:root {
  color-scheme: light;
  --paper: #fafaf6;
  --panel: #f0f1eb;
  --ink: #282b26;
  --muted: #60675a;
  --green: #4f664c;
  --rule: #dcded4;
  --serif:
    Georgia, "Noto Serif CJK SC", "Source Han Serif SC", "Songti SC", serif;
  --sans:
    Arial, "Noto Sans CJK SC", "PingFang SC", "Microsoft YaHei", sans-serif;
}
* {
  box-sizing: border-box;
}
html {
  scroll-padding-top: 100px;
}
body {
  margin: 0;
  background: var(--paper);
  color: var(--ink);
  font: 16px/1.9 var(--sans);
}
[hidden] {
  display: none !important;
}
::selection {
  background: #dce5d7;
  color: var(--ink);
}
a {
  color: var(--green);
  text-underline-offset: 4px;
}
a:hover {
  color: var(--ink);
}
:focus-visible {
  outline: 2px solid var(--green);
  outline-offset: 4px;
}
button,
input {
  font: inherit;
  accent-color: var(--green);
  caret-color: var(--green);
}
button {
  cursor: pointer;
  color: var(--green);
  border: 1px solid var(--rule);
  background: transparent;
  padding: 8px 14px;
  border-radius: 4px;
  min-height: 44px;
}
button:hover {
  background: var(--panel);
  border-color: var(--green);
}
button:disabled {
  cursor: default;
  opacity: 0.6;
}
.skip-link {
  position: absolute;
  left: 16px;
  top: -70px;
  z-index: 5;
  background: var(--paper);
  padding: 12px;
}
.skip-link:focus {
  top: 12px;
}
.learn-header {
  height: 80px;
  border-bottom: 1px solid var(--rule);
  display: flex;
  align-items: center;
  gap: 48px;
  padding: 0 40px;
  max-width: 1600px;
  margin: auto;
}
.brand {
  font: 700 27px var(--serif);
  color: var(--ink);
  text-decoration: none;
}
.learn-header nav,
.header-tools {
  display: flex;
  gap: 24px;
  align-items: center;
}
.learn-header a {
  white-space: nowrap;
}
.learn-header nav a,
.header-tools a {
  text-decoration: none;
  font-size: 14px;
}
.learn-header nav [aria-current] {
  font-weight: 700;
}
.header-tools {
  margin-left: auto;
}
.learn-layout {
  display: grid;
  grid-template-columns: 240px minmax(0, 760px) 160px;
  gap: 40px;
  max-width: 1440px;
  padding: 36px 40px 72px;
  margin: auto;
}
.lesson-rail,
.section-rail {
  position: sticky;
  top: 28px;
  align-self: start;
  max-height: calc(100vh - 56px);
  overflow: auto;
}
.chapter-menu summary {
  font-weight: 700;
  cursor: pointer;
  min-height: 44px;
}
.progress-info {
  font-size: 13px;
  color: var(--muted);
  margin: 4px 0 18px;
}
progress {
  display: block;
  width: 100%;
  height: 4px;
  border: 0;
  background: var(--rule);
  margin-top: 8px;
}
progress::-webkit-progress-bar {
  background: var(--rule);
}
progress::-webkit-progress-value {
  background: var(--green);
}
progress::-moz-progress-bar {
  background: var(--green);
}
.lesson-rail ol {
  list-style: none;
  padding: 0;
  margin: 12px 0;
}
.lesson-rail li a {
  display: flex;
  gap: 12px;
  align-items: baseline;
  min-height: 48px;
  padding: 10px 12px;
  text-decoration: none;
  font-size: 14px;
  color: var(--muted);
  border-radius: 4px;
}
.lesson-rail li a[aria-current] {
  background: var(--panel);
  color: var(--green);
  font-weight: 700;
}
.lesson-rail li a:hover {
  color: var(--ink);
  background: var(--panel);
}
.lesson-number {
  font-variant-numeric: tabular-nums;
  min-width: 12px;
}
.read-state {
  font-size: 11px;
  display: block;
  font-weight: 400;
}
.rail-links {
  border-top: 1px solid var(--rule);
  margin-top: 22px;
  padding-top: 18px;
}
.rail-links a,
.resume-link {
  display: block;
  font-size: 13px;
  margin: 8px 0;
}
.section-rail {
  font-size: 13px;
  color: var(--muted);
  padding-left: 12px;
  border-left: 1px solid var(--rule);
}
.section-rail ul {
  list-style: none;
  margin: 0;
  padding: 0;
}
.section-rail > ul > li > a {
  display: none;
}
.section-rail li a {
  display: block;
  text-decoration: none;
  color: var(--muted);
  padding: 6px 0;
}
.section-rail li li li {
  display: none;
}
.lesson-article {
  min-width: 0;
  max-width: 74ch;
}
.reading-controls {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  font-size: 13px;
  color: var(--muted);
  margin-bottom: 20px;
}
.reading-controls button {
  border: 0;
  padding: 4px 0;
  min-height: 32px;
  text-decoration: underline;
  text-underline-offset: 4px;
}
.lesson-article h1 {
  font: 400 36px/1.4 var(--serif);
  letter-spacing: -0.02em;
  margin: 0 0 24px;
  text-wrap: balance;
}
.lesson-article h2 {
  font-size: 24px;
  line-height: 1.5;
  margin: 42px 0 16px;
  font-weight: 600;
}
.lesson-article h3 {
  font-size: 19px;
  margin: 28px 0 12px;
}
.lesson-article p {
  margin: 0 0 18px;
}
.lesson-article ul,
.lesson-article ol {
  padding-left: 24px;
}
.lesson-article li {
  margin: 8px 0;
}
.headerlink {
  visibility: hidden;
  font-size: 0.65em;
  text-decoration: none;
  margin-left: 8px;
}
h1:hover .headerlink,
h2:hover .headerlink,
h3:hover .headerlink,
.headerlink:focus {
  visibility: visible;
}
.lesson-article code {
  font-size: 0.88em;
  overflow-wrap: anywhere;
  background: var(--panel);
  padding: 2px 4px;
}
.lesson-article pre {
  overflow-x: auto;
  padding: 16px;
  background: var(--panel);
  font-size: 14px;
  line-height: 1.7;
}
.lesson-article pre code {
  padding: 0;
}
.lesson-article details {
  border-top: 1px solid var(--rule);
  border-bottom: 1px solid var(--rule);
  padding: 16px 0;
  margin: 24px 0;
}
.lesson-article details + details {
  border-top: 0;
  margin-top: -24px;
}
.lesson-article summary {
  cursor: pointer;
  color: var(--green);
  font-weight: 600;
  min-height: 32px;
}
.lesson-article details[open] summary {
  margin-bottom: 16px;
}
.lesson-article details p:last-child {
  margin-bottom: 0;
}
.lesson-article .note {
  font-size: 14px;
  color: var(--muted);
}
.lesson-article blockquote {
  margin: 24px 0;
  padding: 20px 24px;
  background: var(--panel);
}
.lesson-article blockquote p:last-child {
  margin: 0;
}
.lesson-article table {
  border-collapse: collapse;
  width: 100%;
  font-size: 14px;
}
.lesson-article th,
.lesson-article td {
  padding: 12px 10px;
  border-bottom: 1px solid var(--rule);
  text-align: left;
  vertical-align: top;
}
.lesson-article .toctree-wrapper {
  display: none;
}
.time-lab {
  background: var(--panel);
  padding: 24px;
  margin: 24px 0;
}
.time-lab label {
  display: block;
  font-size: 14px;
}
.time-lab input {
  display: block;
  width: 100%;
  margin: 12px 0 20px;
}
.time-lab output {
  font-variant-numeric: tabular-nums;
  font-weight: 700;
}
.time-results {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 20px;
  border-top: 1px solid var(--rule);
  padding-top: 14px;
}
.time-results span {
  display: block;
  font-size: 13px;
  color: var(--muted);
}
.time-results output {
  font-size: 22px;
}
.timeline {
  display: block;
  width: 100%;
  height: auto;
  margin-top: 10px;
}
.timeline text {
  font: 12px var(--sans);
  fill: var(--muted);
}
.timeline .axis {
  stroke: var(--muted);
  stroke-width: 1;
}
.timeline .physics-tick {
  stroke: var(--muted);
  stroke-width: 1.5;
}
.timeline .control-tick {
  stroke: var(--green);
  stroke-width: 3;
}
.time-lab .note {
  margin: 10px 0 0;
}
.quiz {
  margin: 28px 0;
}
.quiz fieldset {
  border: 0;
  padding: 0;
  margin: 0;
}
.quiz legend {
  font-weight: 600;
  margin-bottom: 12px;
}
.quiz label {
  display: flex;
  gap: 10px;
  align-items: baseline;
  padding: 8px 0;
  cursor: pointer;
}
.quiz input {
  flex-shrink: 0;
}
.quiz button {
  margin-top: 14px;
}
.quiz-feedback {
  margin-top: 16px !important;
  font-weight: 600;
}
.quiz details {
  font-size: 14px;
}
.completion {
  margin: 44px 0 24px;
  border-top: 1px solid var(--rule);
  padding-top: 24px;
}
.completion button[aria-pressed="true"] {
  background: var(--green);
  color: var(--paper);
}
.completion p {
  font-size: 13px;
  color: var(--muted);
  margin-top: 10px;
}
.lesson-pagination {
  display: flex;
  gap: 20px;
  justify-content: space-between;
  border-top: 1px solid var(--rule);
  padding: 24px 0;
}
.lesson-pagination a {
  max-width: 48%;
  text-decoration: none;
  font-weight: 600;
}
.lesson-pagination small {
  display: block;
  color: var(--muted);
  font-size: 12px;
  font-weight: 400;
}
.lesson-pagination .next {
  margin-left: auto;
  text-align: right;
}
.lesson-footer {
  font-size: 12px;
  color: var(--muted);
  margin-top: 32px;
  border-top: 1px solid var(--rule);
  padding-top: 20px;
}
.lesson-footer p {
  margin-top: 8px;
}
@media (max-width: 1200px) {
  .learn-layout {
    grid-template-columns: 220px minmax(0, 1fr);
    gap: 32px;
    max-width: 1120px;
  }
  .section-rail {
    display: none;
  }
  .learn-header {
    gap: 32px;
  }
}
@media (max-width: 800px) {
  html {
    scroll-padding-top: 24px;
  }
  .learn-header {
    height: auto;
    min-height: 72px;
    padding: 18px 22px;
    gap: 16px;
    flex-wrap: wrap;
  }
  .learn-header nav {
    gap: 16px;
  }
  .header-tools {
    gap: 16px;
  }
  .learn-layout {
    display: block;
    padding: 20px 22px 48px;
  }
  .lesson-rail {
    position: static;
    max-height: none;
    overflow: visible;
    margin-bottom: 28px;
    border-bottom: 1px solid var(--rule);
    padding-bottom: 12px;
  }
  .chapter-menu summary {
    font-size: 15px;
  }
  .lesson-rail ol {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 4px;
  }
  .lesson-article {
    max-width: 74ch;
    margin: auto;
  }
  .lesson-article h1 {
    font-size: 30px;
  }
  .lesson-article h2 {
    font-size: 23px;
  }
  .rail-links {
    display: flex;
    gap: 18px;
    flex-wrap: wrap;
  }
  .lesson-article table {
    font-size: 13px;
  }
  .time-lab {
    padding: 18px;
  }
  .learn-header .brand {
    font-size: 25px;
  }
  .header-tools a,
  .learn-header nav a {
    font-size: 13px;
  }
}
@media (max-width: 440px) {
  .timeline text {
    font-size: 22px;
  }
  .learn-header {
    gap: 12px;
  }
  .header-tools {
    width: 100%;
    justify-content: flex-end;
    margin-top: -8px;
  }
  .lesson-rail ol {
    grid-template-columns: 1fr;
  }
  .lesson-article h1 {
    font-size: 28px;
  }
  .time-results {
    gap: 12px;
  }
  .time-results output {
    font-size: 19px;
  }
  .lesson-article th,
  .lesson-article td {
    padding: 10px 6px;
  }
}
@media print {
  .learn-header,
  .lesson-rail,
  .section-rail,
  .reading-controls,
  .completion,
  .lesson-pagination {
    display: none;
  }
  .learn-layout {
    display: block;
    padding: 0;
  }
  .lesson-article {
    max-width: none;
  }
  body {
    background: white;
  }
  .lesson-article details {
    break-inside: avoid;
  }
}