:root {
  color-scheme: light;
  --ink: #1d302e;
  --ink-soft: #3b504c;
  --muted: #687a75;
  --faint: #9aaba6;
  --teal: #087f79;
  --teal-dark: #06635e;
  --teal-soft: #e5f2ef;
  --amber: #e29a14;
  --line: #dfe7e3;
  --page: #f5f7f5;
  --paper: #fff;
  --content: 880px;
  --math: "STIX Two Math", "Cambria Math", "Latin Modern Math", "Times New Roman", serif;
  font-family: "PingFang SC", "Hiragino Sans GB", "Microsoft YaHei", system-ui, sans-serif;
}
* { box-sizing: border-box; }
body {
  margin: 0;
  min-height: 100vh;
  background: radial-gradient(1000px 440px at 88% -160px, #dff0eb 0, rgba(223, 240, 235, 0) 70%), var(--page);
  color: var(--ink);
  line-height: 1.6;
  -webkit-font-smoothing: antialiased;
}
a { color: inherit; text-decoration: none; -webkit-tap-highlight-color: transparent; }
a:focus-visible { outline: 3px solid #82cfc7; outline-offset: 4px; border-radius: 8px; }
var { font-family: var(--math); font-style: italic; font-size: 1.1em; }
.nowrap { white-space: nowrap; }
.skip-link { position: absolute; top: -80px; left: 20px; z-index: 2; padding: 12px; background: white; }
.skip-link:focus { top: 12px; }


.workspace { max-width: calc(var(--content) + 56px); margin: auto; padding: 0 28px; }

.hero { padding: 44px 0 36px; }
.eyebrow { display: flex; align-items: center; gap: 10px; margin: 0 0 12px; font-size: 13px; letter-spacing: .06em; color: var(--muted); }
.eyebrow::before { content: ""; width: 6px; height: 6px; border-radius: 50%; background: var(--amber); }
.eyebrow-sep { color: var(--faint); }
.eyebrow a { color: var(--teal-dark); text-decoration: underline; text-decoration-color: #b9dcd3; text-underline-offset: 4px; transition: color .15s, text-decoration-color .15s; }
.eyebrow a:hover { color: var(--teal); text-decoration-color: currentColor; }
h1 { margin: 0; font-size: clamp(32px, 5vw, 44px); line-height: 1.2; font-weight: 800; letter-spacing: .01em; }
.title-mark { background: linear-gradient(transparent 64%, #cfe9e2 64%, #cfe9e2 92%, transparent 92%); padding: 0 2px; }
.lead { margin: 12px 0 20px; color: var(--muted); font-size: 15px; }

.formula-strip { display: inline-flex; flex-wrap: wrap; align-items: center; gap: 6px 16px; margin: 0; padding: 8px 16px 8px 8px; border: 1px solid var(--line); border-radius: 12px; background: rgba(255, 255, 255, .75); }
.formula-label { padding: 2px 8px; border-radius: 7px; background: var(--teal-soft); color: var(--teal-dark); font-size: 12px; font-weight: 600; letter-spacing: .06em; }
.formula { display: inline-flex; align-items: center; gap: 8px; color: var(--ink); }
.formula math { font-family: var(--math); font-size: 17px; math-style: normal; }
.formula-cond { color: var(--muted); font-size: 13px; }
.formula-tips { padding-left: 16px; border-left: 1px solid var(--line); color: var(--teal-dark); font-size: 13px; font-weight: 600; letter-spacing: .04em; }

.bank-heading { display: flex; align-items: baseline; justify-content: space-between; gap: 16px; margin-bottom: 16px; }
h2 { margin: 0; font-size: 20px; font-weight: 750; }
.bank-count { margin: 0; color: var(--muted); font-size: 13px; font-variant-numeric: tabular-nums; }
.bank-count strong { margin: 0 2px; color: var(--teal); font-size: 18px; font-weight: 750; }

html { scroll-behavior: smooth; }

.toc { margin-bottom: 16px; padding: 18px 22px 14px; border: 1px solid var(--line); border-radius: 18px; background: rgba(255, 255, 255, .8); scroll-margin-top: 16px; }
.toc-heading { display: flex; align-items: baseline; justify-content: space-between; gap: 12px; margin-bottom: 6px; }
.toc-heading h3 { margin: 0; font-size: 15px; font-weight: 700; letter-spacing: .04em; }
.toc-meta { color: var(--faint); font-size: 12px; }
.toc-list { margin: 0; padding: 0; list-style: none; columns: 2; column-gap: 40px; }
.toc-list li { break-inside: avoid; }
.toc-link { display: flex; align-items: center; gap: 10px; min-height: 44px; color: var(--ink); font-size: 15px; transition: color .15s; }
.toc-index { flex-shrink: 0; padding: 4px 7px; border-radius: 7px; background: var(--teal-soft); color: var(--teal); font: 600 12px/1 ui-monospace, "SF Mono", Menlo, Consolas, monospace; letter-spacing: .04em; transition: background .15s, color .15s; }
.toc-name { flex-shrink: 0; font-weight: 600; }
.toc-leader { flex: 1; min-width: 16px; height: 0; margin-top: 4px; border-bottom: 1.5px dotted #c3d2cd; transition: border-color .15s; }
.toc-count { flex-shrink: 0; color: var(--muted); font-size: 13px; font-variant-numeric: tabular-nums; }
.toc-link:hover { color: var(--teal); }
.toc-link:hover .toc-index { background: var(--teal); color: #fff; }
.toc-link:hover .toc-leader { border-color: #8fc8bc; }
.toc-return { position: fixed; right: max(20px, env(safe-area-inset-right)); bottom: max(20px, env(safe-area-inset-bottom)); z-index: 3; display: inline-flex; align-items: center; gap: 6px; padding: 9px 14px; border: 1px solid #cfe0db; border-radius: 999px; background: rgba(255, 255, 255, .94); color: var(--teal-dark); font-size: 13px; font-weight: 600; box-shadow: 0 10px 24px -14px rgba(8, 80, 75, .55); backdrop-filter: blur(6px); transition: background .15s, color .15s; }
.toc-return[hidden] { display: none; }
.toc-return:hover { background: var(--teal); color: #fff; }

.type-group { scroll-margin-top: 16px; padding: 20px; border: 1px solid var(--line); border-radius: 18px; background: var(--paper); box-shadow: 0 18px 40px -32px rgba(20, 60, 55, .35); transition: border-color .3s, box-shadow .3s; }
.type-group:target { border-color: #9fd0c6; box-shadow: 0 0 0 3px rgba(8, 127, 121, .1), 0 18px 40px -32px rgba(20, 60, 55, .35); }
.type-heading { display: flex; align-items: flex-start; gap: 12px; margin-bottom: 16px; }
.type-index { flex-shrink: 0; margin-top: 2px; padding: 5px 8px; border-radius: 8px; background: var(--teal-soft); color: var(--teal); font: 600 13px/1 ui-monospace, "SF Mono", Menlo, Consolas, monospace; letter-spacing: .04em; }
.type-title { flex: 1; min-width: 0; }
.type-title h3 { margin: 0; font-size: 17px; font-weight: 700; line-height: 1.5; }
.type-hint { margin: 2px 0 0; color: var(--muted); font-size: 13px; }
.type-count { flex-shrink: 0; margin-top: 3px; color: var(--muted); font-size: 12px; }
.type-empty { margin: -4px 0 0 46px; padding: 12px 16px; border: 1px dashed #cfdcd7; border-radius: 14px; background: #fafcfb; color: var(--faint); font-size: 13px; }

.type-side { flex-shrink: 0; display: flex; align-items: center; gap: 10px; }
.type-side .type-count { margin-top: 0; }
.method-chip { display: inline-flex; align-items: center; gap: 5px; min-height: 30px; padding: 4px 11px 4px 9px; border: 1px solid #cfe3de; border-radius: 999px; background: var(--teal-soft); color: var(--teal-dark); font: inherit; font-size: 12.5px; font-weight: 600; cursor: pointer; transition: background .15s, color .15s, border-color .15s; }
.method-chip:hover { border-color: var(--teal); background: var(--teal); color: #fff; }
.method-chip:focus-visible { outline: 3px solid #82cfc7; outline-offset: 2px; }
.method-chip[hidden] { display: none; }

.problem-list { display: grid; gap: 10px; margin: 0; padding: 0; list-style: none; }
.problem-card { display: flex; align-items: center; gap: 16px; padding: 16px 16px 16px 18px; border: 1px solid #d3e2dd; border-radius: 14px; background: #fff; box-shadow: 0 1px 2px rgba(20, 50, 45, .05), 0 6px 16px -12px rgba(20, 60, 55, .3); cursor: pointer; transition: border-color .15s, background .15s, box-shadow .15s, transform .15s; }
.problem-card:hover { border-color: #8fc8bc; background: #fbfefd; box-shadow: 0 14px 28px -18px rgba(8, 127, 121, .6); transform: translateY(-1px); }
.problem-card:active { background: #f2f9f7; box-shadow: 0 1px 2px rgba(20, 50, 45, .06); transform: none; }
.problem-number { flex-shrink: 0; display: grid; place-items: center; width: 44px; height: 44px; border: 1.5px solid #c7d6d1; border-radius: 50%; background: #fff; color: var(--ink-soft); font-size: 16px; font-weight: 600; font-variant-numeric: tabular-nums; transition: background .15s, border-color .15s, color .15s; }
.problem-card:hover .problem-number { border-color: var(--teal); background: var(--teal); color: #fff; }
.problem-stem { flex: 1; min-width: 0; color: var(--ink); font-size: 16px; line-height: 1.7; }
.problem-go { flex-shrink: 0; display: grid; place-items: center; width: 32px; height: 32px; border-radius: 50%; background: var(--teal-soft); color: var(--teal); transition: background .15s, color .15s, transform .15s; }
.problem-go svg { display: block; }
.problem-card:hover .problem-go { background: var(--teal); color: #fff; transform: translateX(2px); }
.visually-hidden { position: absolute; width: 1px; height: 1px; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; }

.chapter-footer { display: flex; align-items: center; justify-content: space-between; margin-top: 48px; padding: 20px 2px 84px; border-top: 1px solid var(--line); color: var(--muted); font-size: 12px; }
.chapter-footer a { padding: 10px 0 10px 10px; letter-spacing: .03em; transition: color .15s; }
.chapter-footer a:hover { color: var(--teal); }

@media (max-width: 720px) {
  .workspace { padding: 0 20px; }
  .hero { padding: 28px 0 26px; }
  .lead { font-size: 14px; margin-bottom: 16px; }
  .formula-strip { display: flex; }
  .formula-tips { flex-basis: 100%; padding-left: 0; border-left: 0; }
  .toc { padding: 14px 16px 8px; border-radius: 16px; }
  .toc-list { columns: 1; }
  .toc-link { min-height: 42px; font-size: 14.5px; }
  .type-group { padding: 16px; border-radius: 16px; }
  .type-side { flex-direction: column; align-items: flex-end; gap: 6px; }
  .type-empty { margin-left: 0; }
  .problem-card { gap: 12px; padding: 14px 12px 14px 14px; }
  .problem-number { width: 40px; height: 40px; font-size: 15px; }
  .problem-stem { font-size: 15px; }
  .problem-go { width: 28px; height: 28px; }
  .chapter-footer { margin-top: 36px; }
}
@media (max-width: 380px) {
  .workspace { padding: 0 14px; }
}
@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  .problem-card, .problem-number, .problem-go { transition: none; }
  .problem-card:hover, .problem-card:hover .problem-go { transform: none; }
}