/* Fourth Move — Textbook plate
   A page from an old physics textbook: aged paper, ink-blue rules and type, pencil grey, and one red apple.
   旧物理教科书的一页：旧纸、墨蓝线条与文字、铅笔灰，唯一的彩色是红苹果。 */

:root {
  --paper: #f4ecdc;
  --paper-deep: #ebe0ca;
  --paper-light: #faf5ea;
  --ink: #1b2a4a;
  --ink-soft: #3a4866;
  --pencil: #8a8f99;
  --rule: rgba(27, 42, 74, 0.28);
  --rule-soft: rgba(27, 42, 74, 0.14);
  --red: #d7262c;
  --red-deep: #a81a1f;
  --leaf: #4f9a3f;
  --cream: #fff8ee;

  --serif: 'Spectral', Georgia, 'Times New Roman', serif;
  --sans: 'Source Sans 3', 'Helvetica Neue', Arial, system-ui, sans-serif;
  --mono: 'Cousine', 'Courier New', ui-monospace, monospace;

  --gutter: clamp(20px, 5vw, 72px);
  --max: 1140px;
  --hero-max: 1920px;
  --ease: cubic-bezier(0.2, 0.7, 0.2, 1);
}

*, *::before, *::after { box-sizing: border-box; }
html { scroll-behavior: smooth; -webkit-text-size-adjust: 100%; }
body {
  margin: 0;
  background: var(--paper);
  color: var(--ink);
  font-family: var(--sans);
  font-size: 18px;
  line-height: 1.6;
  -webkit-font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
}
img { display: block; max-width: 100%; height: auto; }
a { color: inherit; text-decoration: none; }
h1, h2, h3, p, figure, figcaption, blockquote, dl, dd, ol, ul { margin: 0; }
h2, h3, .section-lede, .plate-line { text-wrap: balance; }
ol, ul { padding: 0; list-style: none; }
[hidden] { display: none !important; }
:focus-visible { outline: 2px solid var(--red); outline-offset: 3px; }
q { quotes: '“' '”'; font-style: italic; }

/* Header / 页头：像书页顶部的双线页眉 */
.site-header {
  position: absolute; top: 0; left: 0; right: 0; z-index: 5;
  display: grid; grid-template-columns: auto 1fr auto; align-items: center; gap: 28px;
  max-width: var(--hero-max); margin: 0 auto; padding: 18px var(--gutter) 14px;
}
.site-header::after {
  content: ''; position: absolute; left: var(--gutter); right: var(--gutter); bottom: 0; height: 4px;
  border-top: 1px solid var(--ink); border-bottom: 1px solid var(--ink);
}
.monogram { font: 700 26px/1 var(--serif); letter-spacing: 0.02em; color: var(--ink); }
.site-nav { display: flex; justify-content: center; gap: clamp(18px, 2.6vw, 40px); }
.site-nav a { font: 600 15px/1 var(--sans); letter-spacing: 0.04em; text-transform: uppercase; color: var(--ink); }
.site-nav a:hover { color: var(--red); }
.header-right { display: flex; align-items: center; gap: 12px; }
.header-social {
  display: inline-flex; align-items: center; justify-content: center; width: 34px; height: 34px; border-radius: 999px;
  border: 1.5px solid var(--ink); font: 700 14px/1 var(--serif); color: var(--ink);
}
.header-ticker {
  display: inline-flex; align-items: center; gap: 6px; padding: 9px 16px; border-radius: 999px;
  border: 1.5px solid var(--ink); background: var(--paper-light); color: var(--ink); font: 700 14px/1 var(--mono); letter-spacing: 0.04em;
}

/* Hero / 首屏：一整页图版 */
.hero {
  position: relative; min-height: max(760px, 100svh); overflow: hidden; isolation: isolate;
  background: var(--paper); display: flex; align-items: flex-start;
}
.hero-art, .hero-art img { position: absolute; inset: 0; width: 100%; height: 100%; }
.hero-art img { object-fit: cover; object-position: 50% 70%; }
.hero-art.is-running img { animation: redraw 1.4s var(--ease); }
@keyframes redraw {
  0% { opacity: 0.2; transform: translateX(-3%); filter: grayscale(1); }
  60% { opacity: 1; transform: translateX(0); filter: grayscale(1); }
  100% { filter: grayscale(0); }
}
.hero-inner { position: relative; z-index: 1; width: 100%; max-width: var(--hero-max); min-height: inherit; margin: 0 auto; padding: 108px var(--gutter) 64px; }
.hero-copy { max-width: 720px; display: flex; flex-direction: column; align-items: flex-start; gap: 16px; }
.eyebrow { font: 600 13px/1 var(--mono); letter-spacing: 0.2em; text-transform: uppercase; color: var(--ink-soft); }
.eyebrow .dot, .section-head .dot { color: var(--red); margin: 0 6px; }
.hero-quote { font: 400 clamp(16px, 1.5vw, 20px)/1.4 var(--serif); font-style: italic; color: var(--ink-soft); max-width: 640px; }
.footnote { font: 700 0.8em/1 var(--mono); color: var(--red); vertical-align: super; font-style: normal; }
.wordmark { font: 700 clamp(52px, 8vw, 116px)/0.95 var(--serif); letter-spacing: 0.01em; text-transform: uppercase; color: var(--ink); }
.hero-lede { font: 500 clamp(20px, 2.2vw, 28px)/1.3 var(--serif); font-style: italic; color: var(--ink-soft); }
.hero-actions { display: flex; flex-wrap: wrap; gap: 12px; margin-top: 4px; }
.ca-line {
  display: inline-flex; align-items: center; gap: 12px; padding: 9px 14px 9px 14px;
  border: 1.5px solid var(--ink); background: var(--paper-light); border-radius: 4px;
}
.ca-label { font: 700 12px/1 var(--mono); letter-spacing: 0.18em; color: var(--red); }
.ca-short { font: 700 15px/1 var(--mono); color: var(--ink); }
.hero-note { max-width: 520px; font-size: 15px; line-height: 1.5; color: var(--ink-soft); }
.caption { position: absolute; left: var(--gutter); bottom: 26px; z-index: 1; display: flex; flex-wrap: wrap; align-items: center; gap: 14px; font: 400 15px/1.4 var(--mono); color: var(--ink); }
.btn-replay { padding: 6px 12px; border: 1.5px solid var(--ink); border-radius: 4px; background: var(--paper-light); color: var(--ink); font: 700 12px/1 var(--mono); letter-spacing: 0.1em; text-transform: uppercase; cursor: pointer; }
.btn-replay:hover { background: var(--ink); color: var(--paper); }

.stamp {
  display: inline-block; padding: 6px 12px; border: 2px solid var(--ink); border-radius: 4px; color: var(--ink);
  font: 700 12px/1 var(--mono); letter-spacing: 0.14em; text-transform: uppercase; transform: rotate(-3deg);
  box-shadow: inset 0 0 0 1px var(--paper), inset 0 0 0 2.5px var(--ink); opacity: 0.85;
}
.stamp-small { padding: 3px 8px; font-size: 10px; vertical-align: middle; border-color: var(--red); color: var(--red); box-shadow: inset 0 0 0 1px var(--paper), inset 0 0 0 2px var(--red); }
.stamp-hero { position: absolute; left: var(--gutter); bottom: 76px; z-index: 1; border-color: var(--red); color: var(--red); box-shadow: inset 0 0 0 1px var(--paper), inset 0 0 0 2.5px var(--red); transform: rotate(-6deg); }

/* Buttons / 按钮 */
.btn {
  display: inline-flex; align-items: center; gap: 8px; padding: 13px 20px; border-radius: 4px;
  font: 700 15px/1 var(--sans); letter-spacing: 0.02em; transition: transform 0.15s var(--ease), background 0.15s var(--ease);
}
.btn:hover { transform: translateY(-2px); }
.btn-red { background: var(--red); color: var(--cream); }
.btn-red:hover { background: var(--red-deep); }
.btn-ink { border: 1.5px solid var(--ink); color: var(--ink); background: rgba(250, 245, 234, 0.7); }
.btn-ink:hover { background: var(--ink); color: var(--paper); }
.btn-lg { padding: 17px 24px; font-size: 16px; }
.btn-copy {
  display: inline-flex; align-items: center; padding: 9px 13px; border-radius: 4px; border: 1.5px solid var(--ink); background: var(--paper-light);
  color: var(--ink); font: 700 12px/1 var(--mono); letter-spacing: 0.1em; text-transform: uppercase; cursor: pointer;
}
.btn-copy-sm { padding: 6px 10px; font-size: 11px; }
.btn-copy:hover { background: var(--ink); color: var(--paper); }

/* Running head / 页眉跑马灯 */
.running-head { overflow: hidden; border-top: 1px solid var(--ink); border-bottom: 1px solid var(--ink); background: var(--paper-deep); }
.running-track { display: flex; gap: 40px; width: max-content; padding: 10px 0; animation: run 30s linear infinite; }
.running-track span { font: 600 13px/1 var(--mono); letter-spacing: 0.16em; text-transform: uppercase; white-space: nowrap; color: var(--ink); }
.running-track span::after { content: '§'; margin-left: 40px; color: var(--red); }
@keyframes run { from { transform: translateX(0); } to { transform: translateX(-50%); } }

/* Sections / 章节 */
.section { padding: clamp(72px, 9vw, 120px) var(--gutter); background: var(--paper); }
.section > * { max-width: var(--max); margin-left: auto; margin-right: auto; }
.section + .section { border-top: 1px solid var(--rule); }
.section-head { margin-bottom: 44px; padding-bottom: 22px; border-bottom: 3px double var(--ink); }
.section-head .eyebrow { margin-bottom: 14px; }
.section h2 { font: 700 clamp(34px, 4.4vw, 58px)/1.05 var(--serif); letter-spacing: 0.005em; }
.section-lede { margin-top: 14px; font: 400 19px/1.5 var(--serif); font-style: italic; color: var(--ink-soft); max-width: 680px; }

/* References / 参考文献式时间线 */
.refs { display: flex; flex-direction: column; }
.ref { display: grid; grid-template-columns: 64px 1fr; gap: 18px; padding: 28px 0; border-bottom: 1px solid var(--rule-soft); }
.ref-no { font: 700 18px/1.6 var(--mono); color: var(--red); }
.ref-date { display: block; font: 600 13px/1.4 var(--mono); letter-spacing: 0.08em; text-transform: uppercase; color: var(--pencil); margin-bottom: 6px; }
.ref-body h3 { font: 700 clamp(22px, 2.3vw, 30px)/1.2 var(--serif); margin-bottom: 8px; display: flex; flex-wrap: wrap; align-items: center; gap: 12px; }
.ref-body p { max-width: 700px; color: var(--ink-soft); }
.ref-body strong { color: var(--ink); }
.ref-links { margin-top: 10px; display: flex; flex-wrap: wrap; gap: 18px; }
.ref-links a { font: 700 13px/1 var(--mono); letter-spacing: 0.08em; text-transform: uppercase; color: var(--ink); border-bottom: 2px solid var(--red); padding-bottom: 3px; }
.ref-ca .ref-no { color: var(--leaf); }

/* Corollaries / 推论 */
.rules-grid { display: grid; grid-template-columns: 1.15fr minmax(260px, 0.85fr); gap: 48px; align-items: center; }
.rules { display: flex; flex-direction: column; gap: 28px; }
.rules li { display: grid; grid-template-columns: 64px 1fr; gap: 14px; align-items: start; }
.rule-no { font: 700 26px/1.2 var(--mono); color: var(--red); }
.rules h3 { font: 700 clamp(21px, 2.2vw, 28px)/1.2 var(--serif); margin-bottom: 8px; }
.rules p { grid-column: 2; color: var(--ink-soft); font-style: italic; font-family: var(--serif); }
.rules-art { border: 1px solid var(--ink); padding: 10px; background: var(--paper-light); box-shadow: 4px 4px 0 var(--ink); }
.rules-art img { width: 100%; height: auto; }
.rules-art figcaption { margin-top: 10px; text-align: center; font: 400 13px/1.4 var(--mono); color: var(--ink-soft); }
.specs { margin-top: 54px; display: grid; grid-template-columns: repeat(4, 1fr); border: 1px solid var(--ink); }
.specs > div { padding: 18px 20px; border-right: 1px solid var(--rule); background: var(--paper-light); }
.specs > div:last-child { border-right: 0; }
.specs dt { font: 600 12px/1 var(--mono); letter-spacing: 0.16em; text-transform: uppercase; color: var(--red); margin-bottom: 10px; }
.specs dd { font-size: 15px; line-height: 1.45; color: var(--ink); }

/* Plate / 空白图版大字 */
.plate-section { position: relative; min-height: 600px; overflow: hidden; isolation: isolate; display: flex; align-items: center; justify-content: center; background: var(--paper-deep); border-top: 1px solid var(--rule); border-bottom: 1px solid var(--rule); }
.plate-art { position: absolute; inset: 0; width: 100%; height: 100%; object-fit: cover; object-position: 50% 40%; z-index: 0; }
.plate-inner { position: relative; z-index: 1; width: 100%; max-width: 1000px; padding: 96px var(--gutter); text-align: center; display: flex; flex-direction: column; align-items: center; gap: 18px; }
.plate-line { font: 700 clamp(34px, 5.4vw, 80px)/1.05 var(--serif); font-style: italic; color: var(--ink); max-width: 18ch; }
.plate-sign { font: 600 clamp(15px, 1.8vw, 20px)/1 var(--mono); letter-spacing: 0.18em; text-transform: uppercase; color: var(--red); }
.plate-inner .btn { margin-top: 10px; }

/* Enrol / 入学 */
.dispense-note { margin-top: 14px; font: 400 17px/1.5 var(--serif); font-style: italic; color: var(--ink-soft); max-width: 600px; }
.enrol-grid { display: grid; grid-template-columns: minmax(280px, 0.9fr) 1.1fr; gap: 44px; align-items: start; }
.enrol-actions { display: flex; flex-direction: column; gap: 14px; }
.enrol-actions .btn { justify-content: center; }
.errata { padding: 22px 24px; border: 1.5px dashed var(--ink); background: var(--paper-light); display: flex; flex-direction: column; gap: 10px; }
.errata strong { font: 700 13px/1 var(--mono); letter-spacing: 0.14em; text-transform: uppercase; color: var(--red); }
.errata p { color: var(--ink-soft); }
.margin-notes { padding: 26px 28px; border-left: 3px double var(--ink); background: var(--paper-light); }
.margin-head { font: 600 12px/1 var(--mono); letter-spacing: 0.18em; text-transform: uppercase; color: var(--pencil); margin-bottom: 14px; }
.margin-notes ul { display: flex; flex-direction: column; gap: 14px; }
.margin-notes li { font: 400 18px/1.5 var(--serif); font-style: italic; color: var(--ink); padding-left: 22px; position: relative; }
.margin-notes li::before { content: '—'; position: absolute; left: 0; color: var(--red); }

/* Appendix / 附录 */
.plate { position: relative; padding: 40px 44px; background: var(--paper-light); border: 1px solid var(--ink); box-shadow: 6px 6px 0 var(--ink); }
.plate-rows { display: flex; flex-direction: column; }
.plate-rows > div { display: grid; grid-template-columns: 160px 1fr; gap: 20px; padding: 16px 0; border-bottom: 1px solid var(--rule-soft); align-items: start; }
.plate-rows > div:last-child { border-bottom: 0; }
.plate-rows dt { font: 600 12px/1.6 var(--mono); letter-spacing: 0.16em; text-transform: uppercase; color: var(--red); }
.plate-rows dd { font-size: 17px; display: flex; flex-wrap: wrap; align-items: center; gap: 12px; }
.plate-ca { font: 700 15px/1.5 var(--mono); word-break: break-all; }
.plate-note { font-size: 14px; color: var(--pencil); }
.plate-links { margin-top: 26px; display: flex; flex-wrap: wrap; gap: 22px; }
.plate-links a { font: 700 13px/1 var(--mono); letter-spacing: 0.08em; text-transform: uppercase; color: var(--ink); border-bottom: 2px solid var(--red); padding-bottom: 3px; }
.stamp-plate { position: absolute; top: 22px; right: 26px; font-size: 18px; padding: 8px 14px; border-color: var(--red); color: var(--red); box-shadow: inset 0 0 0 1px var(--paper-light), inset 0 0 0 3px var(--red); transform: rotate(7deg); }

/* Footer / 页尾 */
.site-footer { padding: 44px var(--gutter) 56px; background: var(--paper-deep); border-top: 1px solid var(--ink); color: var(--ink-soft); font-size: 14px; text-align: center; }
.footer-links { margin-top: 12px; display: flex; flex-wrap: wrap; justify-content: center; gap: 20px; }
.footer-links a { color: var(--ink); border-bottom: 1px solid var(--red); }

/* Reveal / 渐显 */
[data-reveal] { opacity: 0; transform: translateY(18px); transition: opacity 0.7s var(--ease), transform 0.7s var(--ease); }
[data-reveal].is-in { opacity: 1; transform: none; }

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  .running-track, .hero-art.is-running img { animation: none; }
  [data-reveal] { opacity: 1; transform: none; transition: none; }
}

/* Tablet / 平板 */
@media (max-width: 1080px) {
  .rules-grid, .enrol-grid { grid-template-columns: 1fr; }
  .rules-art { max-width: 520px; }
  .specs { grid-template-columns: repeat(2, 1fr); }
  .specs > div { border-bottom: 1px solid var(--rule); }
  .specs > div:nth-child(2n) { border-right: 0; }
  .specs > div:nth-last-child(-n+2) { border-bottom: 0; }
}

/* Phone / 手机 */
@media (max-width: 760px) {
  body { font-size: 17px; }
  .site-header { padding: 14px var(--gutter) 12px; }
  .site-nav { display: none; }
  /* Phone: copy first, then the plate as a block pulled up under it, so the whole apple is on the first screen
     手机：文字在上，图版作为块级图上提到文字下方，整颗苹果连鞋进第一屏 */
  .hero { display: flex; flex-direction: column; min-height: 0; align-items: stretch; }
  .hero-art { order: 2; position: relative; inset: auto; width: 100%; height: auto; margin-top: -44%; z-index: 0; }
  .hero-art img { position: relative; inset: auto; width: 100%; height: auto; object-fit: contain; object-position: center; -webkit-mask-image: linear-gradient(to bottom, transparent 0, #000 30%); mask-image: linear-gradient(to bottom, transparent 0, #000 30%); }
  .hero-inner { order: 1; position: relative; z-index: 1; min-height: 0; padding: 78px var(--gutter) 0; }
  .hero-copy { gap: 10px; }
  .hero-quote { font-size: 14px; }
  .wordmark { font-size: clamp(40px, 12vw, 64px); }
  .hero-lede { font-size: 18px; }
  .hero-actions .btn { padding: 11px 16px; font-size: 14px; }
  .ca-line { padding: 7px 12px; gap: 10px; }
  .ca-short { font-size: 13px; }
  .hero-note { font-size: 13px; }
  .caption { position: static; margin-top: 12px; font-size: 12px; gap: 10px; }
  .stamp-hero { position: absolute; bottom: auto; top: 70px; left: auto; right: var(--gutter); font-size: 10px; transform: rotate(6deg); }
  .ref { grid-template-columns: 44px 1fr; gap: 12px; padding: 22px 0; }
  .rules li { grid-template-columns: 48px 1fr; }
  .specs { grid-template-columns: 1fr; }
  .specs > div { border-right: 0; }
  .specs > div:nth-last-child(-n+2) { border-bottom: 1px solid var(--rule); }
  .specs > div:last-child { border-bottom: 0; }
  .plate-section { min-height: 520px; }
  .plate { padding: 28px 20px; box-shadow: 4px 4px 0 var(--ink); }
  .plate-rows > div { grid-template-columns: 1fr; gap: 6px; }
  .stamp-plate { top: 12px; right: 12px; font-size: 14px; }
  .margin-notes { padding: 20px; }
}

/* Wide screens / 宽屏（用户 3435×1309） */
@media (min-width: 2000px) {
  :root { --max: 1440px; --hero-max: 2200px; }
  body { font-size: 21px; }
  .site-nav a { font-size: 18px; }
  .hero-copy { max-width: 900px; gap: 20px; }
  .wordmark { font-size: 160px; }
  .hero-lede { font-size: 34px; }
  .btn { font-size: 18px; padding: 16px 24px; }
  .ca-short { font-size: 18px; }
  .hero-note { font-size: 18px; max-width: 640px; }
  .caption { font-size: 18px; }
  .section h2 { font-size: 70px; }
  .section-lede { font-size: 23px; max-width: 820px; }
  .ref-body p, .rules p, .margin-notes li { font-size: 21px; }
  .ref-body p { max-width: 860px; }
  .specs dd { font-size: 18px; }
  .plate-section { min-height: 780px; }
  .plate-line { font-size: 100px; }
  .plate-rows dd { font-size: 20px; }
  .plate-ca { font-size: 18px; }
}
@media (min-width: 3000px) {
  :root { --max: 1720px; --hero-max: 2600px; }
  body { font-size: 23px; }
  .hero-copy { max-width: 1080px; }
  .wordmark { font-size: 200px; }
  .hero-lede { font-size: 40px; }
  .section h2 { font-size: 84px; }
  .plate-line { font-size: 120px; }
  .ref-body p, .rules p, .margin-notes li { font-size: 23px; }
}