/* ============ 数学之窗 · 包豪斯几何风 ============ */
:root {
  --bg: #f4f0e6;
  --ink: #14120d;
  --red: #d0342c;
  --blue: #1d4e9e;
  --yellow: #e9a820;
  --soft: #6f6a5c;
  --line: #14120d;
  --disp: "ZCOOL QingKe HuangYou", "Noto Sans SC", sans-serif;
  --sans: "Noto Sans SC", "PingFang SC", sans-serif;
  --mono: "JetBrains Mono", Consolas, monospace;
}
* { margin: 0; padding: 0; box-sizing: border-box; }
html { scroll-behavior: smooth; }
body { font-family: var(--sans); background: var(--bg); color: var(--ink); line-height: 2; font-size: 17px; }
a { color: var(--blue); text-decoration-thickness: 2px; text-underline-offset: 4px; }
a:hover { color: var(--red); }
code { font-family: var(--mono); background: #fff; border: 1.5px solid var(--ink); padding: 1px 8px; font-size: .87em; }

/* 顶栏：几何条 */
.masthead {
  display: flex; justify-content: space-between; align-items: center; flex-wrap: wrap;
  padding: 0 5vw; border-bottom: 3px solid var(--ink); background: var(--bg);
}
.masthead .logo {
  display: flex; align-items: center; gap: 14px; padding: 14px 0; text-decoration: none;
}
.masthead .logo .shapes { display: flex; gap: 5px; }
.masthead .logo .sq { width: 18px; height: 18px; background: var(--red); }
.masthead .logo .ci { width: 18px; height: 18px; border-radius: 50%; background: var(--blue); }
.masthead .logo .tri { width: 0; height: 0; border-left: 10px solid transparent; border-right: 10px solid transparent; border-bottom: 18px solid var(--yellow); }
.masthead .logo span { font-family: var(--disp); font-size: 24px; color: var(--ink); letter-spacing: 4px; }
.masthead .sub { font-family: var(--mono); font-size: 12px; color: var(--soft); }
nav.dims {
  display: flex; flex-wrap: wrap; border-bottom: 3px solid var(--ink);
  background: var(--bg); position: sticky; top: 0; z-index: 50;
}
nav.dims a {
  flex: 1 1 auto; text-align: center; padding: 12px 10px;
  font-family: var(--disp); font-size: 16px; letter-spacing: 3px; color: var(--ink); text-decoration: none;
  border-right: 3px solid var(--ink);
}
nav.dims a:last-child { border-right: none; }
nav.dims a:hover, nav.dims a.active { background: var(--ink); color: var(--yellow); }
nav.dims a:nth-child(2):hover, nav.dims a:nth-child(2).active { color: var(--red); }
nav.dims a:nth-child(3):hover, nav.dims a:nth-child(3).active { color: var(--blue); }

/* 页头：几何构成 */
.hero { border-bottom: 3px solid var(--ink); position: relative; overflow: hidden; }
.hero .inner { max-width: 1100px; margin: 0 auto; padding: 10vh 5vw 8vh; position: relative; z-index: 2; }
.hero .dim-tag {
  display: inline-block; font-family: var(--mono); font-size: 13px; letter-spacing: 3px;
  border: 2px solid var(--ink); padding: 4px 16px; margin-bottom: 28px; background: #fff;
}
h1.display {
  font-family: var(--disp); font-weight: 400;
  font-size: clamp(40px, 7vw, 84px); line-height: 1.15; letter-spacing: 2px; max-width: 16em;
}
h1.display .r { color: var(--red); } h1.display .b { color: var(--blue); } h1.display .y { background: linear-gradient(transparent 62%, var(--yellow) 62%); }
.standfirst { margin-top: 24px; max-width: 44em; font-size: 18px; color: var(--soft); border-top: 2px solid var(--ink); padding-top: 16px; }
.byline { margin-top: 16px; font-family: var(--mono); font-size: 13px; color: var(--soft); }
.hero .deco { position: absolute; z-index: 1; opacity: .9; }
.hero .deco.c1 { width: 180px; height: 180px; border-radius: 50%; background: var(--red); right: -50px; top: -50px; }
.hero .deco.c2 { width: 90px; height: 90px; background: var(--yellow); right: 160px; bottom: 40px; transform: rotate(12deg); }
.hero .deco.c3 { width: 0; height: 0; border-left: 70px solid transparent; border-right: 70px solid transparent; border-bottom: 120px solid var(--blue); left: -60px; bottom: -30px; transform: rotate(18deg); }

/* 正文 */
main.body { max-width: 880px; margin: 0 auto; padding: 64px 5vw 40px; }
section.chapter { margin-bottom: 76px; }
.sec-tag {
  display: inline-block; font-family: var(--mono); font-size: 12.5px; letter-spacing: 3px;
  background: var(--ink); color: var(--bg); padding: 3px 12px; margin-bottom: 12px;
}
.sec-tag.red { background: var(--red); } .sec-tag.blue { background: var(--blue); } .sec-tag.yellow { background: var(--yellow); color: var(--ink); }
h2 { font-family: var(--disp); font-weight: 400; font-size: clamp(26px, 3.4vw, 38px); line-height: 1.3; margin-bottom: 20px; }
h3 { font-family: var(--disp); font-size: 21px; margin: 34px 0 12px; display: flex; align-items: center; gap: 10px; }
h3::before { content: ""; width: 14px; height: 14px; background: var(--red); flex: none; }
h3.b::before { background: var(--blue); border-radius: 50%; }
h3.y::before { background: var(--yellow); transform: rotate(45deg); border-radius: 3px; }
p { margin-bottom: 18px; }
ul, ol { padding-left: 1.7em; margin-bottom: 20px; }
li { margin-bottom: 10px; }
li::marker { color: var(--red); font-weight: 700; }
strong { background: linear-gradient(transparent 60%, rgba(233,168,32,.45) 60%); }

/* 观察窗 */
.window {
  margin: 30px 0; border: 3px solid var(--ink); background: #fff; padding: 22px 26px 18px;
  position: relative; box-shadow: 8px 8px 0 var(--yellow);
}
.window::before {
  content: attr(data-label); position: absolute; top: -16px; left: 18px;
  font-family: var(--disp); font-size: 15px; letter-spacing: 2px;
  background: var(--ink); color: var(--yellow); padding: 2px 16px;
}
.window p { margin: 0; }

/* 双栏对比 */
.compare { display: grid; grid-template-columns: 1fr 1fr; gap: 0; border: 3px solid var(--ink); margin: 28px 0; }
.compare > div { padding: 18px 20px; }
.compare > div:first-child { border-right: 3px solid var(--ink); background: #fff; }
.compare > div:last-child { background: #fdf6e3; }
.compare h4 { font-family: var(--disp); font-size: 17px; margin-bottom: 8px; letter-spacing: 1px; }
.compare .before h4 { color: var(--red); } .compare .after h4 { color: var(--blue); }
.compare p { font-size: 14.5px; margin: 0; color: var(--soft); line-height: 1.8; }
@media (max-width: 640px) { .compare { grid-template-columns: 1fr; } .compare > div:first-child { border-right: none; border-bottom: 3px solid var(--ink); } }

/* 时间线 */
ol.timeline { list-style: none; padding: 0; position: relative; margin: 30px 0; }
ol.timeline::before { content: ""; position: absolute; left: 89px; top: 0; bottom: 0; width: 3px; background: var(--ink); }
ol.timeline li { position: relative; padding: 0 0 28px 130px; margin: 0; }
ol.timeline li::before {
  content: ""; position: absolute; left: 82px; top: 8px;
  width: 16px; height: 16px; background: var(--red); transform: rotate(45deg);
}
ol.timeline li:nth-child(3n)::before { background: var(--blue); border-radius: 50%; transform: none; }
ol.timeline li:nth-child(3n+2)::before { background: var(--yellow); transform: none; }
ol.timeline .era { position: absolute; left: 0; top: 2px; width: 68px; text-align: right; font-family: var(--mono); font-size: 13px; font-weight: 700; color: var(--ink); }
ol.timeline b { font-family: var(--disp); font-size: 17px; font-weight: 400; display: block; }
ol.timeline p { font-size: 14.5px; color: var(--soft); margin: 4px 0 0; line-height: 1.8; }

/* 步骤 */
ol.steps { counter-reset: s; list-style: none; padding: 0; }
ol.steps li { counter-increment: s; position: relative; padding: 16px 18px 16px 60px; border-left: 3px solid var(--ink); margin-bottom: 6px; }
ol.steps li::before {
  content: counter(s); position: absolute; left: -17px; top: 15px;
  width: 32px; height: 32px; background: var(--blue); color: #fff;
  font-family: var(--disp); font-size: 15px; display: flex; align-items: center; justify-content: center;
}
ol.steps li b { font-family: var(--disp); font-weight: 400; display: block; font-size: 17px; }

/* 表格 */
table { width: 100%; border-collapse: collapse; margin: 26px 0; font-size: 14.5px; background: #fff; }
th { background: var(--ink); color: var(--bg); text-align: left; padding: 10px 14px; font-family: var(--disp); font-weight: 400; letter-spacing: 1px; }
td { border: 2px solid var(--ink); padding: 10px 14px; vertical-align: top; }

/* FAQ */
.faq details { border: 2px solid var(--ink); background: #fff; padding: 14px 18px; margin-bottom: 12px; }
.faq summary { font-family: var(--disp); font-size: 17px; cursor: pointer; }
.faq summary::marker { color: var(--red); }
.faq details p { margin: 10px 0 2px; color: var(--soft); }

/* 互链 */
.crosslinks { border-top: 3px solid var(--ink); padding-top: 36px; }
.dim-cards { display: grid; grid-template-columns: repeat(auto-fit, minmax(240px, 1fr)); gap: 0; border: 3px solid var(--ink); margin-top: 20px; }
.dim-cards a {
  display: block; text-decoration: none; color: var(--ink); padding: 22px 20px;
  border-right: 3px solid var(--ink); background: var(--bg); transition: background .15s;
}
.dim-cards a:last-child { border-right: none; }
.dim-cards a:hover { background: #fff; color: var(--ink); }
.dim-cards .no { font-family: var(--mono); font-size: 12px; color: var(--red); letter-spacing: 2px; }
.dim-cards .ti { font-family: var(--disp); font-size: 19px; margin: 6px 0; }
.dim-cards .de { font-size: 13px; color: var(--soft); line-height: 1.7; }

/* 首页三大窗 */
.triptych { display: grid; grid-template-columns: repeat(auto-fit, minmax(280px, 1fr)); border-top: 3px solid var(--ink); }
.pane {
  display: block; text-decoration: none; color: var(--ink); padding: 44px 32px 36px;
  border-bottom: 3px solid var(--ink); border-right: 3px solid var(--ink); position: relative;
  transition: background .2s;
}
.pane:hover { background: #fff; color: var(--ink); }
.pane .glyph { width: 64px; height: 64px; margin-bottom: 22px; }
.pane .glyph.sq { background: var(--red); }
.pane .glyph.ci { background: var(--blue); border-radius: 50%; }
.pane .glyph.tri { width: 0; height: 0; border-left: 38px solid transparent; border-right: 38px solid transparent; border-bottom: 66px solid var(--yellow); }
.pane .kicker { font-family: var(--mono); font-size: 12px; letter-spacing: 3px; color: var(--soft); }
.pane h2 { font-family: var(--disp); font-size: 30px; margin: 8px 0 12px; }
.pane p { font-size: 14.5px; color: var(--soft); line-height: 1.85; }
.pane .more { display: inline-block; margin-top: 16px; font-family: var(--disp); font-size: 15px; letter-spacing: 2px; color: var(--blue); }
.pane:hover .more { color: var(--red); }

footer {
  padding: 26px 5vw 44px; font-size: 13px; color: var(--soft); font-family: var(--mono);
  display: flex; justify-content: space-between; flex-wrap: wrap; gap: 10px;
}
footer nav a { margin-right: 16px; color: var(--soft); }

@keyframes rise { from { opacity: 0; transform: translateY(16px); } to { opacity: 1; transform: none; } }
.pane, .hero .inner > *, section.chapter > * { animation: rise .55s ease both; }
@media (prefers-reduced-motion: reduce) { * { animation: none !important; } }