/* ==========================================================================
   Notes — site styles (shared by every subject)
   Palette: set through the variables in :root; each subject overrides them below.
   ========================================================================== */
:root {
  --navy: #002b80;
  --blue: #007bff;
  --blue-hover: #0044cc;
  --blue-logo: #0056b3;
  --bg: #f8f9fa;
  --ink: #333;
  --muted: #5b6573;
  --card: #fff;
  --line: #dfe5ee;
  --shadow: 0 8px 8px rgba(0, 0, 0, 0.2);
  --shadow-soft: 0 4px 14px rgba(0, 32, 96, 0.10);
  --ease: cubic-bezier(.2, .9, .2, 1);

  --offspec: #e8590c;
  --offspec-bg: #fff4e6;
  --tip: #007bff;
  --tip-bg: #e7f1ff;
  --warn: #e03131;
  --warn-bg: #fff0f0;
  --define: #2f9e44;
  --define-bg: #ebfbee;
  --key: #002b80;
  --key-bg: #edf2ff;
  --blue-soft: #9fc0ff;
  --blue-scroll: #2a56b0;
  --line-strong: #c7d2e3;
  --surface-2: #eef2f8;
  --ink-2: #3d4652;
  --tint-1: #eef3ff;
  --tint-2: #e3ebfb;
  --tint-3: #fbfcff;
  --tint-4: #f7f9fd;
}

body {
    font-family: bryant-web, sans-serif;
    margin: 0;
    padding: 0;
    background-color: var(--bg);
    color: var(--ink);
    -webkit-text-size-adjust: 100%;
    text-size-adjust: 100%;
}
.sidebar {
  width: 250px;
  background-color: var(--navy);
  color: white;
  display: flex;
  flex-direction: column;
  padding: 20px;
  box-sizing: border-box;
  position: fixed;
  top: 0;
  left: 0;
  height: 100vh;
  height: 100dvh; /* iOS Safari: the visible height, not the height behind the toolbars */
  overflow-y: auto;
  -webkit-overflow-scrolling: touch;
  overscroll-behavior: contain;
  box-shadow:10px 10px 10px rgba(0, 0, 0, 0.1);
  z-index: 50;
  scrollbar-width: thin;
  scrollbar-color: var(--blue-scroll) transparent;
}
.logo {
  display: block;
  color: white;
  text-decoration: none;
  font-size: 24px;
  font-weight: bold;
  margin-bottom: 20px;
  text-align: center;
  border-radius: 6px;
  padding: 4px 0;
  transition: transform 0.35s cubic-bezier(.2,.9,.2,1),
        background-color 0.5s ease;
    will-change: transform;
}
.bank-back {
  display: block;
  margin: -10px 0 18px;
  color: #cfe0ff;
  font-size: 14px;
  text-decoration: none;
}

.bank-back:hover { color: #fff; text-decoration: underline; }

.logo:hover {
  background-color: var(--blue-logo);
  transform: scale(1.05) translateY(3px);
}
.nav-links {
  list-style: none;
  padding: 0;
  margin: 0;
}
.nav-links li {
  margin-bottom: 6px;
}
.nav-links li a {
  color: white;
  text-decoration: none;
  display: block;
  padding: 8px 10px;
  border-radius: 6px;
  font-size: 15px;
  line-height: 1.3;
  transition: transform 0.35s cubic-bezier(.2,.9,.2,1),
        background-color 0.5s ease;
    will-change: transform;
}

.nav-links li a:hover {
  background-color: var(--blue-hover);
    transform: scale(1.05) translateX(3px) rotate(3deg);
}
.nav-links li a.active {
  background-color: var(--blue);
  font-weight: bold;
  box-shadow: 0 3px 6px rgba(0, 0, 0, 0.25);
}

/* Sidebar unit groups */
.nav-group {
  margin-top: 14px;
  border-top: 1px solid rgba(255, 255, 255, 0.15);
  padding-top: 10px;
}
.nav-group-head {
  all: unset;
  box-sizing: border-box;
  display: grid;
  grid-template-columns: 1fr auto;
  width: 100%;
  cursor: pointer;
  padding: 6px 10px;
  border-radius: 6px;
  transition: background-color 0.3s ease;
}
.nav-group-head:hover { background-color: rgba(255, 255, 255, 0.08); }
.nav-group-head:focus-visible { outline: 2px solid #8fb6ff; }
.nav-level {
  font-size: 11px;
  font-weight: bold;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--blue-soft);
}
.nav-group-name {
  grid-column: 1;
  font-size: 15px;
  font-weight: bold;
  line-height: 1.25;
  margin-top: 2px;
}
.nav-group-head .chev {
  grid-column: 2;
  grid-row: 1 / span 2;
  align-self: center;
  font-size: 18px;
  transition: transform 0.3s var(--ease);
}
.nav-group.open .chev { transform: rotate(90deg); }
.nav-group .nav-links { display: none; margin-top: 8px; padding-left: 6px; }
.nav-group.open .nav-links { display: block; }

/* Mobile menu */
.menu-toggle {
  display: none;
  position: fixed;
  top: 12px;
  left: 12px;
  z-index: 60;
  background: var(--navy);
  color: #fff;
  border: 0;
  border-radius: 10px;
  padding: 10px 14px;
  font: inherit;
  font-weight: bold;
  box-shadow: 0 4px 8px rgba(0, 0, 0, 0.25);
  cursor: pointer;
}
.nav-scrim {
  display: none;
  position: fixed;
  inset: 0;
  background: rgba(0, 20, 60, 0.4);
  z-index: 40;
}

.content {
  margin-left: 250px;
  padding: 40px;
  min-height: 100vh;
  min-height: 100dvh;
  background-color: var(--bg);
  box-sizing: border-box;
}
.page {
  max-width: 980px;
}
.IntroductionText {
    font-size: 20px;
    display: block;
    line-height: 1.6;
    margin-bottom: 20px;
}
.main-title {
    display: inline-block;
    background-color: var(--blue);
    color: white;
    padding: 15px 25px;
    border-radius: 12px;
    font-size: 2rem;
    font-weight: bold;
    box-shadow: 0 4px 8px rgba(0, 0, 0, 0.2);
    transition: transform 0.35s cubic-bezier(.2,.9,.2,1),
        background-color 0.5s ease;
    will-change: transform;
    text-align: center;
    margin: 12px 0 20px;
}
.main-title:hover {
  transform: scale(1.05) rotate(-3deg) translateY(-2px);

}
.mainpage-image {
    display: inline-block;
    transform: rotate(-5deg);
    border-radius: 5px;
}
.bookimg {
    box-shadow: 0px 8px 8px rgba(0, 0, 0, 0.2);
    max-width: 100%;
    height: auto;
}
.animations {
  width: 600px;
  max-width: 100%;
  height: auto;
  display: block;
  margin: 20px auto;
  border-radius: 10px;
  box-shadow: 0px 8px 8px rgba(0, 0, 0, 0.2);
}
.example {
  width: 300px;
  max-width: 100%;
  height: auto;
  display: block;
  margin-left: auto;
  margin-right: auto;
}
.question {
  width: 600px;
  max-width: 100%;
  height: auto;
  display: block;
  margin: 16px auto;
}

/* ==========================================================================
   Headings & text
   ========================================================================== */
.section-title {
  color: var(--navy-text, var(--navy));
  font-size: 1.65rem;
  margin: 56px 0 18px;
  padding-bottom: 8px;
  border-bottom: 3px solid var(--blue);
  scroll-margin-top: 20px;
  transition: transform 0.35s var(--ease);
}
.section-title:hover { transform: translateX(4px); }
.sub-title {
  color: var(--blue-hover);
  font-size: 1.25rem;
  margin: 32px 0 10px;
  scroll-margin-top: 20px;
}
.explanation,
.content p,
.content li {
  font-size: 18px;
  line-height: 1.7;
}
.content ul, .content ol { padding-left: 26px; }
.key-concepts li, .equations-list li { margin-bottom: 6px; }
.content a { color: var(--blue-hover); }
.content a:hover { color: var(--blue); }
.muted { color: var(--muted); }
.center { text-align: center; }
mjx-container[display="true"] { overflow-x: auto; overflow-y: hidden; padding: 2px 0; }

/* Topic meta chips (filled in by script.js) */
.topic-meta {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin-bottom: 4px;
}
.chip {
  display: inline-block;
  font-size: 13px;
  font-weight: bold;
  letter-spacing: 0.04em;
  padding: 5px 12px;
  border-radius: 999px;
  background: var(--tint-2);
  color: var(--navy-text, var(--navy));
}
.chip.level-as { background: var(--blue); color: #fff; }
.chip.level-a2 { background: var(--navy); color: #fff; }

/* Download link for the page's ready-made PDF (added by script.js). */
.content a.pdf-download {
  box-sizing: border-box;
  display: inline-flex;
  align-items: center;
  gap: 6px;
  margin-left: auto;
  font-size: 13px;
  font-weight: bold;
  letter-spacing: 0.04em;
  padding: 4px 14px;
  border: 2px solid var(--blue);
  border-radius: 999px;
  background: var(--card);
  color: var(--blue-hover);
  text-decoration: none;
  transition: background-color 0.3s ease, color 0.3s ease;
}
.content a.pdf-download:hover { background: var(--blue); color: #fff; }
.content a.pdf-download:focus-visible { outline: 3px solid var(--blue-soft); outline-offset: 2px; }

/* On-this-page list */
.toc {
  background: var(--card);
  border-radius: 12px;
  box-shadow: var(--shadow-soft);
  padding: 14px 18px;
  margin: 10px 0 28px;
}
.toc:empty { display: none; }
.toc-label {
  display: block;
  font-size: 12px;
  font-weight: bold;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--muted);
  margin-bottom: 8px;
}
.toc ol {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
}
.toc li { font-size: 15px; line-height: 1.2; }
.toc a {
  display: inline-block;
  text-decoration: none;
  color: var(--navy-text, var(--navy));
  background: var(--tint-1);
  padding: 6px 12px;
  border-radius: 999px;
  transition: transform 0.35s var(--ease), background-color 0.3s ease;
}
.toc a:hover { background: var(--blue); color: #fff; transform: scale(1.05) rotate(-2deg); }

/* ==========================================================================
   Cards, equations, callouts
   ========================================================================== */
.card {
  background: var(--card);
  border-radius: 12px;
  box-shadow: var(--shadow-soft);
  padding: 18px 22px;
  margin: 22px 0;
}
.cols {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(270px, 1fr));
  gap: 18px;
  margin: 22px 0;
}
.cols > .card { margin: 0; }

.eq-card {
  position: relative;
  background: var(--card);
  border-left: 6px solid var(--blue);
  border-radius: 12px;
  box-shadow: var(--shadow-soft);
  padding: 14px 22px 16px;
  margin: 22px 0;
}
.eq-card .eq { font-size: 1.15em; }
.eq-card .eq-name {
  font-weight: bold;
  color: var(--navy-text, var(--navy));
  margin: 0 0 4px;
  font-size: 17px;
}
.eq-card .tag { position: absolute; top: 12px; right: 14px; }
.eq-legend {
  display: flex;
  flex-wrap: wrap;
  gap: 6px 18px;
  font-size: 15px;
  color: var(--muted);
  border-top: 1px dashed var(--line);
  padding-top: 10px;
  margin-top: 4px;
}

.callout {
  position: relative;
  margin: 26px 0;
  padding: 44px 22px 16px;
  border-radius: 12px;
  background: var(--key-bg);
  border-left: 6px solid var(--key);
  box-shadow: var(--shadow-soft);
}
.callout::before {
  content: "Key idea";
  position: absolute;
  top: 12px;
  left: 18px;
  font-size: 12px;
  font-weight: bold;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: #fff;
  background: var(--key);
  padding: 4px 11px;
  border-radius: 999px;
}
.callout > :first-child { margin-top: 0; }
.callout > :last-child { margin-bottom: 0; }
.callout h4 { margin: 0 0 6px; font-size: 18px; color: var(--navy-text, var(--navy)); }
.callout.tip { background: var(--tip-bg); border-left-color: var(--tip); }
.callout.tip::before { content: "Exam tip"; background: var(--tip); }
.callout.warn { background: var(--warn-bg); border-left-color: var(--warn); }
.callout.warn::before { content: "Common mistake"; background: var(--warn); }
.callout.define { background: var(--define-bg); border-left-color: var(--define); }
.callout.define::before { content: "Definition"; background: var(--define); }
.callout.core { background: #f3f0ff; border-left-color: #6741d9; }
.callout.core::before { content: "Core practical"; background: #6741d9; }
.callout[data-badge]::before { content: attr(data-badge); }
.callout.offspec,
.callout.offspec[data-badge] {
  background: var(--offspec-bg);
  border-left-color: var(--offspec);
}
.callout.offspec::before,
.callout.offspec[data-badge]::before {
  content: "\26A0  Beyond the spec \00B7  not part of the Edexcel IAL specification";
  background: var(--offspec);
  text-transform: none;
  letter-spacing: 0.02em;
}
.callout.offspec h4 { color: #a33e06; }

.tag {
  display: inline-block;
  font-size: 11.5px;
  font-weight: bold;
  line-height: 1.4;
  padding: 3px 9px;
  border-radius: 999px;
  vertical-align: middle;
  letter-spacing: 0.05em;
  text-transform: uppercase;
  background: var(--tint-2);
  color: var(--navy-text, var(--navy));
  white-space: nowrap;
}
.tag.offspec { background: var(--offspec); color: #fff; }
.tag.sheet { background: var(--tip-bg); color: var(--blue-hover); }
.tag.req { background: var(--navy); color: #fff; }
.tag.use { background: var(--surface-2); color: var(--ink-2); }
.tag.core { background: #6741d9; color: #fff; }

/* Diagrams (static SVG / images) */
.diagram {
  background: var(--card);
  border-radius: 12px;
  box-shadow: var(--shadow);
  padding: 16px;
  margin: 26px auto;
  text-align: center;
}
.diagram svg, .diagram img { max-width: 100%; height: auto; }
/* Plain figures and inline diagrams written straight into a page (biology and
   chemistry notes) fit the screen too; interactive .sim figures style themselves. */
.page img, .page svg[viewBox] { max-width: 100%; height: auto; }
.page figure:not(.sim):not(.diagram) { margin: 22px auto; text-align: center; }
.page figure:not(.sim) > figcaption {
  color: var(--muted);
  font-size: 15px;
  line-height: 1.5;
  margin-top: 8px;
}
.diagram figcaption {
  color: var(--muted);
  font-size: 15px;
  line-height: 1.5;
  margin-top: 8px;
}

/* Tables */
table.data {
  border-collapse: collapse;
  width: 100%;
  background: var(--card);
  border-radius: 12px;
  overflow: hidden;
  box-shadow: var(--shadow-soft);
  margin: 22px 0;
  font-size: 16px;
}
table.data th {
  background: var(--navy);
  color: #fff;
  text-align: left;
  padding: 10px 14px;
}
table.data td {
  padding: 9px 14px;
  border-top: 1px solid var(--line);
  vertical-align: top;
}
table.data tr:nth-child(even) td { background: var(--tint-4); }
.table-wrap { overflow-x: auto; }

/* ==========================================================================
   Buttons
   ========================================================================== */
.btn {
  background: var(--blue);
  color: #fff;
  border: 2px solid var(--blue);
  border-radius: 8px;
  padding: 7px 15px;
  font: inherit;
  font-size: 15px;
  font-weight: bold;
  cursor: pointer;
  box-shadow: 0 3px 6px rgba(0, 0, 0, 0.15);
  transition: transform 0.35s var(--ease), background-color 0.3s ease, color 0.3s ease;
}
.btn:hover { background: var(--blue-hover); border-color: var(--blue-hover); transform: scale(1.05) rotate(-2deg); }
.btn:focus-visible { outline: 3px solid var(--blue-soft); outline-offset: 2px; }
.btn.ghost { background: var(--card); color: var(--blue); }
.btn.ghost:hover { background: var(--blue); color: #fff; }
.btn:disabled { opacity: 0.5; cursor: default; transform: none; }

/* ==========================================================================
   Interactive diagrams (built by js/sim.js)
   ========================================================================== */
.sim {
  background: var(--card);
  border-radius: 12px;
  box-shadow: var(--shadow);
  margin: 30px 0;
  overflow: hidden;
}
.sim-head {
  padding: 12px 18px;
  background: var(--navy);
  color: #fff;
}
.sim-title { display: block; font-weight: bold; font-size: 17px; line-height: 1.35; }
/* The "Interactive" label in the key on the physics notes home page. */
.sim-badge {
  display: inline-block;
  background: var(--navy);
  color: #fff;
  padding: 3px 10px;
  border-radius: 999px;
  font-size: 11.5px;
  font-weight: bold;
  letter-spacing: 0.08em;
  text-transform: uppercase;
}
/* One plain sentence under the title: what the diagram shows. */
.sim-about {
  margin: 4px 0 0;
  font-size: 15px;
  line-height: 1.45;
  color: rgba(255, 255, 255, 0.86);
}
.sim-stage {
  position: relative;
  background: var(--card);
  border-bottom: 1px solid var(--line);
}
.sim-stage canvas { display: block; width: 100%; }
.sim-controls {
  display: flex;
  flex-wrap: wrap;
  align-items: flex-end;
  gap: 14px 26px;
  padding: 14px 18px 6px;
}
.sim-controls:empty { display: none; }
.ctrl {
  display: flex;
  flex-direction: column;
  gap: 4px;
  flex: 1 1 190px;
  min-width: 170px;
}
.ctrl-label {
  display: flex;
  justify-content: space-between;
  gap: 8px;
  font-size: 14.5px;
  color: var(--muted);
}
.ctrl-label output {
  font-weight: bold;
  color: var(--navy-text, var(--navy));
  font-variant-numeric: tabular-nums;
}
.ctrl input[type="range"] { width: 100%; accent-color: var(--blue); margin: 4px 0; }
.ctrl select {
  font: inherit;
  font-size: 16px; /* 16px or more stops iOS Safari zooming in when the menu is tapped */
  padding: 6px 8px;
  border-radius: 8px;
  border: 1px solid var(--line-strong);
  background: var(--card);
  color: var(--ink);
}
.ctrl.check {
  flex: 0 1 auto;
  min-width: 0;
  flex-direction: row;
  align-items: center;
  gap: 8px;
  font-size: 15px;
  color: var(--ink);
  padding-bottom: 6px;
}
.ctrl.check input { width: 18px; height: 18px; accent-color: var(--blue); margin: 0; }
.sim-buttons {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
  padding-bottom: 6px;
}
/* Settings that are not needed at first, folded away under "More settings". */
.sim-more { flex-basis: 100%; padding-bottom: 6px; }
.sim-more > summary {
  display: inline-block;
  cursor: pointer;
  font-size: 14.5px;
  font-weight: bold;
  color: var(--blue);
}
.sim-more > summary::before { content: "+ "; }
.sim-more[open] > summary::before { content: "− "; }
.sim-more-body {
  display: flex;
  flex-wrap: wrap;
  align-items: flex-end;
  gap: 14px 26px;
  padding-top: 12px;
}
.sim-readout {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  padding: 8px 18px 14px;
}
.sim-readout:empty { display: none; }
.ro {
  background: var(--tint-1);
  border-radius: 8px;
  padding: 5px 11px;
  font-size: 15px;
  font-variant-numeric: tabular-nums;
}
.ro b { color: var(--navy-text, var(--navy)); }
.ro.hl { background: var(--blue); color: #fff; }
.ro.hl b { color: #fff; }
.sim figcaption {
  padding: 2px 18px 16px;
  color: var(--muted);
  font-size: 15.5px;
  line-height: 1.55;
}
.sim figcaption::before {
  content: "Try it: ";
  font-weight: bold;
  color: var(--blue);
}

/* ==========================================================================
   Step-by-step derivations (built by script.js)
   ========================================================================== */
.derive {
  background: var(--card);
  border-radius: 12px;
  box-shadow: var(--shadow-soft);
  border-top: 5px solid var(--navy);
  margin: 28px 0;
  padding: 16px 22px 18px;
}
.derive-head {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 8px 12px;
  margin-bottom: 8px;
}
.derive-title { font-weight: bold; font-size: 18px; color: var(--navy-text, var(--navy)); }
.derive-head .tag { white-space: normal; max-width: 100%; }
.derive-steps { counter-reset: step; }
.derive .step {
  position: relative;
  padding: 6px 0 6px 44px;
  counter-increment: step;
}
.derive .step::before {
  content: counter(step);
  position: absolute;
  left: 0;
  top: 10px;
  width: 28px;
  height: 28px;
  border-radius: 50%;
  background: var(--blue);
  color: #fff;
  font-weight: bold;
  font-size: 14px;
  display: grid;
  place-items: center;
}
.derive .step + .step { border-top: 1px dashed var(--line); }
.derive .step > :first-child { margin-top: 4px; }
.derive .step > :last-child { margin-bottom: 4px; }
.derive .step .why { color: var(--muted); font-style: italic; font-size: 16px; }
.derive.js .step.hidden { display: none; }
.derive.js .step.revealed { animation: stepIn 0.45s var(--ease); }
@keyframes stepIn {
  from { opacity: 0; transform: translateY(8px); }
  to { opacity: 1; transform: none; }
}
.derive-controls {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 10px;
  margin-top: 12px;
}
.derive-count { color: var(--muted); font-size: 14px; margin-left: auto; }

/* ==========================================================================
   Quick-check questions and worked examples
   ========================================================================== */
.mcq {
  background: var(--card);
  border-radius: 12px;
  box-shadow: var(--shadow-soft);
  border-left: 6px solid var(--blue);
  padding: 16px 22px;
  margin: 26px 0;
}
.mcq-label {
  display: inline-block;
  font-size: 12px;
  font-weight: bold;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: #fff;
  background: var(--blue);
  padding: 4px 11px;
  border-radius: 999px;
  margin-bottom: 6px;
}
.mcq-q { font-weight: bold; margin: 6px 0 12px; }
.mcq-options { display: grid; gap: 8px; }
.mcq-options button {
  all: unset;
  box-sizing: border-box;
  cursor: pointer;
  padding: 10px 14px;
  border-radius: 8px;
  border: 2px solid var(--line-strong);
  background: var(--tint-3);
  font-size: 17px;
  line-height: 1.45;
  transition: transform 0.3s var(--ease), border-color 0.2s, background-color 0.2s;
}
.mcq-options button:hover { border-color: var(--blue); transform: translateX(4px); }
.mcq-options button:focus-visible { outline: 3px solid var(--blue-soft); }
.mcq-options button.correct { border-color: #2f9e44; background: #ebfbee; }
.mcq-options button.correct::after { content: "  \2714  Correct"; color: #2f9e44; font-weight: bold; }
.mcq-options button.wrong { border-color: #e03131; background: #fff0f0; animation: shake 0.35s; }
.mcq-options button.wrong::after { content: "  \2718  Not quite"; color: #e03131; font-weight: bold; }
.mcq.done .mcq-options button { cursor: default; }
.mcq.done .mcq-options button:hover { transform: none; }
.mcq-why { display: none; margin-top: 12px; padding-top: 10px; border-top: 1px dashed var(--line); }
.mcq.done .mcq-why { display: block; }
@keyframes shake {
  0%, 100% { transform: translateX(0); }
  25% { transform: translateX(-6px); }
  75% { transform: translateX(6px); }
}

.question-card {
  background: var(--card);
  border-radius: 12px;
  box-shadow: var(--shadow-soft);
  padding: 16px 22px;
  margin: 26px 0;
  border-left: 6px solid var(--navy);
}
.question-card .q-label {
  display: inline-block;
  font-size: 12px;
  font-weight: bold;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: #fff;
  background: var(--navy);
  padding: 4px 11px;
  border-radius: 999px;
}
details.solution {
  margin-top: 10px;
  border-top: 1px dashed var(--line);
  padding-top: 8px;
}
details.solution > summary {
  cursor: pointer;
  font-weight: bold;
  color: var(--blue);
  list-style: none;
  display: inline-block;
  padding: 6px 14px;
  border-radius: 8px;
  border: 2px solid var(--blue);
  transition: transform 0.35s var(--ease), background-color 0.3s ease, color 0.3s;
}
details.solution > summary::-webkit-details-marker { display: none; }
details.solution > summary:hover { background: var(--blue); color: #fff; transform: scale(1.04) rotate(-2deg); }
details.solution[open] > summary { background: var(--blue); color: #fff; margin-bottom: 8px; }

/* ==========================================================================
   Home page
   ========================================================================== */
.home-hero {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 24px 40px;
}
.home-hero .hero-text { flex: 1 1 380px; }
.home-hero .mainpage-image { flex: 0 1 260px; margin: 10px auto; }
.legend {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(210px, 1fr));
  gap: 12px;
  margin: 18px 0 8px;
}
.legend .card { margin: 0; padding: 14px 16px; font-size: 16px; line-height: 1.5; }
.legend .card p { font-size: 16px; margin: 8px 0 0; }
.unit-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(290px, 1fr));
  gap: 22px;
  margin: 20px 0 40px;
}
.unit-card {
  background: var(--card);
  border-radius: 12px;
  box-shadow: var(--shadow);
  overflow: hidden;
  transition: transform 0.35s var(--ease);
}
.unit-card:hover { transform: translateY(-4px) rotate(-0.6deg); }
.unit-card header {
  background: var(--blue);
  color: #fff;
  padding: 14px 18px;
}
.unit-card.a2 header { background: var(--navy); }
.unit-card.prac header { background: #6741d9; }
.unit-card header .unit-code {
  font-size: 12px;
  font-weight: bold;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  opacity: 0.85;
}
.unit-card header h3 { margin: 4px 0 0; font-size: 20px; }
.unit-card ul { list-style: none; margin: 0; padding: 10px 10px 14px; }
.unit-card li { margin: 0; font-size: 16px; line-height: 1.4; }
.unit-card li a {
  display: block;
  text-decoration: none;
  color: var(--ink);
  padding: 8px 10px;
  border-radius: 8px;
  transition: background-color 0.3s ease, transform 0.35s var(--ease);
}
.unit-card li a:hover { background: var(--tint-1); transform: translateX(4px); }
.unit-card li a strong { color: var(--navy-text, var(--navy)); display: block; }
.unit-card li a span { color: var(--muted); font-size: 14.5px; line-height: 1.4; }

/* Prev / next */
.page-nav {
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
  gap: 14px;
  margin: 60px 0 10px;
  padding-top: 22px;
  border-top: 2px solid var(--line);
}
.page-nav a {
  text-decoration: none;
  background: var(--card);
  border-radius: 12px;
  box-shadow: var(--shadow-soft);
  padding: 12px 18px;
  color: var(--navy-text, var(--navy));
  flex: 1 1 240px;
  max-width: 420px;
  transition: transform 0.35s var(--ease), background-color 0.3s;
}
.page-nav a:hover { background: var(--blue); color: #fff; transform: scale(1.03) rotate(-1deg); }
.page-nav a small { display: block; font-size: 12px; letter-spacing: 0.08em; text-transform: uppercase; opacity: 0.75; }
.page-nav a strong { font-size: 17px; }
.page-nav a.next { text-align: right; margin-left: auto; }

.site-footer {
  color: var(--muted);
  font-size: 14px;
  margin-top: 30px;
}

/* ==========================================================================
   Small screens
   ========================================================================== */
@media (max-width: 900px) {
  .menu-toggle { display: block; }
  .sidebar {
    transform: translateX(-105%);
    transition: transform 0.35s var(--ease);
    box-shadow: none;
  }
  body.nav-open .sidebar { transform: none; box-shadow: 10px 0 30px rgba(0, 0, 0, 0.3); }
  body.nav-open .nav-scrim { display: block; }
  .content { margin-left: 0; padding: 72px 16px 40px; }
  .main-title { font-size: 1.6rem; padding: 12px 20px; }
  .IntroductionText { font-size: 18px; }
  .explanation, .content p, .content li { font-size: 17px; }
  .section-title { font-size: 1.4rem; }
  /* Jumping to a section lands below the fixed Topics button. */
  .section-title, .sub-title { scroll-margin-top: 72px; }
  .callout, .derive, .mcq, .question-card, .eq-card, .card { padding-left: 16px; padding-right: 16px; }
  .derive .step { padding-left: 38px; }
}
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { transition: none !important; animation: none !important; }
}

/* ---------- Pressing and moving between pages ----------
   js/motion.js makes buttons sink while pressed. Here: no grey tap flash on phones
   (the press shows instead), and no wait for a double-tap zoom before a tap counts. */
button, summary, [role="button"], .btn, .nav-links li a, .page-nav a, .unit-card li a, .toc a,
.subject-tab, .subject-links a, .paper-pdf, .drawer-link {
  -webkit-tap-highlight-color: transparent;
  touch-action: manipulation;
}

/* Changing page slides the old page up and away while the new one rises in. The top
   bar, sidebar and theme button stay where they are. Browsers without cross-page view
   transitions (marked html.no-vt by motion.js) just fade the new page in. The fill is
   "backwards" so nothing is left on <main> afterwards to upset fixed or sticky parts. */
@media (prefers-reduced-motion: no-preference) {
  @view-transition { navigation: auto; }

  .bank-top { view-transition-name: top-bar; }
  .sidebar { view-transition-name: sidebar; }
  .theme-toggle { view-transition-name: theme-toggle; }

  ::view-transition-old(root) { animation: page-out 0.2s cubic-bezier(.4, 0, 1, 1) both; }
  ::view-transition-new(root) { animation: page-in 0.38s cubic-bezier(.2, .9, .2, 1) both; }

  html.no-vt main { animation: page-in 0.4s cubic-bezier(.2, .9, .2, 1) backwards; }
}

@keyframes page-out { to { opacity: 0; transform: translateY(-12px); } }
@keyframes page-in { from { opacity: 0; transform: translateY(16px); } }

/* ---------- Subject palettes ----------
   Each subject's notes config sets <html data-subject="...">. Physics keeps the
   default blues above; the others swap the same variables. Text-on-colour pairs
   stay above 4.5:1 contrast. */
html[data-subject="biology"] {
  --navy: #173d17;
  --blue: #2a7a2f;
  --blue-hover: #1f5f23;
  --blue-logo: #256b29;
  --blue-soft: #a9d8a4;
  --blue-scroll: #3f8f44;
  --tip: #2a7a2f;
  --tip-bg: #e8f5e9;
  --key: #173d17;
  --key-bg: #edf7ed;
  --tint-1: #eef7ee;
  --tint-2: #e1efe1;
  --tint-3: #fbfdfb;
  --tint-4: #f6faf6;
  --shadow-soft: 0 4px 14px rgba(20, 70, 20, 0.10);
}

html[data-subject="chemistry"] {
  --navy: #063f33;
  --blue: #0b7a60;
  --blue-hover: #085c48;
  --blue-logo: #096b54;
  --blue-soft: #9fd9c9;
  --blue-scroll: #1f8f73;
  --tip: #0b7a60;
  --tip-bg: #e3f6f0;
  --key: #063f33;
  --key-bg: #e9f6f2;
  --tint-1: #ebf7f3;
  --tint-2: #dcefe9;
  --tint-3: #fbfdfc;
  --tint-4: #f5faf8;
  --shadow-soft: 0 4px 14px rgba(5, 60, 48, 0.10);
}

/* ---------- Dark mode ----------
   js/theme.js sets data-theme="dark" on <html>, from the reader's choice or the
   device setting. Surfaces and text swap; coloured bars keep their colour.
   Diagrams and interactive figures sit on white panels so their lines show. */
:root[data-theme="dark"] {
  color-scheme: dark;
  --bg: #0e131d;
  --card: #161d2a;
  --ink: #e2e7ef;
  --ink-2: #c5cedb;
  --muted: #9aa6b8;
  --line: #273246;
  --line-strong: #34405a;
  --surface-2: #1d2636;
  --shadow: 0 8px 8px rgba(0, 0, 0, 0.5);
  --shadow-soft: 0 4px 14px rgba(0, 0, 0, 0.45);
  --navy-text: #a9c2ff;
  --key: #a9c2ff;
  --key-bg: #171f3a;
  --tip-bg: #13233d;
  --offspec-bg: #33210f;
  --warn-bg: #351717;
  --define-bg: #132b19;
  --tint-1: #1a2436;
  --tint-2: #1f2a40;
  --tint-3: #182030;
  --tint-4: #192131;
}

:root[data-theme="dark"][data-subject="biology"] {
  --navy-text: #b5e3b1;
  --key: #b5e3b1;
  --key-bg: #142a16;
  --tip-bg: #132b16;
}

:root[data-theme="dark"][data-subject="chemistry"] {
  --navy-text: #a3e3d2;
  --key: #a3e3d2;
  --key-bg: #0f2a24;
  --tip-bg: #0f2a24;
}

:root[data-theme="dark"] .page svg[viewBox],
:root[data-theme="dark"] .page canvas {
  background: #fff;
  border-radius: 6px;
}

:root[data-theme="dark"] .callout.core { background: #211a3d; }
:root[data-theme="dark"] .mcq-options button.correct { background: #132b19; }
:root[data-theme="dark"] .mcq-options button.wrong { background: #351717; }

/* The light/dark switch, added to every page by js/theme.js. */
.theme-toggle {
  position: fixed;
  right: 16px;
  bottom: 16px;
  z-index: 45;
  display: grid;
  place-items: center;
  width: 44px;
  height: 44px;
  padding: 0;
  border: 1px solid var(--line-strong);
  border-radius: 50%;
  background: var(--card);
  color: var(--ink);
  box-shadow: var(--shadow-soft);
  cursor: pointer;
}

.theme-toggle:hover { border-color: var(--muted); }

.theme-toggle svg {
  width: 20px;
  height: 20px;
  fill: currentColor;
}

/* Phones and tablets: bigger tap targets, and no hover effects that stick after a tap. */
@media (pointer: coarse) {
  .btn, .content a.pdf-download, .mcq-options button { min-height: 44px; }
}
@media (hover: none) {
  .main-title:hover, .section-title:hover, .btn:hover,
  details.solution > summary:hover { transform: none; }
}

/* Printing, and the downloadable PDFs (tools/notes/build-pdfs.mjs): just the notes.
   script.js opens every solution and switches to light mode before printing. */
@media print {
  @page { size: A4; margin: 16mm 14mm; }
  .theme-toggle, .sidebar, .menu-toggle, .nav-scrim, .page-nav, .pdf-download,
  .interactive-section, .derive-controls { display: none !important; }
  body, .content { background: #fff; }
  .content { margin-left: 0; padding: 0; min-height: 0; }
  .page { max-width: none; }
  .derive.js .step.hidden { display: block; }
  .mcq-why { display: block; }
  .mcq-options button { font-size: 13.5px; padding: 5px 12px; min-height: 0; }
  .mcq-options button[data-correct] { border-color: #2f9e44; background: #ebfbee; }
  .main-title, .main-title:hover, .section-title { transform: none; box-shadow: none; }
  .IntroductionText { font-size: 15px; }
  .explanation, .content p, .content li { font-size: 13.5px; line-height: 1.55; }
  .main-title { font-size: 1.6rem; }
  .section-title { font-size: 1.35rem; margin-top: 28px; }
  .section-title, .sub-title, h3, h4 { break-after: avoid; }
  .eq-card, .mcq, figure, .derive .step { break-inside: avoid; }
  mjx-container[display="true"] { overflow: visible; }
  .table-wrap { overflow: visible; }
}

/* Sub-headings, links and sheet tags use the dark hover colour as text in light mode. */
:root[data-theme="dark"] .sub-title,
:root[data-theme="dark"] .content a,
:root[data-theme="dark"] .tag.sheet { color: var(--navy-text); }

/* This page's sections, as a drop-down under its link in the sidebar (js/script.js). */
.nav-links li.has-sections { position: relative; }
.nav-links li.has-sections > a.nav-link { padding-right: 40px; }

.nav-sections-toggle {
  all: unset;
  box-sizing: border-box;
  position: absolute;
  top: 3px;
  right: 3px;
  display: grid;
  place-items: center;
  width: 30px;
  height: 30px;
  border-radius: 6px;
  color: #fff;
  font-size: 18px;
  cursor: pointer;
}

.nav-sections-toggle span { transition: transform 0.3s var(--ease); }
.has-sections.open > .nav-sections-toggle span { transform: rotate(90deg); }
.nav-sections-toggle:hover { background-color: rgba(255, 255, 255, 0.14); }
.nav-sections-toggle:focus-visible { outline: 2px solid #8fb6ff; }

.nav-sections {
  display: none;
  margin: 6px 0 4px 12px;
  padding: 0 0 0 8px;
  border-left: 1px solid rgba(255, 255, 255, 0.25);
  list-style: none;
}

.has-sections.open > .nav-sections { display: block; }
.nav-links .nav-sections li { margin-bottom: 2px; }

.nav-links .nav-sections a {
  padding: 5px 8px;
  color: rgba(255, 255, 255, 0.82);
  font-size: 13.5px;
}

.nav-links .nav-sections a:hover { transform: translateX(3px); }

.nav-links .nav-sections a.current {
  background-color: rgba(255, 255, 255, 0.13);
  color: #fff;
  font-weight: 600;
}

.nav-links .nav-sections .nav-sections-note {
  padding: 5px 8px;
  color: rgba(255, 255, 255, 0.7);
  font-size: 13px;
}