:root {
  color-scheme: light;
  --ink: #101a34;
  --ink-soft: #526078;
  --paper: #f4f6fa;
  --surface: #ffffff;
  --line: #dfe4ed;
  --line-dark: #c9d1df;
  --navy: #101a34;
  --navy-2: #1b294d;
  --accent: #ffca58;
  --accent-strong: #eca91e;
  --cyan: #41c4d7;
  --cyan-dark: #16788b;
  --blue-wash: #eaf7fa;
  --green: #19875d;
  --green-wash: #eaf7f1;
  --red: #b8424c;
  --red-wash: #fff0f1;
  --purple: #6c54c7;
  --shadow: 0 18px 55px rgba(28, 44, 79, 0.1);
  --radius: 18px;
  font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
}

* { box-sizing: border-box; }
[hidden] { display: none !important; }
html { scroll-behavior: smooth; scroll-padding-top: 96px; }
body {
  margin: 0;
  min-width: 320px;
  background: var(--paper);
  color: var(--ink);
  font-size: 16px;
  line-height: 1.62;
}
button, input, select { font: inherit; }
button, a { -webkit-tap-highlight-color: transparent; }
a { color: inherit; }
button { color: inherit; }

.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}

.skip-link {
  position: fixed;
  z-index: 100;
  top: .75rem;
  left: .75rem;
  padding: .65rem .9rem;
  border-radius: 8px;
  background: white;
  color: var(--navy);
  text-decoration: none;
  transform: translateY(-160%);
}
.skip-link:focus { transform: translateY(0); }

.topbar {
  position: sticky;
  z-index: 30;
  top: 0;
  height: 72px;
  display: grid;
  grid-template-columns: 280px 1fr auto;
  align-items: center;
  gap: 1rem;
  padding: 0 clamp(1rem, 3vw, 2.5rem);
  border-bottom: 1px solid rgba(255,255,255,.12);
  background: rgba(16, 26, 52, .97);
  color: white;
  backdrop-filter: blur(16px);
}
.brand {
  display: inline-flex;
  align-items: center;
  gap: .75rem;
  width: fit-content;
  color: white;
  font-size: 1.08rem;
  font-weight: 780;
  letter-spacing: -.02em;
  text-decoration: none;
}
.brand-mark {
  width: 36px;
  height: 36px;
  display: grid;
  place-items: center;
  flex: 0 0 auto;
  border: 1px solid rgba(255,255,255,.2);
  border-radius: 10px;
  background: var(--accent);
  color: var(--navy);
  font-family: Georgia, serif;
  font-size: 1.25rem;
  font-weight: 800;
}
.topbar-context { display: flex; gap: .65rem; align-items: baseline; color: #aeb9cf; font-size: .88rem; }
.topbar-context strong { color: white; }
.topbar-actions { display: flex; gap: .55rem; }
.ghost-button, .print-button, .test-actions button {
  min-height: 38px;
  padding: .45rem .75rem;
  border: 1px solid rgba(255,255,255,.22);
  border-radius: 9px;
  background: transparent;
  color: #dce3ef;
  font-size: .78rem;
  font-weight: 760;
  cursor: pointer;
}
.print-button { border-color: var(--accent); background: var(--accent); color: var(--navy); }
.ghost-button:hover { border-color: rgba(255,255,255,.5); color: white; }
.print-button:hover { background: #ffd675; }

.reading-progress {
  position: fixed;
  z-index: 35;
  top: 72px;
  left: 0;
  width: 100%;
  height: 3px;
  background: rgba(255,255,255,.1);
}
.reading-progress span { display: block; width: 0; height: 100%; background: linear-gradient(90deg, var(--accent), var(--cyan)); }

.chapter-hero {
  position: relative;
  overflow: hidden;
  background:
    radial-gradient(circle at 82% 22%, rgba(65,196,215,.18) 0, rgba(65,196,215,0) 29%),
    radial-gradient(circle at 65% 105%, rgba(255,202,88,.1) 0, rgba(255,202,88,0) 32%),
    var(--navy);
  color: white;
}
.chapter-hero::after {
  content: "Σ";
  position: absolute;
  right: -1vw;
  bottom: -9rem;
  color: rgba(255,255,255,.025);
  font-family: Georgia, serif;
  font-size: min(42vw, 35rem);
  line-height: 1;
}
.hero-inner {
  position: relative;
  z-index: 1;
  width: min(1500px, 100%);
  margin: 0 auto;
  padding: 2rem clamp(1.25rem, 5vw, 5rem) 2.4rem;
}
.breadcrumb { display: flex; gap: .5rem; color: #aeb8cc; font-size: .78rem; }
.breadcrumb a { text-decoration: none; }
.breadcrumb a:hover { color: var(--accent); }
.hero-grid {
  display: grid;
  grid-template-columns: minmax(0, 1fr) minmax(300px, .45fr);
  gap: clamp(2rem, 6vw, 6rem);
  align-items: end;
  margin-top: 2.4rem;
}
.eyebrow {
  margin: 0 0 .7rem;
  color: #6c7990;
  font-size: .72rem;
  font-weight: 850;
  letter-spacing: .13em;
  text-transform: uppercase;
}
.eyebrow.accent { color: #9b6a00; }
.chapter-hero .eyebrow.accent { color: var(--accent); }
.chapter-hero h1 {
  margin: 0;
  font-family: Georgia, "Times New Roman", serif;
  font-size: clamp(3.2rem, 8vw, 7.2rem);
  font-weight: 500;
  letter-spacing: -.06em;
  line-height: .88;
}
.chapter-hero h1 span { color: var(--accent); font-style: italic; }
.hero-copy { max-width: 800px; margin: 1.4rem 0 0; color: #c5cedd; font-size: clamp(.98rem, 1.6vw, 1.12rem); }
.hero-actions { display: flex; flex-wrap: wrap; gap: .75rem; margin-top: 1.6rem; }
.primary-button, .secondary-button {
  min-height: 44px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: .55rem;
  padding: .62rem .9rem;
  border-radius: 10px;
  font-size: .84rem;
  font-weight: 820;
  text-decoration: none;
  cursor: pointer;
}
.primary-button { border: 1px solid var(--accent); background: var(--accent); color: var(--navy); }
.primary-button:hover { background: #ffd675; }
.secondary-button { border: 1px solid var(--line-dark); background: white; color: var(--navy); }
.chapter-hero .secondary-button { border-color: rgba(255,255,255,.23); background: rgba(255,255,255,.07); color: white; }
.secondary-button:hover { border-color: var(--navy); }
.hero-panel {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  overflow: hidden;
  border: 1px solid rgba(255,255,255,.15);
  border-radius: 17px;
  background: rgba(255,255,255,.055);
  backdrop-filter: blur(12px);
}
.hero-stat { min-height: 105px; display: grid; align-content: center; padding: 1rem 1.2rem; }
.hero-stat:nth-child(odd) { border-right: 1px solid rgba(255,255,255,.12); }
.hero-stat:nth-child(-n+2) { border-bottom: 1px solid rgba(255,255,255,.12); }
.hero-stat strong { color: var(--accent); font-family: Georgia, serif; font-size: 1.75rem; font-weight: 500; }
.hero-stat span { color: #aeb9ce; font-size: .72rem; letter-spacing: .05em; text-transform: uppercase; }
.syllabus-strip {
  display: grid;
  grid-template-columns: auto 1fr auto;
  gap: .75rem;
  align-items: center;
  margin-top: 2.1rem;
  padding-top: 1.1rem;
  border-top: 1px solid rgba(255,255,255,.13);
  color: #b9c3d6;
  font-size: .78rem;
}
.syllabus-strip p { margin: 0; }
.syllabus-strip strong { color: white; }
.syllabus-strip a { color: var(--accent); font-weight: 780; text-decoration: none; }
.verified-dot { width: 8px; height: 8px; border-radius: 50%; background: var(--cyan); box-shadow: 0 0 0 5px rgba(65,196,215,.12); }

.chapter-shell {
  width: min(1500px, 100%);
  display: grid;
  grid-template-columns: 255px minmax(0, 1fr);
  gap: clamp(2rem, 4vw, 4.5rem);
  margin: 0 auto;
  padding: 2.3rem clamp(1.25rem, 5vw, 5rem) 4rem;
}
.chapter-nav { position: relative; }
.nav-card {
  position: sticky;
  top: 96px;
  padding: 1.15rem;
  border: 1px solid var(--line);
  border-radius: 14px;
  background: rgba(255,255,255,.78);
  box-shadow: 0 8px 30px rgba(33,48,79,.05);
  backdrop-filter: blur(12px);
}
.nav-card nav { display: grid; gap: .2rem; }
.nav-card nav a {
  position: relative;
  padding: .47rem .55rem .47rem .75rem;
  border-radius: 7px;
  color: #667288;
  font-size: .76rem;
  font-weight: 650;
  line-height: 1.3;
  text-decoration: none;
}
.nav-card nav a::before {
  content: "";
  position: absolute;
  left: 0;
  top: 50%;
  width: 3px;
  height: 0;
  border-radius: 3px;
  background: var(--cyan-dark);
  transform: translateY(-50%);
  transition: height 140ms ease;
}
.nav-card nav a:hover { background: #f1f4f8; color: var(--navy); }
.nav-card nav a.active { background: var(--blue-wash); color: #164e60; font-weight: 800; }
.nav-card nav a.active::before { height: 55%; }
.nav-mastery { display: grid; margin-top: 1rem; padding-top: .9rem; border-top: 1px solid var(--line); }
.nav-mastery span { color: #8490a3; font-size: .66rem; text-transform: uppercase; letter-spacing: .07em; }
.nav-mastery strong { margin-top: .2rem; font-size: .78rem; }

.chapter-article { min-width: 0; max-width: 1020px; }
.chapter-section {
  margin-bottom: 2rem;
  padding: clamp(1.4rem, 3.2vw, 2.8rem);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--surface);
  box-shadow: 0 5px 24px rgba(34,49,78,.045);
}
.section-header {
  display: grid;
  grid-template-columns: 52px 1fr;
  gap: 1rem;
  align-items: start;
  margin-bottom: 1.7rem;
  padding-bottom: 1.2rem;
  border-bottom: 1px solid var(--line);
}
.section-number {
  width: 48px;
  height: 48px;
  display: grid;
  place-items: center;
  border-radius: 12px;
  background: var(--navy);
  color: var(--accent);
  font-family: Georgia, serif;
  font-size: 1.1rem;
}
.section-header .eyebrow { margin: .15rem 0 .25rem; }
.section-header h2 {
  margin: 0;
  font-family: Georgia, "Times New Roman", serif;
  font-size: clamp(1.8rem, 4vw, 2.65rem);
  font-weight: 500;
  letter-spacing: -.035em;
  line-height: 1.06;
}
.chapter-section h3 { margin: 2rem 0 .65rem; font-size: 1.02rem; letter-spacing: -.01em; }
.chapter-section > h3:first-of-type { margin-top: 1rem; }
.chapter-section p { color: var(--ink-soft); }
.chapter-section .lead { margin-top: 0; color: #35435b; font-size: 1.03rem; line-height: 1.72; }

.route-grid { display: grid; grid-template-columns: repeat(3, 1fr); gap: .8rem; margin: 1.5rem 0; }
.route-card { padding: 1rem; border: 1px solid var(--line); border-radius: 12px; background: #fafbfd; }
.route-card > span { display: grid; place-items: center; width: 27px; height: 27px; border-radius: 8px; background: var(--accent); font-weight: 850; }
.route-card h3 { margin: .8rem 0 .3rem; }
.route-card p { margin: 0; font-size: .82rem; }

.callout { margin: 1.3rem 0; padding: 1rem 1.1rem; border-left: 4px solid var(--cyan); border-radius: 0 11px 11px 0; background: #f1f9fa; }
.callout strong { color: #174f60; }
.callout p { margin: .25rem 0 0; font-size: .86rem; }
.callout.warning { border-left-color: var(--accent-strong); background: #fff8e8; }
.callout.warning strong { color: #7f5906; }
.callout.advanced { border-left-color: var(--purple); background: #f4f1ff; }
.callout.advanced strong { color: #5542a5; }
.callout.source-note { border: 1px solid #d6e8eb; border-left: 4px solid var(--cyan); }

.definition-grid { display: grid; grid-template-columns: repeat(3, 1fr); gap: .75rem; }
.definition-grid > div { padding: 1rem; border: 1px solid var(--line); border-radius: 12px; background: #fafbfd; }
.definition-grid .term { color: var(--cyan-dark); font-size: .69rem; font-weight: 850; letter-spacing: .08em; text-transform: uppercase; }
.definition-grid strong { display: block; margin-top: .4rem; font-family: Georgia, serif; font-size: 1.13rem; font-weight: 500; }
.definition-grid p { margin: .3rem 0 0; font-size: .78rem; }

.formula-grid { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: .75rem; margin: 1rem 0; }
.formula-grid.three { grid-template-columns: repeat(3, minmax(0, 1fr)); }
.formula-card {
  overflow-x: auto;
  padding: 1rem;
  border: 1px solid var(--line);
  border-radius: 12px;
  background: #f7f9fc;
  color: var(--navy);
}
.formula-card.key { border-color: #d5b155; background: #fffaeb; }
.formula-card.wide { margin: 1rem 0; text-align: center; }
.formula-card p { margin: .3rem 0 0; font-size: .8rem; }
.formula-label { margin: 0 0 .6rem !important; color: #7c889b !important; font-family: Inter, sans-serif; font-size: .65rem !important; font-weight: 850; letter-spacing: .09em; text-transform: uppercase; }
mjx-container { overflow-x: auto; overflow-y: hidden; max-width: 100%; }

.worked-example, .pyq-card {
  overflow: hidden;
  margin: .85rem 0;
  border: 1px solid var(--line);
  border-radius: 12px;
  background: white;
}
.worked-example > summary, .pyq-card > summary {
  position: relative;
  display: flex;
  align-items: center;
  gap: .75rem;
  min-height: 58px;
  padding: .85rem 2.8rem .85rem 1rem;
  color: #26354e;
  font-weight: 720;
  list-style: none;
  cursor: pointer;
}
.worked-example > summary::-webkit-details-marker, .pyq-card > summary::-webkit-details-marker { display: none; }
.worked-example > summary::after, .pyq-card > summary::after {
  content: "+";
  position: absolute;
  right: 1rem;
  color: #758196;
  font-size: 1.25rem;
  font-weight: 400;
}
.worked-example[open] > summary::after, .pyq-card[open] > summary::after { content: "−"; }
.worked-example[open] > summary { border-bottom: 1px solid var(--line); background: #fbfcfe; }
.worked-example summary > span {
  flex: 0 0 auto;
  padding: .18rem .45rem;
  border-radius: 999px;
  background: var(--blue-wash);
  color: var(--cyan-dark);
  font-size: .66rem;
  font-weight: 850;
  letter-spacing: .04em;
  text-transform: uppercase;
}
.solution { padding: 1rem 1.1rem 1.15rem; }
.solution p { margin: .35rem 0; font-size: .87rem; }
.solution .method { margin-top: .9rem; padding: .7rem .8rem; border-radius: 8px; background: #f0f7fa; color: #31596a; }
.problem-large > summary { font-size: .91rem; }

.clean-list { display: grid; gap: .45rem; padding-left: 1.15rem; }
.clean-list li { padding-left: .25rem; color: var(--ink-soft); }
.clean-list strong { color: var(--ink); }

.strategy-table { overflow: hidden; margin: 1rem 0; border: 1px solid var(--line); border-radius: 12px; }
.table-row { display: grid; grid-template-columns: minmax(180px, .75fr) 1fr; border-top: 1px solid var(--line); }
.table-row:first-child { border-top: 0; }
.table-row span { padding: .72rem .85rem; color: var(--ink-soft); font-size: .82rem; }
.table-row span + span { border-left: 1px solid var(--line); color: var(--ink); }
.table-head { background: var(--navy); }
.table-head span { color: white !important; font-weight: 800; }
.strategy-table.compact { max-width: 720px; }

.compare-grid { display: grid; grid-template-columns: repeat(4, 1fr); gap: .6rem; margin: 1rem 0; }
.compare-grid > div { display: grid; padding: .8rem; border-radius: 10px; }
.compare-grid strong { font-family: Georgia, serif; font-size: 1.1rem; }
.compare-grid span { margin-top: .2rem; font-size: .7rem; }
.compare-grid .good { background: var(--green-wash); color: #176847; }
.compare-grid .bad { background: var(--red-wash); color: #8d3841; }

.formula-stack { overflow: hidden; margin: 1rem 0; border: 1px solid var(--line); border-radius: 14px; }
.formula-stack div { display: grid; grid-template-columns: 1fr 1fr; gap: 1rem; padding: .8rem 1rem; border-top: 1px solid var(--line); }
.formula-stack div:first-child { border-top: 0; }
.formula-stack div:nth-child(odd) { background: #fafbfd; }
.formula-stack span { color: var(--ink-soft); }
.formula-stack strong { color: var(--navy); font-family: Georgia, serif; font-weight: 500; }
.difference-demo { display: grid; gap: .5rem; max-width: 720px; margin: 1rem 0; }
.difference-demo div { display: grid; grid-template-columns: 150px 1fr; gap: 1rem; align-items: center; padding: .7rem .8rem; border-radius: 9px; background: #f4f6fa; }
.difference-demo span { color: #6d798d; font-size: .74rem; font-weight: 780; text-transform: uppercase; }
.difference-demo strong { word-spacing: .7rem; font-family: Georgia, serif; font-weight: 500; }

.strategy-ladder { display: grid; gap: .6rem; }
.strategy-ladder > div { display: grid; grid-template-columns: 34px 1fr; gap: .75rem; align-items: center; }
.strategy-ladder span { width: 32px; height: 32px; display: grid; place-items: center; border-radius: 8px; background: var(--navy); color: var(--accent); font-size: .75rem; font-weight: 850; }
.strategy-ladder p { margin: 0; padding: .65rem .8rem; border: 1px solid var(--line); border-radius: 9px; background: #fafbfd; font-size: .84rem; }
.trap-grid { display: grid; grid-template-columns: repeat(2, 1fr); gap: .7rem; margin-top: 1.5rem; }
.trap-grid > div { padding: .85rem; border-left: 3px solid #e26b74; border-radius: 0 9px 9px 0; background: var(--red-wash); }
.trap-grid strong { color: #963e47; font-size: .72rem; letter-spacing: .08em; text-transform: uppercase; }
.trap-grid p { margin: .2rem 0 0; font-size: .78rem; }

.level-divider { display: flex; justify-content: space-between; gap: 1rem; margin: 1.8rem 0 .7rem; padding-bottom: .45rem; border-bottom: 2px solid var(--navy); }
.level-divider span:first-child { font-weight: 850; }
.level-divider span:last-child { color: #7b879a; font-size: .75rem; }

.bank-heading { display: flex; align-items: end; justify-content: space-between; gap: 1rem; margin: 2rem 0 .8rem; }
.bank-heading > div { display: flex; align-items: center; gap: .65rem; }
.bank-heading h3 { margin: 0; }
.bank-heading p { margin: 0; color: #7f8b9e; font-size: .75rem; }
.exam-badge { padding: .23rem .5rem; border-radius: 999px; font-size: .65rem; font-weight: 850; letter-spacing: .06em; text-transform: uppercase; }
.exam-badge.main { background: var(--blue-wash); color: var(--cyan-dark); }
.exam-badge.advanced { background: #f1edff; color: #5e49b2; }
.advanced-bank { margin-top: 2.8rem; }
.pyq-list { display: grid; gap: .7rem; }
.pyq-card { margin: 0; }
.pyq-card > summary { display: grid; grid-template-columns: auto 1fr auto; gap: .85rem; }
.pyq-index { width: 34px; height: 34px; display: grid; place-items: center; border-radius: 8px; background: var(--navy); color: var(--accent); font-size: .72rem; font-weight: 850; }
.pyq-card summary > span:nth-child(2) { display: grid; }
.pyq-card summary strong { font-size: .88rem; }
.pyq-card summary small { margin-top: .12rem; color: #7d899c; font-size: .69rem; font-weight: 600; }
.difficulty { padding: .2rem .45rem; border-radius: 999px; background: var(--blue-wash); color: var(--cyan-dark); font-size: .62rem; font-weight: 850; text-transform: uppercase; }
.difficulty.advanced { background: #f1edff; color: #5e49b2; }
.pyq-card[open] > summary { border-bottom: 1px solid var(--line); background: #fbfcfe; }
.pyq-body { padding: 1rem 1.1rem; }
.pyq-body > p:first-child { margin-top: 0; color: #39465d; }
.answer-line { padding: .7rem .8rem; border-radius: 8px; background: var(--green-wash); color: #176847 !important; }
.solution-block { padding: .1rem .85rem; border-left: 3px solid var(--cyan); }
.solution-block p { font-size: .84rem; }
.source-link { display: inline-flex; margin-top: .8rem; color: var(--cyan-dark); font-size: .72rem; font-weight: 800; text-decoration: none; }
.source-link:hover { text-decoration: underline; }

.test-switcher { display: grid; grid-template-columns: repeat(2, 1fr); gap: .7rem; margin: 1.4rem 0; }
.test-tab {
  display: grid;
  gap: .15rem;
  padding: .9rem 1rem;
  border: 1px solid var(--line-dark);
  border-radius: 11px;
  background: #f8f9fc;
  color: #4c5a71;
  text-align: left;
  font-weight: 800;
  cursor: pointer;
}
.test-tab span { color: #8691a3; font-size: .68rem; font-weight: 650; }
.test-tab.active { border-color: var(--navy); background: var(--navy); color: white; }
.test-tab.active span { color: #b7c2d5; }
.test-toolbar {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 1rem;
  padding: .8rem 1rem;
  border-radius: 11px;
  background: #edf1f7;
}
.test-toolbar > div:first-child { display: grid; }
.timer-label { color: #738096; font-size: .61rem; font-weight: 850; letter-spacing: .08em; text-transform: uppercase; }
.timer { font-variant-numeric: tabular-nums; font-size: 1.25rem; }
.test-actions { display: flex; gap: .45rem; }
.test-actions button { border-color: var(--line-dark); background: white; color: var(--navy); }
.test-toolbar.time-up { background: var(--red-wash); }
.test-toolbar.time-up .timer { color: var(--red); }
.quiz { display: grid; gap: .9rem; margin-top: 1rem; }
.quiz-question { padding: 1.05rem; border: 1px solid var(--line); border-radius: 12px; background: white; }
.quiz-question.correct { border-color: #84c9ad; background: #fbfefc; }
.quiz-question.incorrect { border-color: #e3a0a6; background: #fffdfd; }
.answer-status { margin: .7rem 0 0; color: var(--navy) !important; font-size: .76rem; font-weight: 850; }
.answer-status[hidden] { display: none; }
.question-kicker { display: flex; align-items: center; justify-content: space-between; gap: .7rem; margin-bottom: .55rem; }
.question-kicker span:first-child { color: var(--cyan-dark); font-size: .67rem; font-weight: 850; letter-spacing: .08em; text-transform: uppercase; }
.format-badge { padding: .14rem .4rem; border-radius: 999px; background: #eff2f7; color: #707d91; font-size: .59rem; font-weight: 800; text-transform: uppercase; }
.question-text { margin: 0 0 .75rem; color: #29374e !important; font-size: .91rem; font-weight: 650; }
.options { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: .5rem; }
.option {
  display: grid;
  grid-template-columns: auto 1fr;
  gap: .55rem;
  align-items: start;
  padding: .62rem .7rem;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: #fafbfd;
  color: #4e5b70;
  font-size: .8rem;
  cursor: pointer;
}
.option:hover { border-color: #a9b5c6; }
.option input { margin-top: .2rem; accent-color: var(--cyan-dark); }
.numeric-answer { width: min(100%, 300px); height: 43px; padding: 0 .75rem; border: 1px solid var(--line-dark); border-radius: 8px; outline: none; }
.numeric-answer:focus { border-color: var(--cyan-dark); box-shadow: 0 0 0 3px rgba(65,196,215,.15); }
.hint { margin-top: .65rem; }
.hint summary { width: fit-content; color: var(--cyan-dark); font-size: .72rem; font-weight: 780; cursor: pointer; }
.hint p { margin: .35rem 0 0; color: #6c788c; font-size: .75rem; }
.quiz-solution { margin-top: .8rem; padding: .75rem .85rem; border-radius: 9px; background: #f0f3f8; color: #4e5b70; font-size: .8rem; }
.quiz-solution[hidden] { display: none; }
.quiz-solution strong { color: var(--navy); }
.score-dock {
  position: sticky;
  bottom: 1rem;
  z-index: 5;
  display: flex;
  align-items: center;
  gap: 1rem;
  margin-top: 1rem;
  padding: .8rem;
  border: 1px solid var(--line);
  border-radius: 13px;
  background: rgba(255,255,255,.94);
  box-shadow: 0 16px 45px rgba(20,34,61,.16);
  backdrop-filter: blur(12px);
}
.score-dock .primary-button { flex: 0 0 auto; }
.score-output { margin: 0 !important; font-size: .8rem; }

.revision-section { background: #fffdf7; border-color: #e3d7b7; }
.revision-grid { display: grid; grid-template-columns: repeat(2, 1fr); gap: .75rem; }
.revision-grid > div { padding: 1rem; border: 1px solid #e4ddc9; border-radius: 11px; background: rgba(255,255,255,.72); }
.revision-grid h3 { margin: 0 0 .55rem; color: #8b650e; font-size: .82rem; text-transform: uppercase; letter-spacing: .07em; }
.revision-grid p { margin: .2rem 0; color: #48546a; font-size: .78rem; }
.mastery-checklist { margin-top: 1rem; padding: 1rem; border-radius: 11px; background: var(--navy); color: white; }
.mastery-checklist h3 { margin: 0 0 .7rem; color: var(--accent); }
.mastery-checklist label { display: flex; gap: .6rem; align-items: start; margin: .45rem 0; color: #c6cedd; font-size: .8rem; }
.mastery-checklist input { margin-top: .25rem; accent-color: var(--accent); }

.sources-section { background: #f8fafc; }
.source-grid { display: grid; grid-template-columns: repeat(2, 1fr); gap: .7rem; }
.source-grid a { display: grid; padding: .9rem; border: 1px solid var(--line); border-radius: 10px; background: white; text-decoration: none; }
.source-grid a:hover { border-color: #aeb9c9; }
.source-grid span { color: var(--cyan-dark); font-size: .64rem; font-weight: 850; text-transform: uppercase; letter-spacing: .07em; }
.source-grid strong { margin-top: .22rem; font-size: .84rem; }
.source-grid small { margin-top: .12rem; color: #7d899c; font-size: .7rem; }

.lesson-block { margin: 1.1rem 0; padding: 1rem 1.1rem; border: 1px solid var(--line); border-left: 4px solid var(--accent); border-radius: 12px; background: #fffdf7; }
.lesson-block h3 { margin: .15rem 0 .55rem; }
.lesson-block p { margin: .45rem 0; }
.lesson-kicker { margin: 0 !important; color: var(--cyan-dark) !important; font-size: .72rem; font-weight: 850; letter-spacing: .08em; text-transform: uppercase; }
.concept-visual { overflow: hidden; margin: 1.25rem 0; border: 1px solid #cadde2; border-left: 4px solid var(--cyan); border-radius: 13px; background: white; box-shadow: 0 8px 24px rgba(24,38,70,.055); }
.visual-heading { display: grid; grid-template-columns: minmax(0,1fr) minmax(250px,.55fr); gap: 1rem; align-items: end; padding: 1rem 1.1rem .8rem; background: linear-gradient(100deg,#fff 55%,#f7fcfd); }
.visual-heading h3 { margin: 0; font: 600 1.15rem/1.3 Georgia,serif; }
.visual-heading .eyebrow { margin-bottom: .2rem; color: var(--cyan-dark); }
.visual-controls { display: grid; grid-template-columns: repeat(auto-fit,minmax(125px,1fr)); gap: .65rem; }
.visual-control { display: grid; gap: .35rem; color: #4d5d74; font-size: .86rem; font-weight: 760; }
.visual-control span { display: flex; justify-content: space-between; align-items: baseline; gap: .6rem; }
.visual-control output { color: var(--navy); font-variant-numeric: tabular-nums; }
.visual-control input, .visual-control select { min-height: 44px; }
.visual-control input[type="range"] { width: 100%; accent-color: var(--cyan-dark); }
.visual-control input[type="number"], .visual-control select { width: 100%; padding: .55rem .6rem; border: 1px solid var(--line-dark); border-radius: 8px; background: white; color: var(--ink); }
.visual-stage { min-height: 330px; border-top: 1px solid var(--line); border-bottom: 1px solid var(--line); background: linear-gradient(180deg,#fbfdff,#f4f8fb); }
.visual-stage canvas { display: block; width: 100%; max-width: 100%; height: auto; }
.visual-readout { margin: 0 !important; padding: .75rem 1.1rem; border-bottom: 1px solid #d9e7ea; background: var(--blue-wash); color: #184e5a !important; font-size: .88rem; font-weight: 720; font-variant-numeric: tabular-nums; }
.visual-caption { margin: 0 !important; padding: .9rem 1.1rem .45rem; color: #526078 !important; font-size: .9rem; }
.visual-insights { margin: 0; padding: .2rem 2rem 1rem 2.3rem !important; font-size: .88rem; }
.visual-insights li { margin: .32rem 0; }
.visual-worked { margin: 0 1rem 1rem; border: 1px solid var(--line); border-radius: 9px; background: #fffdf7; }
.visual-worked > summary { padding: .75rem .85rem; font-size: .88rem; font-weight: 800; cursor: pointer; }
.visual-worked > div { padding: 0 .85rem .8rem; color: #4d5b70; font-size: .88rem; }

.chapter-footer { display: flex; align-items: center; justify-content: space-between; gap: 1rem; padding: 1.1rem; border-radius: 14px; background: var(--navy); color: white; }
.chapter-footer > div { display: flex; align-items: center; gap: .8rem; }
.chapter-footer p { margin: .1rem 0 0; color: #acb8cd; font-size: .75rem; }
.noscript-note { position: fixed; z-index: 50; right: 1rem; bottom: 1rem; max-width: 390px; padding: .8rem 1rem; border-radius: 10px; background: #fff2cd; color: #5b4200; box-shadow: var(--shadow); }

:focus-visible { outline: 3px solid rgba(33,156,179,.45); outline-offset: 2px; }

@media (max-width: 1080px) {
  .chapter-shell { grid-template-columns: 210px minmax(0, 1fr); gap: 1.6rem; }
  .topbar { grid-template-columns: 220px 1fr auto; }
  .compare-grid { grid-template-columns: repeat(2, 1fr); }
}

@media (max-width: 840px) {
  html { scroll-padding-top: 72px; }
  .topbar { height: 64px; grid-template-columns: 1fr auto; }
  .topbar-context, .ghost-button { display: none; }
  .reading-progress { top: 64px; }
  .chapter-shell { display: block; padding: 1rem; }
  .chapter-nav { position: sticky; z-index: 20; top: 67px; margin-bottom: 1rem; }
  .nav-card { position: static; overflow-x: auto; padding: .45rem; border-radius: 10px; }
  .nav-card > .eyebrow, .nav-mastery { display: none; }
  .nav-card nav { display: flex; width: max-content; gap: .35rem; }
  .nav-card nav a { padding: .5rem .65rem; border: 1px solid var(--line); white-space: nowrap; background: white; }
  .nav-card nav a::before { display: none; }
  .hero-inner { padding: 1.5rem 1rem 1.8rem; }
  .hero-grid { grid-template-columns: 1fr; margin-top: 1.8rem; }
  .hero-panel { max-width: 560px; }
  .syllabus-strip { grid-template-columns: auto 1fr; }
  .syllabus-strip a { grid-column: 2; }
  .chapter-section { padding: 1.4rem; }
}

@media (max-width: 620px) {
  .topbar { padding: 0 .8rem; }
  .brand { font-size: .98rem; }
  .brand-mark { width: 34px; height: 34px; }
  .print-button { width: 38px; overflow: hidden; padding: 0; color: transparent; }
  .print-button::after { content: "↧"; color: var(--navy); font-size: 1rem; }
  .chapter-hero h1 { font-size: clamp(3rem, 17vw, 5.4rem); }
  .hero-copy { font-size: .92rem; }
  .hero-actions { display: grid; grid-template-columns: 1fr; }
  .hero-panel { grid-template-columns: repeat(2, 1fr); }
  .hero-stat { min-height: 86px; }
  .section-header { grid-template-columns: 42px 1fr; gap: .75rem; }
  .section-number { width: 40px; height: 40px; }
  .section-header h2 { font-size: 1.75rem; }
  .route-grid, .definition-grid, .formula-grid, .formula-grid.three, .revision-grid, .source-grid { grid-template-columns: 1fr; }
  .visual-heading { grid-template-columns: 1fr; }
  .visual-stage { min-height: 300px; }
  .compare-grid, .trap-grid { grid-template-columns: 1fr; }
  .table-row { grid-template-columns: 1fr; }
  .table-row span + span { border-left: 0; border-top: 1px solid var(--line); }
  .formula-stack div { grid-template-columns: 1fr; gap: .2rem; }
  .difference-demo div { grid-template-columns: 1fr; gap: .2rem; }
  .bank-heading { display: grid; }
  .bank-heading > div { align-items: start; }
  .pyq-card > summary { grid-template-columns: auto 1fr; padding-right: 2.6rem; }
  .difficulty { display: none; }
  .test-switcher { grid-template-columns: 1fr; }
  .test-toolbar { align-items: flex-start; }
  .options { grid-template-columns: 1fr; }
  .score-dock { align-items: stretch; flex-direction: column; }
  .chapter-footer { align-items: stretch; flex-direction: column; }
}

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  *, *::before, *::after { transition-duration: .01ms !important; animation-duration: .01ms !important; }
}

@media print {
  @page { size: A4; margin: 15mm 13mm; }
  body { background: white; color: black; font-size: 10.5pt; }
  .topbar, .reading-progress, .chapter-nav, .hero-actions, .test-toolbar, .score-dock, .noscript-note { display: none !important; }
  .chapter-hero { background: white; color: black; }
  .chapter-hero::after { display: none; }
  .hero-inner { padding: 0 0 12mm; }
  .breadcrumb, .hero-panel, .syllabus-strip { color: #333; }
  .hero-panel { border-color: #bbb; }
  .hero-stat { border-color: #ddd !important; }
  .hero-stat strong, .chapter-hero h1 span { color: #000; }
  .chapter-shell { display: block; width: 100%; padding: 0; }
  .chapter-article { max-width: none; }
  .chapter-section { break-before: page; margin: 0; padding: 0; border: 0; box-shadow: none; }
  .chapter-section:first-child { break-before: auto; }
  .worked-example, .pyq-card, .formula-card, .route-card { break-inside: avoid; }
  details > * { display: block !important; }
  details > summary::after { display: none; }
  .section-number, .mastery-checklist, .chapter-footer { background: white !important; color: black !important; border: 1px solid #aaa; }
  .chapter-footer p, .mastery-checklist label { color: #333; }
  .quiz-question { break-inside: avoid; }
  .quiz-solution { display: block !important; }
  .concept-visual { break-inside: avoid; box-shadow: none; }
  .visual-controls { display: none !important; }
  a { text-decoration: none; }
}