub-hero-right img {
  width: 100%;
  max-width: 680px;
  height: auto;
  border-radius: 16px;
  background: var(--hs-bg-alt);
  filter: drop-shadow(0 4px 12px rgba(15,20,25,0.06));
}

/* Hero title with English subtitle line */
.hs-sub-title {
  font-family: var(--hs-font-display);
  font-size: 56px;
  font-weight: 600;
  line-height: 64px;
  letter-spacing: -0.02em;
  color: var(--hs-text);
  margin: 0;
}
.hs-sub-title-en {
  display: block;
  font-family: var(--hs-font-text);
  font-size: 22px;
  font-weight: 400;
  line-height: 32px;
  color: var(--hs-text-3);
  margin-top: 10px;
  letter-spacing: normal;
}
.hs-sub-desc {
  font-family: var(--hs-font-text);
  font-size: 18px;
  line-height: 30px;
  color: var(--hs-text-2);
  max-width: 480px;
  margin: 0;
}

/* Hero stats */
.hs-sub-stats {
  display: flex;
  gap: 32px;
  align-items: flex-end;
}
.hs-sub-stat {
  display: flex;
  flex-direction: column;
  gap: 4px;
}
.hs-sub-stat-num {
  font-family: var(--hs-font-display);
  font-size: 24px;
  font-weight: 600;
  color: var(--hs-text);
}
.hs-sub-stat-label {
  font-family: var(--hs-font-text);
  font-size: 13px;
  color: var(--text-desc);
}

/* Hero CTA (track-colored) */
.hs-sub-cta {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: fit-content;
  background: var(--hs-accent);
  color: #fff;
  border: none;
  border-radius: 8px;
  padding: 14px 28px;
  font-family: var(--hs-font-text);
  font-size: 15px;
  font-weight: 600;
  text-decoration: none;
  transition: background 200ms var(--hs-easing), transform 200ms var(--hs-easing);
}
.hs-sub-cta:hover {
  background: var(--hs-accent-deep);
  color: #fff;
  transform: translateY(-1px);
  text-decoration: none;
}

/* ── Chapters section ── */
.hs-sub-chapters {
  padding: 80px 0;
  background: var(--hs-bg);
}
.hs-sub-chapters-inner {
  max-width: 1440px;
  margin: 0 auto;
  padding: 0 64px;
}
.hs-sub-section-header {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  margin-bottom: 40px;
}
.hs-sub-section-title {
  font-family: var(--hs-font-display);
  font-size: 32px;
  font-weight: 600;
  letter-spacing: -0.02em;
  color: var(--hs-text);
  margin: 0;
}
.hs-sub-section-meta {
  font-family: var(--hs-font-text);
  font-size: 15px;
  color: var(--hs-text-2);
}

/* Chapter grid + cards */
.hs-sub-grid {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: 20px;
}
.hs-sub-card {
  display: flex;
  gap: 16px;
  align-items: flex-start;
  padding: 20px;
  border-radius: 12px;
  background: var(--hs-bg);
  border: 1px solid var(--hs-border);
  box-shadow: 0 1px 2px rgba(15,20,25,0.04);
  text-decoration: none;
  color: inherit;
  transition: transform 200ms var(--hs-easing), box-shadow 200ms var(--hs-easing), border-color 200ms var(--hs-easing);
}
.hs-sub-card:hover {
  transform: translateY(-3px);
  box-shadow: 0 8px 24px rgba(15,20,25,0.08);
  border-color: var(--hs-accent);
  text-decoration: none;
}
.hs-sub-num {
  flex: 0 0 40px;
  height: 40px;
  border-radius: 10px;
  background: var(--hs-accent-soft);
  color: var(--hs-accent);
  font-family: var(--hs-font-display);
  font-weight: 600;
  font-size: 14px;
  display: grid;
  place-items: center;
}
.hs-sub-chapter-body { flex: 1; min-width: 0; }
.hs-sub-chapter-title {
  font-family: var(--hs-font-display);
  font-size: 17px;
  font-weight: 600;
  color: var(--hs-text);
  margin-bottom: 6px;
}
.hs-sub-chapter-desc {
  font-family: var(--hs-font-text);
  font-size: 13px;
  color: var(--hs-text-2);
  line-height: 1.5;
  margin: 0;
}
/* ── Sub-page phase headings (v1.10.0: rl.html 阶段划分) ── */
.hs-sub-phase {
  grid-column: 1 / -1;
  padding: 16px 2px 2px;
  margin-top: 10px;
  border-top: 1px solid var(--hs-border);
}
.hs-sub-grid > .hs-sub-phase:first-child {
  border-top: none;
  margin-top: 0;
  padding-top: 0;
}
.hs-sub-phase-title {
  font-family: var(--hs-font-display);
  font-size: 15px;
  font-weight: 600;
  color: var(--hs-accent);
  margin: 0 0 2px;
}
.hs-sub-phase-desc {
  font-family: var(--hs-font-text);
  font-size: 12px;
  color: var(--hs-text-2);
  margin: 0;
}

/* ── Sub-page section lists (v1.16.0: ai-start style fine-grained outline) ── */
.hs-sub-chapter-title a { color: inherit; text-decoration: none; }
.hs-sub-chapter-title a:hover { color: var(--hs-accent); }
.hs-sub-sections {
  list-style: none;
  margin: 12px 0 0;
  padding: 10px 0 0;
  border-top: 1px dashed var(--hs-border);
  display: grid;
  grid-template-columns: 1fr;
  gap: 3px;
}
.hs-sub-sections li {
  font-family: var(--hs-font-text);
  font-size: 12.5px;
  line-height: 1.5;
}
.hs-sub-sections a {
  color: var(--hs-text-2);
  text-decoration: none;
  transition: color 150ms;
}
.hs-sub-sections a:hover { color: var(--hs-accent); }
.hs-sub-level {
  position: absolute;
  top: 10px;
  right: 12px;
  font-family: var(--hs-font-text);
  font-size: 11px;
  font-weight: 500;
  padding: 2px 8px;
  border-radius: 999px;
  background: rgba(15,20,25,0.05);
  color: var(--hs-text-2);
}
.hs-sub-card[data-level="beginner"] .hs-sub-level { color: #059669; background: rgba(16,185,129,0.12); }
.hs-sub-card[data-level="intermediate"] .hs-sub-level { color: #B45309; background: rgba(245,158,11,0.14); }
.hs-sub-card[data-level="advanced"] .hs-sub-level { color: #BE185D; background: rgba(236,72,153,0.10); }


/* ── Sub-page entrance animations ── */
body.hs-page .hs-sub-hero-left .hs-badge     { animation: hsFadeUp   400ms var(--hs-easing) 0ms   both; }
body.hs-page .hs-sub-hero-left .hs-sub-title { animation: hsFadeUp16 500ms var(--hs-easing) 100ms both; }
body.hs-page .hs-sub-hero-left .hs-sub-desc  { animation: hsFadeUp   400ms var(--hs-easing) 250ms both; }
body.hs-page .hs-sub-hero-left .hs-sub-stats { animation: hsFadeUp   400ms var(--hs-easing) 350ms both; }
body.hs-page .hs-sub-hero-left .hs-sub-cta   { animation: hsFadeUp   400ms var(--hs-easing) 430ms both; }
body.hs-page .hs-sub-hero-right              { animation: hsScaleIn  600ms var(--hs-easing) 200ms both; }
body.hs-page .hs-sub-card                    { animation: hsFadeUp   400ms var(--hs-easing) 100ms both; }

/* ── Sub-page responsive ── */
@media (max-width: 1024px) {
  .hs-sub-hero-inner { flex-direction: column; padding: 0 32px; align-items: center; }
  .hs-sub-hero-left { width: 100%; max-width: 680px; }
  .hs-sub-hero-right { width: 100%; max-width: 700px; }
  .hs-sub-title { font-size: 44px; line-height: 52px; }
  .hs-sub-chapters-inner { padding: 0 32px; }
}
@media (max-width: 768px) {
  .hs-sub-hero { padding: 40px 0; }
  .hs-sub-hero-inner { padding: 0 20px; align-items: center; text-align: center; }
  .hs-sub-hero-left { align-items: center; }
  .hs-sub-title { font-size: 32px; line-height: 40px; }
  .hs-sub-desc { font-size: 16px; line-height: 24px; }
  .hs-sub-grid { grid-template-columns: 1fr; }
  .hs-sub-chapters { padding: 48px 0; }
  .hs-sub-chapters-inner { padding: 0 20px; }
  .hs-sub-cta { width: 100%; }
}

/* ═════════════════════════════════════════
   ML Chapter pages v0.5.0 — Light theme 5-section layout
   Based on design-handoff/ml-chapters design-specs.md
   Scoped under body.hs-chapter to override dark :root
   ═════════════════════════════════════════ */

/* ── Light theme variable overrides for chapter pages ── */
body.hs-chapter {
  --bg:          #FFFFFF;
  --bg-card:     #FFFFFF;
  --bg-card-hover: #F9F9FB;
  --bg-input:    #F9F9FB;
  --border:      #E7E7EA;
  --border-focus:#5046E5;
  --text:        #0F1419;
  --text-muted:  #6B7280;
  --text-dim:    #9CA3AF;
  --text-desc:   #4B5563;
  --accent:      #5046E5;
  --accent-hover:#4338CA;
  --accent-soft: #F3F2FF;
  --accent-rgb:  80,70,229;
  --green:       #10B981;
  --green-dim:   #059669;
  --amber:       #F59E0B;
  --red:         #EF4444;
  --purple:      #5046E5;
  --pink:        #EC4899;
  --radius:      16px;
  --radius-sm:   12px;
  --shadow-card: 0 4px 16px rgba(15,20,25,0.06);
  --shadow-lg:   0 8px 32px rgba(15,20,25,0.08);
  --font-sans:   'DM Sans','PingFang SC','Noto Sans SC',-apple-system,sans-serif;
  background: #FFFFFF;
  color: var(--text);
}

body.hs-chapter a { color: var(--accent); }
body.hs-chapter a:hover { color: var(--accent-hover); }

/* ── Track-specific accent overrides (DL=green, RL=amber) ── */
body.hs-chapter.hs-track-dl {
  --accent:      #10B981;
  --accent-hover:#059669;
  --accent-soft: #E7F8F0;
  --accent-rgb:  16,185,129;
  --border-focus:#10B981;
}
body.hs-chapter.hs-track-rl {
  --accent:      #F59E0B;
  --accent-hover:#D97706;
  --accent-soft: #FEF3C7;
  --accent-rgb:  245,158,11;
  --border-focus:#F59E0B;
}
body.hs-chapter.hs-track-math {
  --accent:      #14B8A6;
  --accent-hover:#0D9488;
  --accent-soft: #E6FFFA;
  --accent-rgb:  20,184,166;
  --border-focus:#14B8A6;
}

/* ── Chapter topbar (sticky nav, 64px) ── */
.hs-chap-topbar {
  position: sticky;
  top: 0;
  z-index: 20;
  height: 64px;
  background: rgba(255,255,255,0.92);
  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);
  border-bottom: 1px solid var(--border);
}
.hs-chap-topbar-inner {
  max-width: 1440px;
  height: 100%;
  margin: 0 auto;
  padding: 0 64px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 24px;
}
.hs-chap-brand {
  display: flex;
  align-items: center;
  gap: 10px;
  font-family: 'Inter Tight','PingFang SC',sans-serif;
  font-weight: 600;
  font-size: 17px;
  letter-spacing: -0.02em;
  color: var(--text);
  text-decoration: none;
}
.hs-chap-brand .mark {
  width: 28px; height: 28px;
  border-radius: 7px;
  background: var(--accent);
  color: #fff;
  display: grid;
  place-items: center;
  font-size: 12px;
  font-weight: 700;
}
.hs-chap-crumbs {
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 14px;
  color: var(--text-muted);
}
.hs-chap-crumbs .sep { color: #D1D5DB; }
.hs-chap-crumbs .here { color: var(--text); font-weight: 600; }
.hs-chap-topbar-right { display: flex; align-items: center; gap: 20px; }
.hs-chap-step-progress { display: flex; align-items: center; gap: 10px; }
.hs-chap-step-pill {
  font-family: 'Inter Tight',sans-serif;
  font-size: 13px;
  font-weight: 600;
  color: var(--accent);
  background: var(--accent-soft);
  padding: 5px 12px;
  border-radius: 100px;
}
.hs-chap-step-pill.completed {
  background: rgba(16, 185, 129, 0.12);
  color: var(--green, #10B981);
}
.hs-chap-track {
  width: 120px;
  height: 4px;
  border-radius: 2px;
  background: var(--border);
  overflow: hidden;
}
.hs-chap-track i {
  display: block;
  height: 100%;
  background: var(--accent);
  border-radius: 2px;
}
.hs-chap-lang {
  display: flex;
  align-items: center;
  gap: 6px;
  font-size: 14px;
  font-weight: 500;
  cursor: pointer;
  background: none;
  border: none;
  font-family: inherit;
}
.hs-chap-lang .lang-zh,
.hs-chap-lang .lang-en {
  display: inline-block;
  padding: 2px 10px;
  border-radius: 12px;
  transition: background 0.2s, color 0.2s;
}
.hs-chap-lang .cur {
  background: var(--accent);
  color: #fff;
  font-weight: 600;
}
.hs-chap-lang span:not(.cur):not(.sep) {
  color: var(--text-dim);
}
.hs-chap-lang .sep { color: #D1D5DB; }

/* ── Chapter Hero ── */
.hs-chap-hero {
  padding: 56px 0 36px;
  max-width: 1440px;
  margin: 0 auto;
  padding-left: 64px;
  padding-right: 64px;
}
.hs-chap-hero-inner {
  display: flex;
  align-items: flex-end;
  justify-content: space-between;
  gap: 48px;
  flex-wrap: wrap;
}
.hs-chap-hero-left {
  max-width: 760px;
  display: flex;
  flex-direction: column;
  gap: 18px;
}
.hs-chap-badge {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  width: fit-content;
  padding: 7px 14px;
  background: var(--accent-soft);
  border-radius: 100px;
  color: var(--accent);
  font-size: 13px;
  font-weight: 500;
}
.hs-chap-badge .dot {
  width: 7px; height: 7px;
  border-radius: 50%;
  background: var(--accent);
}
.hs-chap-h1 {
  font-family: 'Inter Tight','PingFang SC',sans-serif;
  font-size: 48px;
  font-weight: 700;
  line-height: 1.1;
  letter-spacing: -0.02em;
  color: var(--text);
  margin: 0;
}
.hs-chap-h1 .en {
  display: block;
  font-family: 'DM Sans',sans-serif;
  font-size: 20px;
  font-weight: 400;
  color: var(--text-desc);
  margin-top: 8px;
  letter-spacing: 0;
}
/* Hide English subtitle when page is already in English (avoid duplicate title) */
html[lang="en"] .hs-chap-h1 .en { display: none; }
.hs-chap-lead {
  font-size: 18px;
  color: var(--text-muted);
  max-width: 640px;
  margin: 0;
}
.hs-chap-hero-stats { display: flex; gap: 14px; flex-wrap: wrap; }
.hs-chap-stat {
  background: #F9F9FB;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  padding: 12px 16px;
  min-width: 104px;
}
.hs-chap-stat .n {
  font-family: 'Inter Tight',sans-serif;
  font-size: 20px;
  font-weight: 600;
  color: var(--text);
}
.hs-chap-stat .l {
  font-size: 12px;
  color: var(--text-desc);
  margin-top: 2px;
}

/* ── Chapter Lab (core two-column) ── */
.hs-chap-lab {
  padding: 8px 0 48px;
  max-width: 1440px;
  margin: 0 auto;
  padding-left: 64px;
  padding-right: 64px;
}
.hs-chap-lab-grid {
  display: grid;
  grid-template-columns: 1.55fr 1fr;
  gap: 24px;
  align-items: start;
}
.hs-chap-card {
  background: #fff;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  box-shadow: var(--shadow-card);
}
.hs-chap-chart-card { padding: 20px; }
.hs-chap-chart-head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-bottom: 14px;
  flex-wrap: wrap;
  gap: 10px;
}
.hs-chap-chart-head h3 {
  font-family: 'Inter Tight',sans-serif;
  font-size: 16px;
  font-weight: 600;
  color: var(--text);
  margin: 0;
}
.hs-chap-readouts { display: flex; gap: 10px; flex-wrap: wrap; }
.hs-chap-chip {
  background: #F9F9FB;
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 8px 12px;
  font-family: 'Inter Tight',sans-serif;
}
.hs-chap-chip .k { font-size: 11px; color: var(--text-desc); display: block; }
.hs-chap-chip .v { font-size: 16px; font-weight: 600; color: var(--accent); font-variant-numeric:tabular-nums; min-width:56px; display:inline-block; }
/* v1.4.0 #6 — Loss 数值保护：长位数收缩为省略号，避免换行破版 */
#lossVal { max-width: 9.5em; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; vertical-align: bottom; }

/* Canvas containers — preserve existing chart-box sizing */
.hs-chap-canvas {
  height: 360px;
  border-radius: 12px;
  background: #F9F9FB;
  border: 1px solid var(--border);
  display: grid;
  place-items: center;
  overflow: hidden;
}
.hs-chap-canvas canvas { width: 100% !important; height: 100% !important; }
.hs-chap-canvas-small {
  height: 160px;
  border-radius: 12px;
  background: #F9F9FB;
  border: 1px solid var(--border);
  overflow: hidden;
  margin-top: 14px;
}
.hs-chap-canvas-small canvas { width: 100% !important; height: 100% !important; }

.hs-chap-toolbar {
  display: flex;
  align-items: center;
  gap: 16px;
  flex-wrap: wrap;
  margin-top: 16px;
  padding-top: 16px;
  border-top: 1px solid var(--border);
}
.hs-chap-slider {
  display: flex;
  align-items: center;
  gap: 10px;
  font-size: 13px;
  color: var(--text-muted);
}
.hs-chap-slider input[type="range"] {
  width: 120px;
  accent-color: var(--accent);
}
.hs-chap-btns { display: flex; gap: 8px; margin-left: auto; flex-wrap: wrap; }
.hs-chap-btn {
  border: 1px solid var(--border);
  background: #fff;
  border-radius: 8px;
  padding: 8px 14px;
  font-family: 'DM Sans',sans-serif;
  font-size: 13px;
  font-weight: 600;
  color: var(--text);
  cursor: pointer;
  transition: background 0.15s, border-color 0.15s;
}
.hs-chap-btn:hover { border-color: var(--text-muted); background: #F9F9FB; }
.hs-chap-btn.primary {
  background: var(--accent);
  border-color: var(--accent);
  color: #fff;
}
.hs-chap-btn.primary:hover { background: var(--accent-hover); }

/* ── v0.11.0 A11y: focus-visible for chapter interactive elements ── */
.hs-chap-btn:focus-visible,
.hs-chap-navcard:focus-visible,
.hs-chap-lang:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

/* Side panels */
.hs-chap-side {
  display: flex;
  flex-direction: column;
  gap: 16px;
}
.hs-chap-side .hs-chap-card { padding: 18px; }
.hs-chap-side h4 {
  font-family: 'Inter Tight',sans-serif;
  font-size: 14px;
  font-weight: 600;
  margin-bottom: 8px;
  display: flex;
  align-items: center;
  gap: 8px;
  color: var(--text);
}
.hs-chap-side h4 .live {
  width: 8px; height: 8px;
  border-radius: 50%;
  background: var(--green);
  box-shadow: 0 0 0 4px rgba(16,185,129,0.15);
}
.hs-chap-side p { font-size: 13.5px; color: var(--text-muted); line-height: 1.55; }
.hs-chap-formula {
  font-family: 'Inter Tight',sans-serif;
  background: #F9F9FB;
  border: 1px dashed var(--border);
  border-radius: 10px;
  padding: 12px;
  font-size: 15px;
  text-align: center;
  color: var(--text);
}
.hs-chap-formula + .hs-chap-formula { margin-top: 10px; }

/* Param rows (preserve existing IDs) */
.hs-chap-param-row {
  display: flex;
  align-items: center;
  gap: 8px;
  margin: 4px 0;
  font-family: 'SF Mono','Cascadia Code',monospace;
  font-size: 0.85rem;
  color: var(--text);
}
.hs-chap-param-row .arrow { color: var(--text-muted); }
.hs-chap-param-row .val { color: var(--accent); font-weight: 600; }
.hs-chap-param-row .grad { font-size: 0.7rem; color: var(--red); margin-left: 4px; }

/* Loss bar (preserve existing IDs) */
.hs-chap-loss-bar {
  height: 6px;
  background: var(--border);
  border-radius: 3px;
  margin-top: 6px;
  overflow: hidden;
}
.hs-chap-loss-bar-fill {
  height: 100%;
  background: var(--accent);
  border-radius: 3px;
  transition: width 0.3s, background 0.3s;
}
.hs-chap-loss-bar-fill.low { background: var(--green); }

/* Step indicator */
.hs-chap-step-indicator {
  font-family: 'SF Mono','Cascadia Code',monospace;
  font-size: 0.82rem;
  color: var(--text-muted);
  text-align: center;
  padding: 6px 0;
}

/* Section title in side panels */
.hs-chap-section-title {
  font-family: 'Inter Tight',sans-serif;
  font-size: 14px;
  font-weight: 600;
  color: var(--text);
  margin-bottom: 10px;
  padding-bottom: 6px;
  border-bottom: 1px solid var(--border);
}

/* Mode buttons (preserve existing .mode-btn behavior) */
body.hs-chapter .mode-btn {
  background: #F9F9FB;
  border: 1px solid var(--border);
  color: var(--text-muted);
}
body.hs-chapter .mode-btn:hover {
  border-color: var(--text-muted);
  background: #fff;
  color: var(--text);
}
body.hs-chapter .mode-btn.active {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
}

/* Select/inputs in light theme */
body.hs-chapter select,
body.hs-chapter input[type="number"],
body.hs-chapter input[type="text"] {
  background: #F9F9FB;
  color: var(--text);
  border: 1px solid var(--border);
}
body.hs-chapter select:focus,
body.hs-chapter input:focus {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
  border-color: var(--accent);
  box-shadow: 0 0 0 2px rgba(var(--accent-rgb),0.12);
}

/* Buttons in light theme */
body.hs-chapter button {
  background: var(--accent);
  color: #fff;
  border: none;
  border-radius: 8px;
  padding: 8px 18px;
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.15s;
}
body.hs-chapter button:hover { background: var(--accent-hover); }
body.hs-chapter button.secondary {
  background: #fff;
  border: 1px solid var(--border);
  color: var(--text);
}
body.hs-chapter button.secondary:hover { border-color: var(--text-muted); background: #F9F9FB; }

/* Slider in light theme */
body.hs-chapter input[type="range"] { accent-color: var(--accent); }

/* Explain box (preserve existing class) */
body.hs-chapter .explain-box {
  background: rgba(var(--accent-rgb),0.04);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  padding: 12px 14px;
  font-size: 0.8rem;
  line-height: 1.55;
  color: var(--text-muted);
}
body.hs-chapter .explain-box strong { color: var(--text); }
body.hs-chapter .formula-inline {
  font-family: 'SF Mono',monospace;
  background: rgba(var(--accent-rgb),0.06);
  padding: 2px 6px;
  border-radius: 3px;
  font-size: 0.82rem;
  white-space: nowrap;
}

/* ── Explain band (3-panel: what/pitfalls/summary) ── */
.hs-chap-explain {
  background: #F9F9FB;
  border-top: 1px solid var(--border);
  border-bottom: 1px solid var(--border);
  padding: 48px 0;
}

/* ── Chapter TOC block (v1.16.0: 本步小节路线图) ── */
.hs-chap-toc {
  padding: 22px 0 6px;
  background: var(--bg);
}
.hs-chap-toc-inner {
  max-width: 1440px;
  margin: 0 auto;
  padding: 18px 24px;
  display: flex;
  gap: 24px;
  align-items: flex-start;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  flex-wrap: wrap;
}
.hs-chap-toc-inner h2 {
  font-family: 'Inter Tight',sans-serif;
  font-size: 15px;
  font-weight: 600;
  color: var(--accent);
  margin: 0;
  flex: 0 0 auto;
  padding-top: 2px;
}
.hs-chap-toc-inner ol {
  list-style: none;
  display: flex;
  flex-wrap: wrap;
  gap: 6px 14px;
  margin: 0;
  padding: 0;
  flex: 1 1 420px;
}
.hs-chap-toc-inner ol li {
  font-size: 13px;
  line-height: 1.5;
}
.hs-chap-toc-inner ol a {
  color: var(--text-muted);
  text-decoration: none;
  transition: color 150ms;
}
.hs-chap-toc-inner ol a:hover { color: var(--accent); text-decoration: underline; }
.hs-chap-toc-inner ol a b {
  color: var(--accent);
  font-family: var(--font-mono);
  font-weight: 600;
  margin-right: 2px;
}
.hs-chap-explain-inner {
  max-width: 1440px;
  margin: 0 auto;
  padding: 0 64px;
  display: flex;
  flex-direction: column;
  gap: 20px;
}
.hs-chap-why h2 {
  font-family: 'Inter Tight',sans-serif;
  font-size: 24px;
  font-weight: 600;
  margin-bottom: 10px;
  color: var(--text);
}
.hs-chap-why p {
  font-size: 15px;
  color: var(--text-muted);
  max-width: 880px;
  line-height: 1.6;
}
/* v1.5.0 — "next step preview" callout appended to the why band */
.hs-chap-why .hs-chap-next {
  display: inline-block;
  margin-top: 12px;
  padding: 8px 14px;
  border-left: 3px solid var(--accent);
  background: var(--accent-soft);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  font-size: 14px;
  line-height: 1.55;
  color: var(--text);
}
.hs-chap-tri {
  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
  gap: 20px;
}
.hs-chap-panel {
  background: #fff;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 22px;
  box-shadow: var(--shadow-card);
}
.hs-chap-panel h3 {
  font-family: 'Inter Tight',sans-serif;
  font-size: 16px;
  font-weight: 600;
  margin-bottom: 12px;
  color: var(--text);
}
.hs-chap-panel.pit { border-left: 3px solid var(--amber); }
.hs-chap-panel.sum { border-left: 3px solid var(--green); }
.hs-chap-panel ul {
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: 10px;
  padding: 0;
  margin: 0;
}
.hs-chap-panel li {
  font-size: 13.5px;
  color: var(--text-muted);
  display: flex;
  gap: 10px;
  align-items: flex-start;
  line-height: 1.5;
}
.hs-chap-panel.sum li::before {
  content: "✓";
  color: var(--green);
  font-weight: 700;
  flex-shrink: 0;
}
.hs-chap-panel.pit li::before {
  content: "!";
  color: var(--amber);
  font-weight: 700;
  background: rgba(245,158,11,0.12);
  width: 18px; height: 18px;
  border-radius: 50%;
  display: grid;
  place-items: center;
  flex: 0 0 18px;
  font-size: 11px;
}

/* ── Content depth band (v0.9.0: derivation + code + hyperparams + refs) ── */
.hs-chap-depth {
  max-width: 1440px;
  margin: 0 auto;
  padding: 48px 64px;
  border-top: 1px solid var(--border);
}
.hs-chap-depth-inner {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 24px;
}
.hs-chap-depth-block {
  background: #fff;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 24px 28px;
}
.hs-chap-depth-block.full { grid-column: 1 / -1; }
.hs-chap-depth-block h2 {
  font-family: 'Inter Tight',sans-serif;
  font-size: 18px;
  font-weight: 600;
  margin: 0 0 14px;
  color: var(--text);
}
.hs-chap-depth-block p {
  font-size: 14px;
  color: var(--text-muted);
  line-height: 1.7;
  margin: 0 0 10px;
}
.hs-chap-depth-block .hs-chap-formula { margin: 8px 0; }

/* Code block */
.hs-chap-code {
  background: #1e1e2e;
  color: #cdd6f4;
  border-radius: var(--radius-sm);
  padding: 16px 20px;
  overflow-x: auto;
  font-family: 'Fira Code','Cascadia Code',Consolas,monospace;
  font-size: 13px;
  line-height: 1.6;
  margin: 8px 0;
}
.hs-chap-code code { font-family: inherit; background: none; padding: 0; color: inherit; }

/* Hyperparameter comparison table */
.hs-chap-hyper-table {
  width: 100%;
  border-collapse: collapse;
  font-size: 13px;
  margin: 8px 0;
}
.hs-chap-hyper-table th,
.hs-chap-hyper-table td {
  text-align: left;
  padding: 8px 12px;
  border-bottom: 1px solid var(--border);
}
.hs-chap-hyper-table th { font-weight: 600; color: var(--text); background: #F9F9FB; }
.hs-chap-hyper-table td { color: var(--text-muted); }
.hs-chap-hyper-table .ok   { color: var(--green); font-weight: 600; }
.hs-chap-hyper-table .warn { color: var(--amber); font-weight: 600; }
.hs-chap-hyper-table .bad  { color: var(--red);   font-weight: 600; }

/* References list */
.hs-chap-refs {
  list-style: none;
  padding: 0;
  margin: 0;
  display: flex;
  flex-direction: column;
  gap: 10px;
}
.hs-chap-refs li {
  font-size: 13.5px;
  color: var(--text-muted);
  padding-left: 20px;
  position: relative;
  line-height: 1.6;
}
.hs-chap-refs li::before {
  content: "▸";
  position: absolute;
  left: 0;
  color: var(--accent);
}
.hs-chap-refs a { color: var(--accent); text-decoration: none; }
.hs-chap-refs a:hover { text-decoration: underline; }

/* ── Step navigation footer ── */
.hs-chap-stepnav {
  padding: 32px 0 64px;
  max-width: 1440px;
  margin: 0 auto;
  padding-left: 64px;
  padding-right: 64px;
}
.hs-chap-stepnav-inner {
  display: flex;
  justify-content: space-between;
  gap: 16px;
}
.hs-chap-navcard {
  flex: 1;
  display: flex;
  flex-direction: column;
  gap: 4px;
  padding: 18px 22px;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  background: #fff;
  transition: border-color 0.2s, box-shadow 0.2s;
  text-decoration: none;
  color: var(--text);
}
.hs-chap-navcard:hover {
  border-color: var(--accent);
  box-shadow: var(--shadow-card);
  text-decoration: none;
  color: var(--text);
}
.hs-chap-navcard .dir { font-size: 12px; color: var(--text-desc); }
.hs-chap-navcard .ttl {
  font-family: 'Inter Tight',sans-serif;
  font-size: 16px;
  font-weight: 600;
}
.hs-chap-navcard.next {
  text-align: right;
  align-items: flex-end;
  background: var(--accent-soft);
}
.hs-chap-navcard.next .ttl { color: var(--accent-hover); }

/* ── Chapter responsive ── */
@media (max-width: 1199px) {
  .hs-chap-lab-grid { grid-template-columns: 1fr; }
  .hs-chap-tri { grid-template-columns: 1fr; }
  .hs-chap-depth-inner { grid-template-columns: 1fr; }
  .hs-chap-topbar-inner,
  .hs-chap-hero,
  .hs-chap-lab,
  .hs-chap-explain-inner,
  .hs-chap-depth,
  .hs-chap-stepnav { padding-left: 32px; padding-right: 32px; }
}
@media (max-width: 768px) {
  .hs-chap-topbar-inner { padding: 0 20px; }
  .hs-chap-crumbs { display: none; }
  .hs-chap-track { display: none; }
  .hs-chap-hero { padding: 32px 20px; }
  .hs-chap-hero-inner { flex-direction: column; align-items: flex-start; }
  .hs-chap-h1 { font-size: 32px; }
  .hs-chap-lab { padding: 8px 20px 32px; }
  .hs-chap-explain-inner { padding: 0 20px; }
  .hs-chap-depth { padding: 32px 20px; }
  .hs-chap-stepnav { padding: 24px 20px 40px; }
  .hs-chap-stepnav-inner { flex-direction: column; }
  .hs-chap-navcard.next { text-align: left; align-items: flex-start; }
  .hs-chap-hero-stats { gap: 10px; }
  .hs-chap-stat { min-width: 88px; padding: 10px 12px; }
}

/* ═══════════════════════════════════════════ */
/* v0.10.0 — Search bar & track filter chips   */
/* ═══════════════════════════════════════════ */

.hs-search-bar {
  position: relative;
  max-width: 480px;
  margin: 16px 0 24px;
}
.hs-search-input {
  width: 100%;
  padding: 10px 14px 10px 40px;
  font-size: 0.92rem;
  font-family: inherit;
  color: var(--text);
  background: #fff;
  border: 1px solid var(--border);
  border-radius: 10px;
  transition: border-color 0.15s, box-shadow 0.15s;
  box-sizing: border-box;
}
.hs-search-input::placeholder { color: var(--text-muted); }
.hs-search-input:focus {
  outline: none;
  border-color: var(--accent);
  box-shadow: 0 0 0 3px rgba(80, 70, 229, 0.12);
}
.hs-search-icon {
  position: absolute;
  left: 14px;
  top: 50%;
  transform: translateY(-50%);
  font-size: 0.95rem;
  opacity: 0.55;
  pointer-events: none;
}
.hs-search-empty {
  margin: 12px 0;
  padding: 16px 20px;
  font-size: 0.88rem;
  color: var(--text-muted);
  background: var(--bg-alt, #F9F9FB);
  border: 1px dashed var(--border);
  border-radius: 10px;
  text-align: center;
}
.hs-search-empty p { margin: 0 0 10px; }
.hs-search-clear {
  display: inline-block;
  margin: 0 0 10px;
  padding: 6px 16px;
  font-family: inherit;
  font-size: 0.8rem;
  font-weight: 500;
  color: var(--accent);
  background: transparent;
  border: 1px solid var(--border);
  border-radius: 100px;
  cursor: pointer;
  transition: border-color 200ms var(--hs-easing, ease), color 200ms var(--hs-easing, ease);
}
.hs-search-clear:hover { border-color: var(--accent); color: var(--accent); }
.hs-search-suggest {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: center;
  gap: 8px;
}
.hs-search-suggest-label { font-size: 0.8rem; color: var(--text-muted); }
.hs-search-suggestion {
  font-family: inherit;
  font-size: 0.8rem;
  color: var(--text);
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 100px;
  padding: 5px 14px;
  cursor: pointer;
  transition: border-color 200ms var(--hs-easing, ease), color 200ms var(--hs-easing, ease);
}
.hs-search-suggestion:hover { border-color: var(--accent); color: var(--accent); }

/* Track filter chips (curriculum.html) */
.hs-track-filter {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin: 0 0 24px;
  padding: 0;
}
.hs-track-filter-chip {
  font-family: inherit;
  font-size: 0.82rem;
  font-weight: 500;
  padding: 6px 16px;
  border-radius: 16px;
  border: 1px solid var(--border);
  background: #fff;
  color: var(--text-muted);
  cursor: pointer;
  transition: all 0.15s;
}
.hs-track-filter-chip:hover {
  border-color: var(--accent);
  color: var(--accent);
}
.hs-track-filter-chip.active {
  background: var(--accent);
  border-color: var(--accent);
  color: #fff;
}
.hs-track-filter-chip:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

/* ═════════════════════════════════════════
   v0.11.0 — Quiz section styles
   ═════════════════════════════════════════ */

.hs-chap-quiz {
  max-width: 1440px;
  margin: 0 auto;
  padding: 32px 24px;
  border-top: 1px solid var(--border);
}
.hs-chap-quiz-inner {
  max-width: 760px;
  margin: 0 auto;
}
.hs-chap-quiz-title {
  font-size: 1.3rem;
  margin: 0 0 4px;
  color: var(--text);
}
.hs-chap-quiz-sub {
  font-size: 0.85rem;
  color: var(--text-muted);
  margin: 0 0 20px;
}

.hs-quiz-score {
  font-size: 0.85rem;
  font-weight: 600;
  color: var(--accent);
  padding: 8px 14px;
  background: rgba(80, 70, 229, 0.06);
  border-radius: 8px;
  margin-bottom: 16px;
  display: inline-block;
}

.hs-quiz-q {
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 18px 20px;
  margin-bottom: 14px;
  background: #fff;
  transition: border-color 0.15s;
}
.hs-quiz-q.hs-quiz-answered {
  border-color: var(--border);
}
.hs-quiz-q-text {
  font-size: 0.92rem;
  font-weight: 600;
  color: var(--text);
  margin-bottom: 12px;
  line-height: 1.5;
  padding: 0;
}
.hs-quiz-q-num {
  display: inline-block;
  background: var(--accent);
  color: #fff;
  font-size: 0.72rem;
  padding: 2px 8px;
  border-radius: 10px;
  margin-right: 6px;
  font-weight: 700;
  letter-spacing: 0.03em;
}

.hs-quiz-options {
  display: flex;
  flex-direction: column;
  gap: 6px;
  margin-bottom: 12px;
}
.hs-quiz-option {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 10px 14px;
  min-height: 48px;
  border: 1px solid var(--border);
  border-radius: 8px;
  cursor: pointer;
  font-size: 0.85rem;
  color: var(--text);
  transition: background 0.12s, border-color 0.12s;
}
.hs-quiz-option:hover {
  background: rgba(80, 70, 229, 0.04);
  border-color: var(--accent);
}
.hs-quiz-option input {
  margin-top: 2px;
  cursor: pointer;
  accent-color: var(--accent);
}
.hs-quiz-option.hs-quiz-locked {
  cursor: default;
  opacity: 0.7;
}
.hs-quiz-option.hs-quiz-locked:hover {
  background: transparent;
  border-color: var(--border);
}
.hs-quiz-option-text {
  flex: 1;
  line-height: 1.45;
}

.hs-quiz-submit {
  font-size: 0.9rem;
  padding: 10px 24px;
  min-height: 44px;
  border: 1px solid var(--accent);
  border-radius: 8px;
  background: var(--accent);
  color: #fff;
  cursor: pointer;
  font-weight: 600;
  transition: background 0.12s;
}
.hs-quiz-submit:hover {
  background: #4338CA;
}
.hs-quiz-submit:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

.hs-quiz-feedback {
  margin-top: 10px;
  min-height: 0;
}
.hs-quiz-correct,
.hs-quiz-wrong {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  font-size: 0.82rem;
  font-weight: 700;
  padding: 4px 10px;
  border-radius: 8px;
  margin-bottom: 6px;
}
.hs-quiz-correct {
  background: rgba(16, 185, 129, 0.1);
  color: var(--green, #10B981);
}
.hs-quiz-wrong {
  background: rgba(239, 68, 68, 0.1);
  color: var(--red, #EF4444);
}
.hs-quiz-correct-ans {
  font-size: 0.8rem;
  color: var(--text);
  margin-bottom: 6px;
  font-weight: 600;
}
.hs-quiz-explanation {
  font-size: 0.8rem;
  color: var(--text-muted);
  line-height: 1.55;
  padding: 8px 12px;
  background: rgba(107, 114, 128, 0.06);
  border-left: 3px solid var(--border);
  border-radius: 0 6px 6px 0;
}

.hs-quiz-reset {
  font-size: 0.78rem;
  padding: 4px 14px;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: transparent;
  color: var(--text-muted);
  cursor: pointer;
  margin-top: 8px;
  transition: border-color 0.12s, color 0.12s;
}
.hs-quiz-reset:hover {
  border-color: var(--accent);
  color: var(--accent);
}
.hs-quiz-reset:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

/* ── A11y: visually-hidden announcer ── */
.hs-a11y-announcer {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}

/* ── A11y: quiz keyboard focus on options ── */
.hs-quiz-option input:focus-visible + .hs-quiz-option-text {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
  border-radius: 4px;
}

@media (max-width: 768px) {
  .hs-chap-quiz { padding: 24px 16px; }
  .hs-quiz-q { padding: 14px 16px; }
}

/* ═════════════════════════════════════════ */
/* v1.1.0 — Mobile hamburger navigation       */
/* Works on .hs-header (index/ml/dl/rl),      */
/* .topbar (about), .top-nav (curriculum) and */
/* .hs-chap-topbar (28 chapters).             */
/* ═════════════════════════════════════════ */

.hs-burger {
  display: none;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  gap: 5px;
  width: 44px;
  height: 44px;
  padding: 0;
  background: none;
  border: 1px solid var(--hs-border, var(--border, #E7E7EA));
  border-radius: 10px;
  cursor: pointer;
  flex-shrink: 0;
}
.hs-burger-bar {
  display: block;
  width: 18px;
  height: 2px;
  border-radius: 2px;
  background: var(--hs-text, var(--text, #0F1419));
  transition: transform 200ms var(--hs-easing, ease), opacity 200ms var(--hs-easing, ease);
}
.hs-burger:focus-visible {
  outline: 2px solid var(--hs-purple, var(--accent, #5046E5));
  outline-offset: 2px;
}
.hs-burger[aria-expanded="true"] .hs-burger-bar:nth-child(1) { transform: translateY(7px) rotate(45deg); }
.hs-burger[aria-expanded="true"] .hs-burger-bar:nth-child(2) { opacity: 0; }
.hs-burger[aria-expanded="true"] .hs-burger-bar:nth-child(3) { transform: translateY(-7px) rotate(-45deg); }

.hs-mobile-nav {
  display: none;
}

@media (max-width: 768px) {
  .hs-burger { display: inline-flex; }

  /* In-flow collapsible panel below the header */
  .hs-mobile-nav {
    display: block;
    max-height: 0;
    overflow: hidden;
    transition: max-height 280ms ease;
    background: var(--hs-bg, var(--bg, #FFFFFF));
    border-bottom: 1px solid var(--hs-border, var(--border, #E7E7EA));
  }
  .hs-mobile-nav.open { max-height: 480px; }
  .hs-mobile-nav a {
    display: block;
    padding: 14px 20px;
    font-family: var(--hs-font-text, var(--font-sans, sans-serif));
    font-size: 15px;
    font-weight: 500;
    color: var(--hs-text-2, var(--text-muted, #6B7280));
    text-decoration: none;
    border-top: 1px solid var(--hs-border, var(--border, #E7E7EA));
  }
  .hs-mobile-nav a:hover { color: var(--hs-text, var(--text, #0F1419)); text-decoration: none; }
  .hs-mobile-nav a.active { color: var(--hs-text, var(--text, #0F1419)); font-weight: 600; }
  .hs-mobile-nav .hs-cta {
    margin: 14px 20px 20px;
    justify-content: center;
  }

  /* curriculum.html — replace inline links with the hamburger panel */
  .top-nav-links { display: none; }
}

/* ═════════════════════════════════════════ */
/* v1.1.0 — Mobile chapter-page first screen */
/* Brings controls above the fold, fixes the */
/* 3+2 irregular parameter-panel wrap.       */
/* ═════════════════════════════════════════ */
@media (max-width: 768px) {
  .hs-chap-hero-stats { gap: 8px; }
  .hs-chap-stat { min-width: 0; flex: 1 1 30%; padding: 8px 10px; }
  .hs-chap-stat .n { font-size: 16px; }
  .hs-chap-canvas { height: 260px; }
  .hs-chap-canvas-small { height: 130px; }
  .hs-chap-toolbar { gap: 10px; }
  .hs-chap-slider { width: 100%; }
  .hs-chap-btns { flex-wrap: wrap; gap: 8px; }
  .hs-chap-btns .hs-chap-btn { flex: 1 1 calc(50% - 8px); }
}

/* ═════════════════════════════════════════ */
/* v1.1.0 — Sub-page polish                  */
/* Search bar aligned with card grid;        */
/* hero stats baseline alignment.            */
/* ═════════════════════════════════════════ */
.hs-search-bar { max-width: none; width: 100%; }

/* ═════════════════════════════════════════ */
/* v1.1.0 — Homepage hero balance            */
/* ═════════════════════════════════════════ */
.hs-hero { align-items: flex-start; }
.hs-hero-right { align-self: center; }
.hs-hero-fig { margin: 0; text-align: center; }
.hs-hero-fig figcaption {
  margin-top: 10px;
  font-family: var(--hs-font-text, var(--font-sans, sans-serif));
  font-size: 12.5px;
  color: var(--hs-text-3, #9CA3AF);
}
.hs-footer-inner { padding: 64px 64px; }

/* ═════════════════════════════════════════ */
/* v1.2.0 — Learning progress loop           */
/* Quiz empty-submit feedback, sub-page      */
/* status badges + continue banner, homepage */
/* track progress, custom confirm modal.     */
/* ═════════════════════════════════════════ */

/* ── Quiz: empty-submit feedback ── */
@keyframes hs-quiz-shake {
  0%, 100% { transform: translateX(0); }
  20% { transform: translateX(-6px); }
  40% { transform: translateX(6px); }
  60% { transform: translateX(-4px); }
  80% { transform: translateX(4px); }
}
.hs-quiz-submit.hs-quiz-shake { animation: hs-quiz-shake 0.4s ease; }
.hs-quiz-feedback.hs-quiz-empty-warn {
  color: var(--amber, #F59E0B);
  font-size: 0.85rem;
  font-weight: 600;
  margin-top: 6px;
}

/* ── Sub-pages: chapter status badge ── */
.hs-sub-card { position: relative; }
.hs-sub-status {
  position: absolute;
  top: 12px;
  right: 12px;
  font-size: 0.72rem;
  font-weight: 600;
  padding: 2px 10px;
  border-radius: 12px;
  background: var(--hs-badge-bg, var(--accent-soft, #F3F2FF));
  color: var(--hs-accent, var(--accent, #5046E5));
}
.hs-sub-card.status-completed { border-color: var(--hs-green, var(--green, #10B981)); }
.hs-sub-card.status-completed .hs-sub-status {
  background: rgba(16, 185, 129, 0.12);
  color: var(--hs-green, var(--green, #10B981));
}
.hs-sub-card.status-visited .hs-sub-status {
  background: var(--hs-badge-bg, var(--accent-soft, #F3F2FF));
  color: var(--hs-accent, var(--accent, #5046E5));
}

/* v1.5.0 — Sub-pages: prerequisite knowledge tags */
.hs-sub-prereq {
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: 6px;
  margin-top: 10px;
  font-size: 0.72rem;
}
.hs-sub-prereq-label {
  color: var(--hs-text-2, #6B7280);
  font-weight: 500;
}
.hs-sub-prereq-chip {
  background: var(--hs-accent-soft, rgba(80, 70, 229, 0.08));
  color: var(--hs-accent-deep, var(--hs-accent, #5046E5));
  border: 1px solid var(--hs-accent, #5046E5);
  padding: 1px 10px;
  border-radius: 100px;
  font-weight: 600;
  white-space: nowrap;
}

/* ── Sub-pages: continue-learning banner ── */
.hs-continue {
  display: flex;
  align-items: center;
  gap: 10px;
  margin: 0 0 20px;
  padding: 12px 18px;
  border-radius: 12px;
  background: var(--hs-accent-soft, var(--accent-soft, #F3F2FF));
  border: 1px solid var(--hs-border, var(--border, #E7E7EA));
  font-family: var(--hs-font-text, var(--font-sans, sans-serif));
  font-size: 0.9rem;
}
.hs-continue .hs-continue-label {
  font-weight: 600;
  color: var(--hs-text, var(--text, #0F1419));
  flex-shrink: 0;
}
.hs-continue a {
  color: var(--hs-accent, var(--accent, #5046E5));
  font-weight: 600;
  text-decoration: none;
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
}
.hs-continue a:hover { text-decoration: underline; }

/* ── Homepage: track progress bar ── */
.hs-path-progress {
  margin-top: 12px;
  width: 100%;
}
.hs-path-progress-row {
  display: flex;
  justify-content: space-between;
  align-items: center;
  font-family: var(--hs-font-text);
  font-size: 12px;
  color: var(--hs-text-2, #6B7280);
  margin-bottom: 5px;
}
.hs-path-progress-bar {
  height: 4px;
  border-radius: 2px;
  background: var(--hs-bg-alt, #F9F9FB);
  overflow: hidden;
}
.hs-path-progress-fill {
  height: 100%;
  border-radius: 2px;
  background: var(--hs-purple, #5046E5);
  transition: width 300ms ease;
}
.hs-path-dl .hs-path-progress-fill { background: var(--hs-green, #10B981); }
.hs-path-rl .hs-path-progress-fill { background: var(--hs-amber, #F59E0B); }
.hs-path-math .hs-path-progress-fill { background: var(--hs-teal, #14B8A6); }

/* ── Homepage: custom confirm modal ── */
.hs-confirm {
  display: none;
  position: fixed;
  inset: 0;
  z-index: 200;
  align-items: center;
  justify-content: center;
  background: rgba(15, 20, 25, 0.45);
  backdrop-filter: blur(4px);
  -webkit-backdrop-filter: blur(4px);
  padding: 24px;
}
.hs-confirm.show { display: flex; }
.hs-confirm-card {
  max-width: 420px;
  width: 100%;
  background: #fff;
  border-radius: 16px;
  padding: 28px 24px;
  box-shadow: 0 16px 48px rgba(15, 20, 25, 0.18);
  font-family: var(--hs-font-text, var(--font-sans, sans-serif));
  text-align: center;
}
.hs-confirm-card h3 {
  font-family: var(--hs-font-display, var(--font-sans, sans-serif));
  font-size: 17px;
  font-weight: 600;
  color: var(--hs-text, #0F1419);
  margin: 0 0 10px;
}
.hs-confirm-card p {
  font-size: 14px;
  color: var(--hs-text-2, #6B7280);
  line-height: 1.6;
  margin: 0 0 22px;
}
.hs-confirm-actions { display: flex; gap: 12px; justify-content: center; }
.hs-confirm-actions button {
  font-family: inherit;
  font-size: 14px;
  font-weight: 600;
  padding: 10px 22px;
  border-radius: 10px;
  border: 1px solid var(--hs-border, #E7E7EA);
  background: #fff;
  color: var(--hs-text-2, #6B7280);
  cursor: pointer;
  transition: border-color 200ms ease, color 200ms ease;
}
.hs-confirm-actions button.hs-confirm-ok {
  background: var(--hs-purple, #5046E5);
  border-color: var(--hs-purple, #5046E5);
  color: #fff;
}
.hs-confirm-actions button.hs-confirm-ok:hover { background: var(--hs-purple-deep, #4338CA); }
.hs-confirm-actions button.hs-confirm-cancel:hover { border-color: var(--hs-text-3, #9CA3AF); color: var(--hs-text, #0F1419); }

/* ═════════════════════════════════════════
   v1.14.0 — Mobile reading fixes (≤768px)
   Wide unshrinkable children (KaTeX display
   equations, tables, fixed-size canvases) used
   to force the mobile layout viewport wider
   than the device, zooming the whole page out.
   Now they scroll/shrink inside their cards.
   ═════════════════════════════════════════ */
@media (max-width: 768px) {
  /* Grid/flex children may shrink below content width */
  .hs-chap-depth-block,
  .hs-chap-card,
  .hs-chap-side,
  .hs-sub-card,
  .hs-sub-card > div { min-width: 0; }

  /* Formulas & tables scroll inside their card instead of expanding the page */
  .hs-chap-formula { overflow-x: auto; overflow-y: hidden; }
  .katex-display { overflow-x: auto; overflow-y: hidden; padding-bottom: 4px; }
  .hs-chap-hyper-table { display: block; overflow-x: auto; -webkit-overflow-scrolling: touch; }

  /* Scroll-shadow hint: indicates more content to the right */
  .hs-chap-code,
  .katex-display,
  .hs-chap-hyper-table {
    background:
      linear-gradient(to right, rgba(255,255,255,0) 85%, rgba(15,20,25,0.06) 100%) no-repeat,
      linear-gradient(to right, rgba(15,20,25,0.06), rgba(255,255,255,0) 15%) 0 100% no-repeat;
    background-size: 100% 100%, 200% 100%;
    background-attachment: scroll, local;
  }

  /* Code blocks: smaller type on phones (already overflow-x: auto) */
  .hs-chap-code { font-size: 12.5px; padding: 14px 16px; }

  /* Minimum font sizes for mobile readability (prevent sub-12px text) */
  .hs-chap-chip .k,
  .badge,
  .meta-chip,
  .hs-quiz-q-num { font-size: 12px; }
  .explain-box,
  .prereq-card,
  .hs-chap-stat .l,
  .hs-quiz-explanation,
  .hs-quiz-correct,
  .hs-quiz-wrong { font-size: 0.85rem; }
  .hs-chap-formula { font-size: 14px; }

  /* Canvases cap at card width, keep aspect ratio */
  .hs-chap-lab canvas,
  .hs-chap-card canvas,
  .hs-chap-depth-block canvas,
  canvas.diagram { max-width: 100%; height: auto; }

  /* Chapter topbar: no text wrapping, no clipped toggle */
  .hs-chap-topbar-inner { gap: 10px; }
  .hs-chap-brand { white-space: nowrap; flex-shrink: 0; }
  .hs-chap-topbar-right { gap: 10px; flex-shrink: 0; }
  .hs-chap-step-progress { gap: 6px; }
  .hs-chap-step-pill { white-space: nowrap; }

  /* Track landing cards may wrap their meta row */
  .hs-sub-card { flex-wrap: wrap; }
}

@media (max-width: 480px) {
  /* Brand: keep the mark, hide the wordmark text node */
  .hs-chap-brand { font-size: 0; }
  .hs-chap-brand .mark { font-size: 12px; }
  .hs-chap-h1 { font-size: 28px; }
}