:root {
  color-scheme: light;
  --canvas: #f6f5f1;
  --surface: #fdfdfa;
  --surface-alt: #eeeee7;
  --ink: #1c3039;
  --muted: #536369;
  --line: #dcded5;
  --accent: #176b59;
  --accent-hover: #0f5142;
  --tint: #e5eee5;
  --lime: #e9efbb;
  --focus: #216db0;
  --danger: #a32c32;
  --shadow: 0 10px 30px #24362f06;
  --sidebar-width: 246px;
  --radius: 12px;
  --space-1: 4px;
  --space-2: 8px;
  --space-3: 12px;
  --space-4: 16px;
  --space-5: 24px;
  --space-6: 32px;
  --space-7: 48px;
  --space-8: 64px;
  font-family:
    "Segoe UI",
    -apple-system,
    BlinkMacSystemFont,
    sans-serif;
  font-size: 15px;
  line-height: 1.6;
  font-weight: 400;
}
:root[data-theme="dark"] {
  color-scheme: dark;
  --canvas: #111e25;
  --surface: #17272e;
  --surface-alt: #20343b;
  --ink: #e8eee8;
  --muted: #afbfbe;
  --line: #34474c;
  --accent: #9ed4b4;
  --accent-hover: #bde5ce;
  --tint: #223e35;
  --lime: #354434;
  --focus: #9acdf5;
  --danger: #ffacac;
  --shadow: none;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    color-scheme: dark;
    --canvas: #111e25;
    --surface: #17272e;
    --surface-alt: #20343b;
    --ink: #e8eee8;
    --muted: #afbfbe;
    --line: #34474c;
    --accent: #9ed4b4;
    --accent-hover: #bde5ce;
    --tint: #223e35;
    --lime: #354434;
    --focus: #9acdf5;
    --danger: #ffacac;
    --shadow: none;
  }
}
* {
  box-sizing: border-box;
}
body {
  margin: 0;
  background: var(--canvas);
  color: var(--ink);
}
a {
  color: inherit;
  text-decoration: none;
}
button,
input,
select {
  font: inherit;
}
button,
a,
input,
select,
summary {
  -webkit-tap-highlight-color: transparent;
}
button {
  cursor: pointer;
}
button:disabled {
  cursor: wait;
  opacity: 0.65;
}
button,
input,
select {
  color: var(--ink);
}
button {
  background: transparent;
  border: 0;
}
:focus-visible {
  outline: 3px solid var(--focus);
  outline-offset: 4px;
  border-radius: 3px;
}
main:focus {
  outline: none;
}
::selection {
  background: var(--tint);
  color: var(--ink);
}
[hidden] {
  display: none !important;
}
h1,
h2,
h3,
p {
  margin: 0;
}
h1,
h2,
h3 {
  line-height: 1.18;
  font-weight: 600;
}
h1 {
  font-family: Georgia, "Times New Roman", serif;
  font-weight: 400;
  letter-spacing: -0.045em;
}
h2 {
  font-size: 1.8rem;
  letter-spacing: -0.035em;
}
h3 {
  font-size: 1.15rem;
  letter-spacing: -0.015em;
}
p {
  color: var(--muted);
}
.icon {
  display: inline-block;
  flex-shrink: 0;
  vertical-align: middle;
}
.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
  border: 0;
}
.skip-link {
  position: fixed;
  top: 8px;
  left: 8px;
  z-index: 100;
  transform: translateY(-200%);
  background: var(--ink);
  color: var(--canvas);
  padding: 12px 20px;
}
.skip-link:focus {
  transform: translateY(0);
}
.sidebar {
  position: fixed;
  inset: 0 auto 0 0;
  width: var(--sidebar-width);
  padding: 32px 18px 24px;
  background: var(--surface);
  border-right: 1px solid var(--line);
  display: flex;
  flex-direction: column;
  z-index: 20;
  overflow-y: auto;
}
.brand {
  display: flex;
  gap: 12px;
  align-items: center;
  padding: 0 10px;
  font-size: 1.75rem;
  font-weight: 700;
  letter-spacing: -0.055em;
  line-height: 1.2;
}
.brand-symbol {
  display: grid;
  place-items: center;
  width: 38px;
  height: 38px;
  background: var(--accent);
  color: var(--canvas);
  border-radius: 11px;
}
.brand-symbol .icon {
  width: 25px;
  height: 25px;
}
.brand-domain {
  color: var(--accent);
  font-weight: 400;
}
.brand small {
  display: block;
  margin-top: 6px;
  font-size: 0.47rem;
  letter-spacing: 0.1em;
  color: var(--muted);
  font-weight: 600;
}
.chapter-nav {
  margin-top: 42px;
}
.chapter-nav a {
  display: flex;
  align-items: center;
  gap: 12px;
  min-height: 44px;
  padding: 10px 12px;
  border-radius: 7px;
  font-size: 0.88rem;
  color: var(--muted);
  transition:
    color 150ms,
    background 150ms;
}
.chapter-nav a:hover {
  background: var(--surface-alt);
  color: var(--ink);
}
.chapter-nav a.active {
  color: var(--accent);
  background: var(--tint);
  font-weight: 650;
}
.nav-label {
  font-size: 0.65rem;
  letter-spacing: 0.12em;
  font-weight: 650;
  color: var(--muted);
  margin: 30px 12px 10px;
}
.nav-number {
  font-family: ui-monospace, "Cascadia Code", monospace;
  font-size: 0.7rem;
  opacity: 1;
}
.nav-home .icon {
  width: 16px;
  height: 16px;
}
.sidebar-note {
  margin: auto 12px 0;
  padding-top: 50px;
  font-size: 0.72rem;
  color: var(--ink);
}
.status-dot {
  display: inline-block;
  width: 6px;
  height: 6px;
  border-radius: 50%;
  background: var(--accent);
  margin-right: 6px;
  vertical-align: middle;
}
.sidebar-note p {
  font-size: 0.72rem;
  line-height: 1.8;
  margin-top: 8px;
}
.workspace {
  margin-left: var(--sidebar-width);
}
.topbar {
  min-height: 85px;
  border-bottom: 1px solid var(--line);
  padding: 20px 4.5%;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 20px;
}
.breadcrumb {
  font-size: 0.76rem;
  color: var(--muted);
}
.breadcrumb span {
  margin: 0 12px;
  color: var(--muted);
}
.breadcrumb strong {
  color: var(--ink);
  font-weight: 500;
}
.topbar-actions {
  display: flex;
  align-items: center;
  gap: 16px;
}
.search-form {
  display: flex;
  gap: 8px;
  align-items: center;
  padding: 0 8px 0 12px;
  border: 1px solid var(--line);
  background: var(--surface);
  border-radius: 7px;
}
.search-form > .icon {
  width: 16px;
  color: var(--muted);
}
.search-form input {
  background: transparent;
  border: 0;
  width: 170px;
  min-width: 0;
  height: 38px;
  font-size: 0.78rem;
}
.search-form input::placeholder {
  color: var(--muted);
  opacity: 1;
}
.search-form button {
  width: 30px;
  height: 32px;
  color: var(--muted);
}
.icon-button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  min-width: 44px;
  min-height: 44px;
  color: var(--muted);
  border-radius: 8px;
}
.icon-button:hover {
  background: var(--surface-alt);
  color: var(--ink);
}
.menu-toggle {
  display: none;
}
main {
  max-width: 1310px;
  margin: 0 auto;
  padding: 56px 4.5% 60px;
}
.hero {
  display: grid;
  grid-template-columns: 1.15fr 1fr;
  gap: 35px;
  padding: 4px 0 42px;
  align-items: center;
}
.eyebrow {
  font-size: 0.65rem;
  letter-spacing: 0.14em;
  color: var(--accent);
  font-weight: 650;
}
.hero h1 {
  font-size: clamp(2.7rem, 4.1vw, 4.7rem);
  line-height: 1.04;
  margin: 26px 0 24px;
}
.hero h1 em {
  color: var(--accent);
  font-weight: 400;
  white-space: nowrap;
}
.hero-description {
  max-width: 410px;
  font-size: 0.98rem;
  line-height: 1.85;
  margin-bottom: 26px;
}
.button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 18px;
  min-height: 46px;
  padding: 12px 18px;
  border-radius: 6px;
  font-size: 0.8rem;
  font-weight: 600;
  line-height: 1.5;
  transition:
    background 160ms,
    transform 160ms;
}
.button:hover {
  transform: translateY(-1px);
}
.button.primary {
  background: var(--accent);
  color: var(--canvas);
  border: 1px solid var(--accent);
}
.button.primary:hover {
  background: var(--accent-hover);
}
.button.secondary {
  border: 1px solid var(--line);
  background: var(--surface);
  color: var(--ink);
}
.hero-footnote {
  display: flex;
  align-items: center;
  gap: 9px;
  margin-top: 17px;
  font-size: 0.67rem;
  color: var(--muted);
}
.hero-footnote .icon {
  width: 14px;
}
.stack-illustration {
  position: relative;
  border: 1px solid var(--line);
  border-radius: 16px;
  padding: 28px 26px 22px;
  background: var(--surface);
  background-image: radial-gradient(var(--line) 0.7px, transparent 0.7px);
  background-size: 15px 15px;
  box-shadow: var(--shadow);
}
.diagram-label {
  font-size: 0.59rem;
  letter-spacing: 0.13em;
  font-weight: 600;
  color: var(--muted);
}
.packet-stack {
  display: grid;
  gap: 10px;
  padding: 32px 0 28px;
}
.stack-layer {
  position: relative;
  min-height: 72px;
  display: flex;
  align-items: center;
  gap: 16px;
  padding: 14px 18px;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: var(--surface);
  box-shadow: 0 4px 0 var(--surface-alt);
}
.stack-layer strong {
  display: block;
  font-size: 0.93rem;
  line-height: 1.3;
}
.stack-layer small {
  display: block;
  margin-top: 3px;
  font-family: ui-monospace, monospace;
  font-size: 0.61rem;
  color: var(--muted);
}
.layer-index {
  font-family: ui-monospace, monospace;
  font-size: 0.67rem;
  color: var(--muted);
}
.layer-unit {
  margin-left: auto;
  padding: 4px 6px;
  border: 1px solid var(--line);
  border-radius: 4px;
  font-size: 0.49rem;
  letter-spacing: 0.08em;
  color: var(--muted);
}
.layer-application {
  background: var(--lime);
  border-color: var(--line);
}
.layer-transport {
  background: var(--tint);
}
.layer-internet {
  background: var(--surface-alt);
}
.diagram-bottom {
  font-size: 0.63rem;
  color: var(--muted);
  display: flex;
  align-items: center;
  gap: 8px;
}
.flow-line {
  width: 25px;
  height: 1px;
  background: var(--accent);
}
.principles-strip {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  border-top: 1px solid var(--line);
  border-bottom: 1px solid var(--line);
  padding: 19px 2px;
  font-size: 0.76rem;
  color: var(--muted);
}
.principles-strip span {
  display: flex;
  align-items: center;
  gap: 10px;
}
.principles-strip .icon {
  width: 17px;
  color: var(--accent);
}
.chapter-section {
  margin-top: 44px;
}
.section-heading {
  display: flex;
  justify-content: space-between;
  align-items: flex-end;
  gap: 24px;
  margin-bottom: 24px;
}
.section-heading h2 {
  margin-top: 10px;
}
.section-heading > p {
  font-size: 0.74rem;
  padding-bottom: 4px;
}
.chapter-grid {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 16px;
}
.chapter-card {
  display: flex;
  flex-direction: column;
  padding: 22px;
  border: 1px solid var(--line);
  background: var(--surface);
  border-radius: var(--radius);
  transition:
    border-color 180ms,
    transform 180ms;
}
.chapter-card:hover {
  border-color: var(--accent);
  transform: translateY(-3px);
}
.card-top {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 24px;
}
.chapter-number {
  display: grid;
  place-items: center;
  width: 32px;
  height: 32px;
  border: 1px solid var(--line);
  border-radius: 8px;
  color: var(--accent);
  font-size: 0.7rem;
  font-family: ui-monospace, monospace;
}
.read-time {
  display: inline-flex;
  gap: 8px;
  align-items: center;
  color: var(--muted);
  font-size: 0.65rem;
}
.chapter-card h3 {
  margin-bottom: 12px;
}
.chapter-card p {
  font-size: 0.8rem;
  line-height: 1.75;
  margin-bottom: 26px;
}
.card-link {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-top: auto;
  color: var(--accent);
  font-size: 0.73rem;
  font-weight: 600;
}
.card-link .icon {
  width: 17px;
}
.practice-banner {
  display: flex;
  align-items: center;
  gap: 20px;
  background: var(--tint);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 28px;
  margin-top: 32px;
}
.practice-icon {
  display: grid;
  place-items: center;
  width: 46px;
  height: 46px;
  border-radius: 10px;
  background: var(--surface);
  color: var(--accent);
  flex-shrink: 0;
}
.practice-banner h2 {
  font-size: 1.45rem;
  margin: 9px 0 8px;
}
.practice-banner p:not(.eyebrow) {
  font-size: 0.77rem;
}
.practice-banner .button {
  margin-left: auto;
  flex-shrink: 0;
}
footer {
  border-top: 1px solid var(--line);
  display: flex;
  align-items: center;
  gap: 18px;
  padding: 22px 4.5%;
  color: var(--muted);
  font-size: 0.65rem;
}
footer span:last-child {
  margin-left: auto;
}
.footer-brand {
  font-size: 1rem;
  font-weight: 650;
  letter-spacing: -0.04em;
  color: var(--ink);
}
.page-heading {
  max-width: 800px;
  margin: 4px 0 44px;
}
.page-heading h1 {
  font-size: clamp(2.7rem, 4vw, 4.5rem);
  margin: 18px 0 18px;
}
.page-description {
  font-size: 1.1rem;
  line-height: 1.8;
  max-width: 650px;
}
.page-heading .read-time {
  margin-top: 20px;
  font-size: 0.75rem;
}
.article-layout {
  display: grid;
  grid-template-columns: minmax(0, 1fr) 195px;
  gap: 52px;
}
.article-content {
  max-width: 760px;
  min-width: 0;
}
.content-section {
  position: relative;
  border-top: 1px solid var(--line);
  padding: 32px 0;
  scroll-margin-top: 20px;
}
.section-index {
  display: block;
  font-family: ui-monospace, monospace;
  color: var(--accent);
  font-size: 0.7rem;
  margin-bottom: 14px;
}
.content-section h2 {
  font-size: 1.7rem;
  margin-bottom: 18px;
}
.content-section > p {
  font-size: 1rem;
  line-height: 1.9;
}
.learning-list {
  padding: 0;
  margin: 24px 0 0;
  list-style: none;
}
.learning-list li {
  position: relative;
  padding: 10px 0 10px 24px;
  color: var(--ink);
  font-size: 0.92rem;
  line-height: 1.8;
}
.learning-list li::before {
  content: "";
  position: absolute;
  left: 0;
  top: 21px;
  width: 6px;
  height: 6px;
  background: var(--accent);
  border-radius: 2px;
}
.on-this-page {
  align-self: start;
  position: sticky;
  top: 32px;
  border-left: 1px solid var(--line);
  padding-left: 22px;
}
.on-this-page .nav-label {
  margin: 0 0 20px;
}
.on-this-page > a {
  display: block;
  margin: 16px 0;
  font-size: 0.78rem;
  line-height: 1.6;
  color: var(--muted);
}
.on-this-page > a:hover {
  color: var(--accent);
  text-decoration: underline;
}
.topic-tags {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
  border-top: 1px solid var(--line);
  padding-top: 22px;
  margin-top: 24px;
}
.topic-tags span {
  font-size: 0.62rem;
  background: var(--surface-alt);
  padding: 4px 8px;
  border-radius: 4px;
}
.small-note {
  font-size: 0.72rem;
  line-height: 1.9;
  margin-top: 20px;
}
.takeaway {
  background: var(--tint);
  border-radius: var(--radius);
  padding: 26px;
  margin: 10px 0 32px;
}
.takeaway h2 {
  font-size: 1.4rem;
  margin: 12px 0;
}
.takeaway > p:last-child {
  font-size: 0.9rem;
  line-height: 1.8;
}
.next-chapter {
  display: flex;
  justify-content: space-between;
  align-items: center;
  border-top: 1px solid var(--line);
  padding-top: 28px;
  color: var(--accent);
}
.next-chapter small {
  display: block;
  font-size: 0.65rem;
  letter-spacing: 0.1em;
  color: var(--muted);
  margin-bottom: 8px;
}
.next-chapter strong {
  font-size: 1.15rem;
}
.search-results {
  max-width: 820px;
}
.results-count {
  font-size: 0.85rem;
  margin-bottom: 24px;
}
.search-result {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 24px;
  padding: 26px 0;
  border-top: 1px solid var(--line);
}
.search-result h2 {
  font-size: 1.5rem;
  margin: 8px 0 10px;
}
.search-result p {
  font-size: 0.9rem;
}
.search-result:hover h2 {
  color: var(--accent);
}
.empty-state {
  padding: 55px 30px;
  background: var(--surface);
  border: 1px dashed var(--line);
  border-radius: var(--radius);
  text-align: center;
}
.empty-state > .icon {
  color: var(--accent);
  width: 32px;
  height: 32px;
  margin-bottom: 20px;
}
.empty-state h2 {
  font-size: 1.5rem;
  margin-bottom: 14px;
}
.empty-state p {
  font-size: 0.9rem;
}
.text-link {
  display: inline-flex;
  gap: 12px;
  align-items: center;
  color: var(--accent);
  text-decoration: underline;
  text-underline-offset: 4px;
  margin-top: 20px;
  font-size: 0.9rem;
}
.error-state {
  padding: 60px 0 100px;
  max-width: 700px;
}
.error-state h1 {
  font-size: clamp(2.8rem, 4vw, 4.5rem);
  margin: 24px 0;
}
.error-state > p {
  margin-bottom: 28px;
}
/* Reusable exercise surfaces. */
.exercise {
  background: var(--surface);
  border: 1px solid var(--line);
  padding: 28px;
  border-radius: var(--radius);
  margin-bottom: 36px;
}
.exercise h2 {
  font-size: 1.6rem;
  margin: 10px 0 14px;
}
.exercise > p {
  font-size: 0.9rem;
  line-height: 1.8;
}
.exercise-form {
  margin: 24px 0;
}
.field-row {
  display: flex;
  align-items: flex-end;
  gap: 12px;
}
.field-row > div {
  flex: 1;
  min-width: 0;
}
label {
  display: block;
  font-size: 0.8rem;
  font-weight: 600;
  margin-bottom: 8px;
}
.field-row input {
  width: 100%;
  border: 1px solid var(--line);
  border-radius: 6px;
  padding: 12px;
  background: var(--canvas);
  min-height: 46px;
  font-family: ui-monospace, monospace;
  font-size: 0.85rem;
}
.field-help {
  display: block;
  font-size: 0.72rem;
  color: var(--muted);
  margin: 8px 0;
}
.error-message {
  color: var(--danger);
  font-size: 0.85rem;
  margin: 16px 0;
}
.result-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 0 24px;
  margin: 24px 0 0;
}
.result-grid > div {
  border-top: 1px solid var(--line);
  padding: 14px 0;
  min-width: 0;
}
dt {
  color: var(--muted);
  font-size: 0.72rem;
}
dd {
  margin: 6px 0 0;
  overflow-wrap: anywhere;
  font-family: ui-monospace, monospace;
  font-size: 0.85rem;
}
.result-note {
  font-size: 0.75rem;
  margin-top: 20px;
}
@media (min-width: 1550px) {
  .hero h1 {
    font-size: 4.7rem;
  }
}
@media (max-width: 1190px) {
  :root {
    --sidebar-width: 220px;
  }
  .sidebar {
    padding-left: 12px;
    padding-right: 12px;
  }
  .brand {
    font-size: 1.6rem;
    gap: 8px;
  }
  .brand small {
    font-size: 0.42rem;
  }
  .hero {
    gap: 24px;
  }
  .hero h1 {
    font-size: 3.25rem;
  }
  .hero h1 em {
    white-space: normal;
  }
  .stack-illustration {
    padding: 24px 16px 18px;
  }
  .stack-layer {
    padding: 12px;
    gap: 10px;
  }
  .layer-unit {
    display: none;
  }
  .chapter-grid {
    grid-template-columns: repeat(2, minmax(0, 1fr));
  }
  .section-heading {
    align-items: flex-start;
    flex-direction: column;
    gap: 12px;
  }
  .practice-banner {
    flex-wrap: wrap;
  }
  .practice-banner .button {
    margin-left: 66px;
  }
  .article-layout {
    gap: 28px;
    grid-template-columns: minmax(0, 1fr) 160px;
  }
  .topbar .breadcrumb {
    display: none;
  }
  .topbar-actions {
    margin-left: auto;
  }
}
@media (max-width: 900px) {
  :root {
    --sidebar-width: 0px;
  }
  .sidebar {
    position: relative;
    width: 100%;
    padding: 18px 24px;
    border-right: 0;
    border-bottom: 1px solid var(--line);
    flex-direction: row;
    align-items: center;
    flex-wrap: wrap;
    gap: 10px;
    overflow: visible;
  }
  .brand {
    padding: 0;
  }
  .menu-toggle {
    display: inline-flex;
    margin-left: auto;
  }
  .sidebar-note {
    display: none;
  }
  .chapter-nav {
    flex-basis: 100%;
    margin-top: 12px;
    display: grid;
    grid-template-columns: repeat(2, minmax(0, 1fr));
    gap: 4px 12px;
  }
  .chapter-nav .nav-label {
    grid-column: 1 / -1;
    margin: 16px 12px 5px;
  }
  .chapter-nav.is-collapsed {
    display: none;
  }
  .workspace {
    margin: 0;
  }
  .topbar {
    min-height: 72px;
    padding: 14px 24px;
  }
  .topbar-actions {
    width: 100%;
    margin: 0;
  }
  .search-form {
    flex: 1;
  }
  .search-form input {
    width: 100%;
  }
  main {
    padding: 40px 24px;
  }
  .hero h1 {
    font-size: 3.6rem;
  }
  .article-layout {
    grid-template-columns: minmax(0, 1fr) 180px;
    gap: 36px;
  }
  footer {
    padding: 22px 24px;
  }
}
@media (max-width: 650px) {
  .hero {
    grid-template-columns: 1fr;
    gap: 32px;
    padding: 0 0 32px;
  }
  .hero h1 {
    font-size: clamp(2.65rem, 9vw, 3.9rem);
    margin: 22px 0;
  }
  .hero h1 em {
    white-space: normal;
  }
  .hero-description {
    font-size: 0.95rem;
  }
  .hero-footnote {
    flex-wrap: wrap;
  }
  .stack-illustration {
    padding: 24px;
  }
  .packet-stack {
    padding: 24px 0;
  }
  .layer-unit {
    display: block;
  }
  .stack-layer {
    min-height: 64px;
    padding: 12px 14px;
  }
  .principles-strip {
    flex-direction: column;
    align-items: flex-start;
    gap: 12px;
    padding: 20px 0;
  }
  .chapter-section {
    margin-top: 32px;
  }
  .section-heading h2 {
    font-size: 1.7rem;
  }
  .chapter-grid {
    grid-template-columns: 1fr;
  }
  .chapter-card {
    padding: 24px;
  }
  .card-top {
    margin-bottom: 22px;
  }
  .chapter-card p {
    font-size: 0.88rem;
  }
  .practice-banner {
    padding: 24px;
    display: block;
  }
  .practice-icon {
    margin-bottom: 18px;
  }
  .practice-banner .button {
    margin: 20px 0 0;
  }
  footer {
    flex-wrap: wrap;
    gap: 10px 16px;
  }
  footer span:last-child {
    flex-basis: 100%;
    margin: 0;
  }
  .page-heading {
    margin-bottom: 32px;
  }
  .page-heading h1 {
    font-size: 3rem;
  }
  .page-description {
    font-size: 1rem;
  }
  .article-layout {
    display: flex;
    flex-direction: column;
    gap: 0;
  }
  .on-this-page {
    order: -1;
    position: static;
    border-left: 0;
    border-top: 1px solid var(--line);
    padding: 22px 0;
    width: 100%;
  }
  .on-this-page .nav-label {
    margin-bottom: 10px;
  }
  .on-this-page > a {
    margin: 0;
    padding: 8px 0;
  }
  .topic-tags,
  .small-note {
    display: none;
  }
  .content-section h2 {
    font-size: 1.5rem;
  }
  .content-section > p {
    font-size: 0.96rem;
  }
  .exercise {
    padding: 22px;
  }
  .field-row {
    flex-direction: column;
    align-items: stretch;
  }
  .result-grid {
    grid-template-columns: 1fr;
  }
  .chapter-nav {
    grid-template-columns: 1fr;
  }
}
@media (prefers-reduced-motion: reduce) {
  *,
  *::before,
  *::after {
    animation: none !important;
    transition: none !important;
    scroll-behavior: auto !important;
  }
}
.field-help a {
  color: var(--accent);
  text-decoration: underline;
  text-underline-offset: 3px;
}
#result-title {
  scroll-margin-top: 24px;
}
.quiz-form fieldset {
  border: 0;
  border-top: 1px solid var(--line);
  padding: 24px 0;
  margin: 20px 0 0;
  min-width: 0;
}
.quiz-form legend {
  padding: 0;
  font-size: 0.95rem;
  font-weight: 600;
  line-height: 1.7;
  width: 100%;
  margin-bottom: 12px;
}
.quiz-form legend .eyebrow {
  display: block;
  margin-bottom: 10px;
}
.quiz-option {
  display: flex;
  align-items: center;
  gap: 12px;
  border: 1px solid var(--line);
  border-radius: 6px;
  background: var(--canvas);
  padding: 12px;
  font-size: 0.85rem;
  font-weight: 400;
  cursor: pointer;
}
.quiz-option:has(input:checked) {
  border-color: var(--accent);
  background: var(--tint);
}
.quiz-option input {
  accent-color: var(--accent);
  width: 18px;
  height: 18px;
  margin: 0;
  flex-shrink: 0;
}
.quiz-feedback,
.quiz-score {
  padding: 18px;
  background: var(--tint);
  border-radius: 6px;
  font-size: 0.85rem;
}
.quiz-feedback p,
.quiz-score p {
  margin-top: 8px;
}
.quiz-score {
  margin-top: 24px;
}
.quiz-reset {
  margin: 0 0 0 18px;
}