/* ==========================================================================
   AQA Revision — shared styles
   Tokens first; every colour used by pages and diagrams comes from here so
   light/dark themes stay consistent.
   ========================================================================== */

:root {
  --bg: #f6f6f2;
  --surface: #ffffff;
  --surface-2: #f0f0ea;
  --ink: #1b1c20;
  --muted: #5c606b;
  --line: #e1e1d9;
  --accent: #2c56c9;
  --accent-soft: #e6ecfb;
  --on-accent: #ffffff;
  --good: #17784a;
  --good-soft: #e3f3ea;
  --bad: #b8322b;
  --bad-soft: #fbe7e5;
  --warn-soft: #fff4d6;

  /* subject accents (home tiles, subject pages, search results); --sub is the active one */
  --sub-science: #0f7a63; --sub-science-soft: #e0f3ee;
  --sub-maths: #2c56c9; --sub-maths-soft: #e6ecfb;
  --sub-english: #a23b72; --sub-english-soft: #f8e6ef;
  --sub-chem-al: #b4530f; --sub-chem-al-soft: #fbeadd;
  --sub-maths-al: #6a3fb5; --sub-maths-al-soft: #efe8fb;

  /* tags */
  --tag-ht: #7a3fc4;
  --tag-ht-soft: #f0e8fb;
  --tag-triple: #a35a00;
  --tag-triple-soft: #fdf0dd;

  /* diagram palette */
  --d-ink: #1b1c20;
  --d-muted: #6b7080;
  --d-line: #b9bcc6;
  --d-proton: #d9483f;
  --d-neutron: #8e97a6;
  --d-electron: #2c56c9;
  --d-alpha: #d98a1a;
  --d-water: #cfe3f7;
  --d-glass: #9aa3b5;
  --pt-alkali: #f6c9a3;
  --pt-halogen: #c6e5b3;
  --pt-noble: #c7d6f5;
  --pt-transition: #e8d3f0;
  --pt-metal: #e6e6e1;
  --pt-nonmetal: #fbe7a3;

  --radius: 12px;
  --shadow: 0 1px 2px rgba(0,0,0,.05), 0 4px 16px rgba(0,0,0,.04);
  --font: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  color-scheme: light;
}

@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #131417; --surface: #1c1d22; --surface-2: #24262c; --ink: #ececf0; --muted: #a2a6b2;
    --line: #2f3139; --accent: #7d9cf5; --accent-soft: #232c47; --on-accent: #0f1424; --good: #4cc48a; --good-soft: #17301f;
    --bad: #f07a72; --bad-soft: #3a1d1b; --warn-soft: #3a3118;
    --tag-ht: #c4a2f4; --tag-ht-soft: #2e2340; --tag-triple: #f0b467; --tag-triple-soft: #3a2a14;
    --d-ink: #ececf0; --d-muted: #a2a6b2; --d-line: #4a4d58; --d-proton: #f07a72; --d-neutron: #9aa3b3;
    --d-electron: #7d9cf5; --d-alpha: #f0b467; --d-water: #233a52; --d-glass: #7d8598;
    --pt-alkali: #5a3a22; --pt-halogen: #2d4a24; --pt-noble: #26385e; --pt-transition: #42294e;
    --pt-metal: #2c2e35; --pt-nonmetal: #4f4318;
    --shadow: 0 1px 2px rgba(0,0,0,.3);
    --sub-science: #4fd1b0; --sub-science-soft: #15302a; --sub-maths: #7d9cf5; --sub-maths-soft: #232c47;
    --sub-english: #ec8bbd; --sub-english-soft: #3a1f2d;
  --sub-chem-al: #f2a065; --sub-chem-al-soft: #3a2616;
  --sub-maths-al: #b49af0; --sub-maths-al-soft: #2c2340;
    --sub-chem-al: #f2a065; --sub-chem-al-soft: #3a2616;
  --sub-maths-al: #b49af0; --sub-maths-al-soft: #2c2340;
    --sub-maths-al: #b49af0; --sub-maths-al-soft: #2c2340;
    color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #131417; --surface: #1c1d22; --surface-2: #24262c; --ink: #ececf0; --muted: #a2a6b2;
  --line: #2f3139; --accent: #7d9cf5; --accent-soft: #232c47; --on-accent: #0f1424; --good: #4cc48a; --good-soft: #17301f;
  --bad: #f07a72; --bad-soft: #3a1d1b; --warn-soft: #3a3118;
  --tag-ht: #c4a2f4; --tag-ht-soft: #2e2340; --tag-triple: #f0b467; --tag-triple-soft: #3a2a14;
  --d-ink: #ececf0; --d-muted: #a2a6b2; --d-line: #4a4d58; --d-proton: #f07a72; --d-neutron: #9aa3b3;
  --d-electron: #7d9cf5; --d-alpha: #f0b467; --d-water: #233a52; --d-glass: #7d8598;
  --pt-alkali: #5a3a22; --pt-halogen: #2d4a24; --pt-noble: #26385e; --pt-transition: #42294e;
  --pt-metal: #2c2e35; --pt-nonmetal: #4f4318;
  --shadow: 0 1px 2px rgba(0,0,0,.3);
  --sub-science: #4fd1b0; --sub-science-soft: #15302a; --sub-maths: #7d9cf5; --sub-maths-soft: #232c47;
  --sub-english: #ec8bbd; --sub-english-soft: #3a1f2d;
  --sub-chem-al: #f2a065; --sub-chem-al-soft: #3a2616;
  --sub-maths-al: #b49af0; --sub-maths-al-soft: #2c2340;
  color-scheme: dark;
}

/* ---------- base ---------- */
*, *::before, *::after { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body {
  margin: 0; background: var(--bg); color: var(--ink);
  font: 16px/1.55 var(--font);
}
a { color: var(--accent); }
h1, h2, h3 { line-height: 1.2; margin: 0 0 .5em; }
h1 { font-size: clamp(1.6rem, 4vw, 2.2rem); letter-spacing: -.02em; }
h2 { font-size: 1.3rem; letter-spacing: -.01em; }
h3 { font-size: 1.05rem; }
p { margin: 0 0 .8em; }
button { font: inherit; color: inherit; }
sub, sup { font-size: .72em; }

.wrap { max-width: 920px; margin: 0 auto; padding: 0 16px; }

/* ---------- header ---------- */
.site-header {
  background: var(--surface); border-bottom: 1px solid var(--line);
}
.site-header .wrap { display: flex; align-items: center; gap: 12px; min-height: 56px; }
.brand { font-weight: 700; text-decoration: none; color: var(--ink); display: flex; align-items: center; gap: 8px; }
.brand-mark {
  width: 28px; height: 28px; border-radius: 8px; background: var(--accent); color: var(--on-accent);
  display: grid; place-items: center; font-size: .8rem; font-weight: 800;
}
.header-spacer { flex: 1; }
.icon-btn {
  border: 1px solid var(--line); background: var(--surface); border-radius: 8px;
  padding: 6px 10px; cursor: pointer; font-size: .9rem;
}
.icon-btn:hover { background: var(--surface-2); }

/* ---------- home ---------- */
:root { --sub: var(--accent); --sub-soft: var(--accent-soft); }
.visually-hidden {
  position: absolute; width: 1px; height: 1px; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap;
}
.hero { padding-top: 32px; padding-bottom: 16px; }
.hero p { color: var(--muted); max-width: 60ch; margin: 0; }
/* home: level + exam board picker */
.study-picker { display: flex; flex-wrap: wrap; gap: 10px 22px; margin-top: 18px; }
.study-picker .pick { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; }
.study-picker .label { font-size: .78rem; font-weight: 700; text-transform: uppercase; letter-spacing: .04em; color: var(--muted); }

/* sticky search / filter / settings bar */
.toolbar {
  position: sticky; top: 0; z-index: 20; padding: 10px 0;
  background: color-mix(in srgb, var(--bg) 90%, transparent); backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--line);
}
.toolbar-inner { display: flex; gap: 8px; align-items: center; }
.search-field {
  flex: 1; min-width: 0; display: flex; align-items: center; gap: 8px; height: 42px; padding: 0 12px;
  background: var(--surface); border: 1px solid var(--line); border-radius: 10px; color: var(--muted);
}
.search-field:focus-within { border-color: var(--accent); box-shadow: 0 0 0 3px var(--accent-soft); }
.search-field input {
  flex: 1; min-width: 0; border: 0; outline: 0; background: none; color: var(--ink); font: inherit; font-size: 1rem;
}
.kbd-hint {
  font: 600 .75rem var(--font); border: 1px solid var(--line); border-radius: 5px; padding: 1px 6px; color: var(--muted);
}
.search-field:focus-within .kbd-hint { display: none; }
.type-field select {
  height: 42px; border: 1px solid var(--line); border-radius: 10px; background: var(--surface); color: var(--ink);
  font: inherit; font-size: .92rem; padding: 0 30px 0 12px; cursor: pointer; max-width: 170px;
  appearance: none; -webkit-appearance: none;
  background-image: linear-gradient(45deg, transparent 50%, var(--muted) 50%), linear-gradient(135deg, var(--muted) 50%, transparent 50%);
  background-position: right 15px center, right 10px center; background-size: 5px 5px; background-repeat: no-repeat;
}
.icon-btn.cog { height: 42px; width: 42px; padding: 0; display: grid; place-items: center; border-radius: 10px; color: var(--ink); }
.site-header .icon-btn.cog { height: 36px; width: 36px; }
@media (max-width: 560px) {
  .kbd-hint { display: none; }
  .type-field select { max-width: 120px; }
}

/* subject tiles */
.tiles { display: grid; grid-template-columns: repeat(auto-fit, minmax(260px, 1fr)); gap: 14px; margin: 24px 0 40px; }
.tiles .subject-science, #sections .subject-science, #results .subject-science, body.subject-science { --sub: var(--sub-science); --sub-soft: var(--sub-science-soft); }
.tiles .subject-maths, #sections .subject-maths, #results .subject-maths, body.subject-maths { --sub: var(--sub-maths); --sub-soft: var(--sub-maths-soft); }
.tiles .subject-maths-1ma1, #sections .subject-maths-1ma1, #results .subject-maths-1ma1, body.subject-maths-1ma1 { --sub: var(--sub-maths); --sub-soft: var(--sub-maths-soft); }
.tiles .subject-english, #sections .subject-english, #results .subject-english, body.subject-english { --sub: var(--sub-english); --sub-soft: var(--sub-english-soft); }
.tiles .subject-chemistry-9701, #sections .subject-chemistry-9701, #results .subject-chemistry-9701, body.subject-chemistry-9701 { --sub: var(--sub-chem-al); --sub-soft: var(--sub-chem-al-soft); }
.tiles .subject-maths-7357, #sections .subject-maths-7357, #results .subject-maths-7357, body.subject-maths-7357 { --sub: var(--sub-maths-al); --sub-soft: var(--sub-maths-al-soft); }
.tiles .subject-science-ocr21c, #sections .subject-science-ocr21c, #results .subject-science-ocr21c, body.subject-science-ocr21c { --sub: var(--sub-science); --sub-soft: var(--sub-science-soft); }
.tiles .subject-science-ocrgw, #sections .subject-science-ocrgw, #results .subject-science-ocrgw, body.subject-science-ocrgw { --sub: var(--sub-science); --sub-soft: var(--sub-science-soft); }
.tiles .subject-science-edexcel, #sections .subject-science-edexcel, #results .subject-science-edexcel, body.subject-science-edexcel { --sub: var(--sub-science); --sub-soft: var(--sub-science-soft); }
.tile {
  display: flex; flex-direction: column; background: var(--surface); border: 1px solid var(--line);
  border-radius: 16px; box-shadow: var(--shadow); overflow: hidden; border-top: 4px solid var(--sub);
}
.tile-main {
  flex: 1; display: grid; grid-template-columns: auto 1fr; grid-template-rows: 1fr auto; gap: 6px 14px;
  padding: 18px; text-decoration: none; color: var(--ink);
}
.tile-main:hover { background: var(--sub-soft); }
.tile-icon {
  width: 48px; height: 48px; border-radius: 12px; display: grid; place-items: center;
  background: var(--sub-soft); color: var(--sub); flex: none;
}
.tile-main .tile-icon { grid-row: span 2; align-self: start; }
.tile-text h2 { margin: 2px 0 4px; font-size: 1.35rem; }
.tile-text p { margin: 0; color: var(--muted); font-size: .9rem; }
.tile-meta { grid-column: 2; font-size: .85rem; font-weight: 700; color: var(--sub); margin-top: 6px; }
.sub-tiles { display: grid; grid-template-columns: repeat(3, 1fr); border-top: 1px solid var(--line); }
.sub-tile {
  display: flex; flex-direction: column; align-items: center; gap: 4px; padding: 12px 6px; text-align: center;
  text-decoration: none; color: var(--ink); font-size: .88rem;
}
.sub-tile + .sub-tile { border-left: 1px solid var(--line); }
.sub-tile:hover { background: var(--sub-soft); }
.sub-tile .icon { color: var(--sub); }
.sub-tile small { display: block; color: var(--muted); font-size: .75rem; }

/* subject page */
.subject-title { display: flex; align-items: center; gap: 14px; margin-top: 4px; }
.subject-title h1 { margin: 0; }
.subject-title p { margin: 4px 0 0; color: var(--muted); }
.subject-title .tile-icon { width: 56px; height: 56px; border-radius: 14px; }
.strand-nav { display: flex; flex-wrap: wrap; gap: 8px; margin-top: 18px; }
.strand-nav a {
  display: inline-flex; align-items: center; gap: 6px; padding: 6px 12px; border-radius: 999px;
  border: 1px solid var(--line); background: var(--surface); color: var(--ink); text-decoration: none; font-size: .9rem; font-weight: 600;
}
.strand-nav a:hover { border-color: var(--sub); }
.strand-nav .icon { color: var(--sub); }
.strand-nav span { color: var(--muted); font-weight: 500; font-size: .8rem; }
#sections { padding: 24px 0 16px; }
.strand { margin: 0 0 32px; scroll-margin-top: 76px; }
.strand-head { display: flex; align-items: center; gap: 8px; margin-bottom: 12px; color: var(--sub); flex-wrap: wrap; }
.strand-head h2 { margin: 0; color: var(--ink); font-size: 1.25rem; }
#sections .topic-card:hover { border-color: var(--sub); }
.topic-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(250px, 1fr)); gap: 12px; }
.topic-card {
  display: block; text-decoration: none; color: var(--ink); background: var(--surface);
  border: 1px solid var(--line); border-radius: var(--radius); padding: 16px; box-shadow: var(--shadow);
  transition: border-color .15s, transform .15s;
}
.topic-card:hover { border-color: var(--accent); transform: translateY(-1px); }
.topic-card .card-top { display: flex; align-items: center; gap: 8px; }
/* Topic code label (C1, P3, B7): uses the subject accent where one is set */
.code-badge {
  display: inline-block; font-weight: 800; font-size: .8rem; letter-spacing: .02em; line-height: 1;
  padding: 5px 8px; border-radius: 6px; background: var(--sub, var(--accent)); color: var(--surface);
  vertical-align: middle;
}
h1 .code-badge { font-size: .55em; padding: 6px 10px; margin-right: 10px; position: relative; top: -.12em; }
.result-title .code-badge { font-size: .72rem; padding: 3px 6px; }
.topic-card h3 { margin: 4px 0 6px; }
.topic-card p { color: var(--muted); font-size: .9rem; margin: 0; }
.topic-card .stat { margin-top: 10px; font-size: .8rem; color: var(--good); font-weight: 600; }
.empty-card {
  border: 1px dashed var(--line); border-radius: var(--radius); padding: 16px; color: var(--muted); font-size: .9rem;
}

/* search results */
#results { padding: 20px 0 40px; }
.results-head { font-size: .95rem; margin-bottom: 16px; }
.result-group { margin-bottom: 24px; }
.result-group h2 { font-size: 1rem; margin-bottom: 8px; }
.result {
  display: block; text-decoration: none; color: var(--ink); background: var(--surface); border: 1px solid var(--line);
  border-radius: 10px; padding: 10px 14px; margin-bottom: 6px;
}
.result:hover { border-color: var(--accent); }
.result-meta { font-size: .75rem; color: var(--muted); font-weight: 600; }
.result-title { font-weight: 600; margin: 2px 0; }
.result-snippet { font-size: .87rem; color: var(--muted); }
mark { background: color-mix(in srgb, var(--accent) 25%, transparent); color: inherit; border-radius: 3px; padding: 0 1px; }
.link-btn { border: 0; background: none; padding: 0; color: var(--accent); text-decoration: underline; cursor: pointer; font: inherit; }

/* settings dialog */
dialog.settings {
  width: min(480px, calc(100vw - 32px)); border: 1px solid var(--line); border-radius: 16px; padding: 20px;
  background: var(--surface); color: var(--ink); box-shadow: 0 20px 60px rgba(0,0,0,.25);
}
dialog.settings::backdrop { background: rgba(10, 12, 20, .45); }
.settings-head { display: flex; justify-content: space-between; align-items: center; margin-bottom: 8px; }
.settings-head h2 { margin: 0; }
.setting { display: flex; justify-content: space-between; align-items: center; gap: 12px; padding: 14px 0; border-top: 1px solid var(--line); flex-wrap: wrap; }
.setting > div:first-child { flex: 1 1 200px; }
.setting p { margin: 2px 0 0; }
.btn-danger { color: var(--bad); border-color: color-mix(in srgb, var(--bad) 40%, var(--line)); }

/* deep-link highlight (topic page ?focus=) */
@keyframes flash { from { box-shadow: 0 0 0 3px var(--accent); } to { box-shadow: var(--shadow); } }
.flash { animation: flash 2.2s ease-out; }
.exam-q { scroll-margin-top: 60px; }

/* ---------- topic header ---------- */
.topic-head { padding-top: 24px; padding-bottom: 12px; }
.crumbs { font-size: .85rem; color: var(--muted); margin-bottom: 6px; }
.crumbs a { color: var(--muted); }
.topic-head .lede { color: var(--muted); margin: 0; }

.filters {
  display: flex; flex-wrap: wrap; gap: 8px 16px; align-items: center; margin-top: 14px;
  font-size: .9rem;
}
.seg { display: inline-flex; border: 1px solid var(--line); border-radius: 999px; overflow: hidden; background: var(--surface); }
.seg button {
  border: 0; background: transparent; padding: 5px 12px; cursor: pointer; color: var(--muted);
}
.seg button[aria-pressed="true"] { background: var(--accent); color: var(--on-accent); }
.filters .label { color: var(--muted); margin-right: 6px; }
.filter-group { display: inline-flex; align-items: center; }

/* ---------- tabs ---------- */
.tabs {
  position: sticky; top: 0; z-index: 10; background: color-mix(in srgb, var(--bg) 92%, transparent);
  backdrop-filter: blur(8px); border-bottom: 1px solid var(--line);
}
.tabs .wrap { display: flex; gap: 4px; overflow-x: auto; scrollbar-width: none; }
.tabs .wrap::-webkit-scrollbar { display: none; }
.tab {
  border: 0; background: none; padding: 12px 12px 10px; cursor: pointer; color: var(--muted);
  border-bottom: 2px solid transparent; white-space: nowrap; font-weight: 600; font-size: .95rem;
}
.tab[aria-selected="true"] { color: var(--ink); border-bottom-color: var(--accent); }
.tab .count { font-weight: 500; font-size: .8rem; color: var(--muted); margin-left: 4px; }

.panel { padding: 24px 0 64px; }
.panel[hidden] { display: none; }

/* ---------- shared bits ---------- */
.card {
  background: var(--surface); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 20px; box-shadow: var(--shadow); margin-bottom: 16px;
}
.tag {
  display: inline-block; font-size: .7rem; font-weight: 700; letter-spacing: .03em; padding: 2px 7px;
  border-radius: 999px; vertical-align: middle; margin-left: 6px; text-transform: uppercase;
}
.tag-HT { color: var(--tag-ht); background: var(--tag-ht-soft); }
.tag-Triple { color: var(--tag-triple); background: var(--tag-triple-soft); }
.calc-badge { display: inline-flex; vertical-align: middle; margin-left: 8px; color: var(--muted); }
.calc-badge.no-calc { color: var(--ink); }
.calc-cross { stroke: var(--bad); stroke-width: 2.4; stroke-linecap: round; }
.btn {
  border: 1px solid var(--line); background: var(--surface); padding: 8px 14px; border-radius: 8px;
  cursor: pointer; font-weight: 600; font-size: .92rem;
}
.btn:hover { background: var(--surface-2); }
.btn-primary { background: var(--accent); border-color: var(--accent); color: var(--on-accent); }
.btn-primary:hover { background: var(--accent); filter: brightness(1.08); }
.btn:disabled { opacity: .5; cursor: not-allowed; }
.btn-row { display: flex; flex-wrap: wrap; gap: 8px; align-items: center; }
.muted { color: var(--muted); }
.small { font-size: .85rem; }
.hidden-note {
  font-size: .85rem; color: var(--muted); background: var(--surface-2); border-radius: 8px;
  padding: 8px 12px; margin-bottom: 16px;
}

/* ---------- learn ---------- */
.toc { display: flex; flex-wrap: wrap; gap: 6px; margin-bottom: 20px; }
.toc a {
  font-size: .85rem; text-decoration: none; padding: 4px 10px; border-radius: 999px;
  background: var(--surface); border: 1px solid var(--line); color: var(--ink);
}
.toc a:hover { border-color: var(--accent); }
.learn-section { scroll-margin-top: 60px; }
.learn-section .sec-head { display: flex; align-items: flex-start; gap: 8px; justify-content: space-between; flex-wrap: wrap; }
.learn-section .spec { font-size: .78rem; color: var(--muted); font-weight: 600; }
.learn-section ul.points { padding-left: 1.2em; margin: 8px 0 12px; }
.learn-section ul.points li { margin: 4px 0; }
.learn-section ul.points li.tagged { list-style: none; margin-left: -1.2em; }
.learn-section ul.points li.tagged::before { content: "•"; color: var(--muted); margin-right: .55em; }
.figure { margin: 16px 0; text-align: center; }
.figure svg { max-width: 100%; height: auto; }
.figure figcaption { font-size: .82rem; color: var(--muted); margin-top: 6px; }
.callout {
  border-left: 3px solid var(--accent); background: var(--accent-soft); padding: 10px 14px;
  border-radius: 0 8px 8px 0; margin: 12px 0; font-size: .93rem;
}
.callout strong:first-child { display: block; font-size: .78rem; text-transform: uppercase; letter-spacing: .04em; color: var(--accent); margin-bottom: 2px; }
/* worked examples (maths) */
.worked { border: 1px solid var(--line); border-radius: 10px; margin: 12px 0; overflow: hidden; font-size: .95rem; }
.worked-q { padding: 10px 14px; background: var(--sub-soft); }
.worked-label { display: block; font-size: .72rem; font-weight: 700; text-transform: uppercase; letter-spacing: .04em; color: var(--sub); margin-bottom: 2px; }
.worked-steps { margin: 0; padding: 10px 14px 10px 36px; display: grid; gap: 6px; }
.worked-steps li { display: flex; flex-wrap: wrap; justify-content: space-between; gap: 2px 16px; }
.worked-steps .why { color: var(--muted); font-size: .88rem; }
.worked-answer { padding: 8px 14px; border-top: 1px solid var(--line); font-weight: 700; }
.worked-answer::before { content: "Answer: "; font-weight: 600; color: var(--muted); }
/* nuclide notation: <span class="nuclide"><span>35</span><span>17</span></span>Cl */
.nuclide { display: inline-flex; flex-direction: column; align-items: flex-end; vertical-align: middle; font-size: .68em; line-height: 1.05; margin-right: 1px; }
/* stacked fraction: <span class="frac"><span>3</span><span>4</span></span> */
.frac { display: inline-flex; flex-direction: column; align-items: center; vertical-align: middle; font-size: .9em; line-height: 1.1; margin: 0 2px; }
.frac > span:first-child { border-bottom: 1px solid currentColor; padding: 0 3px; }
.frac > span:last-child { padding: 0 3px; }
.key-terms { display: grid; grid-template-columns: max-content 1fr; gap: 4px 14px; margin: 10px 0 0; font-size: .92rem; }
.key-terms dt { font-weight: 700; }
.key-terms dd { margin: 0; color: var(--muted); }
@media (max-width: 560px) {
  .key-terms { grid-template-columns: 1fr; }
  .key-terms dd { margin-bottom: 6px; }
}
.watch-link { font-size: .85rem; text-decoration: none; white-space: nowrap; }
.data-table { border-collapse: collapse; font-size: .9rem; margin: 8px 0 12px; width: 100%; }
.data-table th, .data-table td { border: 1px solid var(--line); padding: 6px 10px; text-align: left; }
.data-table th { background: var(--surface-2); }
.table-scroll { overflow-x: auto; }

/* ---------- diagram primitives (inline SVG) ---------- */
.dg-text { fill: var(--d-ink); font: 13px var(--font); }
.dg-text-sm { fill: var(--d-muted); font: 11px var(--font); }
.dg-text-b { fill: var(--d-ink); font: 700 13px var(--font); }
.dg-line { stroke: var(--d-line); fill: none; stroke-width: 1.5; }
.dg-ink { stroke: var(--d-ink); fill: none; stroke-width: 1.5; }
.dg-shell { stroke: var(--d-line); fill: none; stroke-width: 1.5; }
.dg-proton { fill: var(--d-proton); }
.dg-neutron { fill: var(--d-neutron); }
.dg-electron { fill: var(--d-electron); }
.dg-alpha { stroke: var(--d-alpha); fill: none; stroke-width: 2; }
.dg-alpha-fill { fill: var(--d-alpha); }
.dg-surface { fill: var(--surface); stroke: var(--d-line); stroke-width: 1.5; }
.dg-surface-2 { fill: var(--surface-2); stroke: var(--d-line); stroke-width: 1.5; }
.dg-glass { stroke: var(--d-glass); fill: none; stroke-width: 2; }
.dg-water { fill: var(--d-water); }
.dg-grid { stroke: var(--d-line); stroke-width: .6; opacity: .6; }
.dg-accent { stroke: var(--sub); fill: none; stroke-width: 2.2; }
.dg-accent-fill { fill: var(--sub); }
.dg-accent-soft { fill: var(--sub-soft); stroke: var(--sub); stroke-width: 1.5; }
.dg-text-accent { fill: var(--sub); font: 700 13px var(--font); }
.pt-alkali { fill: var(--pt-alkali); }
.pt-halogen { fill: var(--pt-halogen); }
.pt-noble { fill: var(--pt-noble); }
.pt-transition { fill: var(--pt-transition); }
.pt-metal { fill: var(--pt-metal); }
.pt-nonmetal { fill: var(--pt-nonmetal); }

/* ---------- quiz ---------- */
.quiz-progress { height: 6px; background: var(--surface-2); border-radius: 999px; overflow: hidden; margin-bottom: 16px; }
.quiz-progress > div { height: 100%; background: var(--accent); transition: width .25s; }
.quiz-meta { display: flex; justify-content: space-between; font-size: .85rem; color: var(--muted); margin-bottom: 8px; }
.quiz-q { font-size: 1.1rem; font-weight: 600; margin-bottom: 14px; }
.options { display: grid; gap: 8px; }
.option {
  text-align: left; border: 1.5px solid var(--line); background: var(--surface); border-radius: 10px;
  padding: 12px 14px; cursor: pointer; display: flex; gap: 10px; align-items: flex-start;
}
.option:hover:not(:disabled) { border-color: var(--accent); }
.option .letter {
  flex: none; width: 24px; height: 24px; border-radius: 6px; background: var(--surface-2);
  display: grid; place-items: center; font-size: .8rem; font-weight: 700;
}
.option.correct { border-color: var(--good); background: var(--good-soft); }
.option.wrong { border-color: var(--bad); background: var(--bad-soft); }
.option:disabled { cursor: default; }
.feedback { margin-top: 14px; padding: 12px 14px; border-radius: 10px; font-size: .95rem; }
.feedback.good { background: var(--good-soft); }
.feedback.bad { background: var(--bad-soft); }
.feedback b { display: block; margin-bottom: 2px; }
.score-big { font-size: 3rem; font-weight: 800; letter-spacing: -.03em; margin: 0; }
.review-list { margin: 16px 0 0; padding: 0; list-style: none; }
.review-list li { padding: 10px 0; border-top: 1px solid var(--line); font-size: .92rem; }

/* ---------- exam ---------- */
.exam-q { margin-bottom: 20px; }
.exam-q .q-head { display: flex; justify-content: space-between; align-items: baseline; gap: 10px; flex-wrap: wrap; }
.exam-q .q-num { font-weight: 800; font-size: 1.1rem; }
.exam-q .source { font-size: .78rem; color: var(--muted); }
.exam-q .context { margin: 10px 0; }
.part { border-top: 1px solid var(--line); padding: 14px 0 4px; }
.part-q { display: flex; gap: 10px; justify-content: space-between; }
.part-q .marks { flex: none; font-weight: 700; font-size: .85rem; color: var(--muted); }
.part textarea {
  width: 100%; min-height: 70px; margin: 8px 0; padding: 10px; border-radius: 8px;
  border: 1px solid var(--line); background: var(--bg); color: var(--ink); font: inherit; font-size: .93rem; resize: vertical;
}
.ms { background: var(--warn-soft); border-radius: 8px; padding: 10px 14px; margin: 8px 0; font-size: .92rem; }
.ms ul { margin: 4px 0 8px; padding-left: 1.2em; }
.ms .ms-title { font-weight: 700; font-size: .78rem; text-transform: uppercase; letter-spacing: .04em; }
.self-mark { display: flex; flex-wrap: wrap; align-items: center; gap: 6px; font-size: .85rem; }
.self-mark button {
  min-width: 34px; border: 1px solid var(--line); background: var(--surface); border-radius: 6px; padding: 4px 8px; cursor: pointer;
}
.self-mark button[aria-pressed="true"] { background: var(--accent); color: var(--on-accent); border-color: var(--accent); }
.self-mark select { font: inherit; padding: 4px 8px; border: 1px solid var(--line); border-radius: 8px; background: var(--surface); color: var(--ink); }
.exam-total {
  position: sticky; bottom: 12px; display: flex; justify-content: space-between; align-items: center; gap: 12px;
  background: var(--surface); border: 1px solid var(--line); border-radius: 999px; padding: 8px 8px 8px 18px;
  box-shadow: var(--shadow); font-weight: 600; margin-top: 8px; font-size: .9rem;
}
.exam-total .btn { padding: 6px 12px; border-radius: 999px; font-size: .85rem; white-space: nowrap; }
.pp-list { display: grid; gap: 6px; margin: 8px 0 0; padding: 0; list-style: none; font-size: .92rem; }
.pp-list li { display: flex; justify-content: space-between; gap: 10px; flex-wrap: wrap; padding: 6px 0; border-top: 1px solid var(--line); }
.pp-list .links { display: flex; gap: 10px; }

/* ---------- flashcards ---------- */
.fc-stage { perspective: 1200px; margin: 0 auto 16px; max-width: 560px; }
.fc {
  position: relative; width: 100%; aspect-ratio: 3 / 2; cursor: pointer; border: 0; padding: 0; background: none;
  transform-style: preserve-3d; transition: transform .45s;
}
.fc.flipped { transform: rotateY(180deg); }
.fc-face {
  position: absolute; inset: 0; backface-visibility: hidden; -webkit-backface-visibility: hidden;
  background: var(--surface); border: 1px solid var(--line); border-radius: 16px; box-shadow: var(--shadow);
  display: flex; flex-direction: column; align-items: center; justify-content: center; padding: 24px; text-align: center;
}
.fc-face .side { position: absolute; top: 12px; left: 16px; font-size: .72rem; font-weight: 700; text-transform: uppercase; letter-spacing: .05em; color: var(--muted); }
.fc-face .fc-text { font-size: clamp(1rem, 3.2vw, 1.3rem); font-weight: 600; }
.fc-back { transform: rotateY(180deg); background: var(--accent-soft); }
.fc-back .fc-text { font-weight: 500; }
.fc-controls { display: flex; justify-content: center; gap: 8px; flex-wrap: wrap; }
.fc-status { text-align: center; color: var(--muted); font-size: .88rem; margin: 10px 0 14px; }
.fc-tally { display: flex; justify-content: center; gap: 16px; font-size: .88rem; margin-bottom: 12px; }

/* ---------- videos ---------- */
.video-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(240px, 1fr)); gap: 14px; }
.video-card {
  display: block; text-decoration: none; color: var(--ink); background: var(--surface);
  border: 1px solid var(--line); border-radius: var(--radius); overflow: hidden; box-shadow: var(--shadow);
}
.video-card:hover { border-color: var(--accent); }
.video-thumb { aspect-ratio: 16 / 9; background: var(--surface-2); display: grid; place-items: center; position: relative; }
.video-thumb img { width: 100%; height: 100%; object-fit: cover; display: block; }
.video-thumb .play {
  position: absolute; width: 44px; height: 44px; border-radius: 50%; background: rgba(0,0,0,.6); color: #fff;
  display: grid; place-items: center; font-size: 1rem;
}
.video-body { padding: 10px 14px 14px; }
.video-body h3 { font-size: .95rem; margin: 0 0 4px; }
.video-body p { margin: 0; font-size: .8rem; color: var(--muted); }

.site-footer { border-top: 1px solid var(--line); padding: 20px 0 32px; font-size: .82rem; color: var(--muted); }

/* ---------- "go to the notes" links and ticked mark schemes ---------- */
.learn-link { flex: none; display: inline-flex; align-items: center; justify-content: center; width: 30px; height: 30px;
  border-radius: 8px; color: var(--accent); text-decoration: none; }
.learn-link:hover, .learn-link:focus-visible { background: var(--accent-soft); }
.part-side { flex: none; display: inline-flex; align-items: center; gap: 4px; align-self: flex-start; margin-top: -4px; }
.quiz-q, .review-q { display: flex; gap: 10px; justify-content: space-between; align-items: flex-start; }
.ms ul.ms-ticks { list-style: none; padding-left: 0; }
.ms-ticks li { margin: 4px 0; }
.ms-ticks label { display: flex; gap: 8px; align-items: flex-start; cursor: pointer; }
.ms-ticks input { margin-top: .3em; accent-color: var(--accent); width: 16px; height: 16px; flex: none; }
.ms-result { font-size: .9rem; }
.ms-result ul { margin: 4px 0 0; }