:root {
  --ink: #1d1d1f;
  --text: #424245;
  --muted: #747478;
  --line: #e7e7ea;
  --code: #f5f5f7;
  --white: #ffffff;
  --display: "Avenir Next", "SF Pro Display", "Helvetica Neue", "PingFang SC", "Noto Sans CJK SC", sans-serif;
  --body: "SF Pro Text", "Avenir Next", "Segoe UI Variable", "PingFang SC", "Noto Sans CJK SC", "Microsoft YaHei", sans-serif;
  --mono: "SFMono-Regular", Consolas, "Liberation Mono", monospace;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; scroll-padding-top: 32px; }
body {
  margin: 0;
  color: var(--text);
  background: var(--white);
  font-family: var(--body);
  font-size: 16px;
  font-weight: 400;
  line-height: 1.82;
  text-rendering: optimizeLegibility;
  -webkit-font-smoothing: antialiased;
}
a { color: inherit; text-decoration: none; }
a:focus-visible, summary:focus-visible { outline: 2px solid var(--ink); outline-offset: 5px; }
main { width: min(880px, calc(100% - 48px)); margin: 0 auto; }
.cover { padding: clamp(86px, 12vw, 124px) 0 clamp(108px, 15vw, 158px); }
.cover-brand { display: flex; align-items: center; gap: 15px; }
.brand-logo { display: block; width: 40px; height: 40px; flex: 0 0 auto; }
.cover h1 {
  margin: 0;
  color: var(--ink);
  font-family: var(--display);
  font-size: clamp(29px, 4vw, 34px);
  font-weight: 600;
  letter-spacing: -0.035em;
  line-height: 1.1;
}
.cover p { margin: 13px 0 0 55px; color: var(--muted); font-size: 16px; letter-spacing: 0.01em; }
.chapter { padding: clamp(78px, 10vw, 108px) 0; border-top: 1px solid var(--line); }
.chapter h2, .chapter h3 { color: var(--ink); font-family: var(--display); }
.chapter h2 {
  max-width: 720px;
  margin: 0 0 44px;
  font-size: clamp(28px, 4.2vw, 34px);
  font-weight: 600;
  letter-spacing: -0.03em;
  line-height: 1.28;
}
.chapter h3 { margin: 68px 0 20px; font-size: 20px; font-weight: 600; letter-spacing: -0.015em; line-height: 1.4; }
.chapter p { max-width: 760px; margin: 22px 0; }
.chapter strong { color: var(--ink); font-weight: 600; }
.chapter ol, .chapter ul { max-width: 780px; margin: 22px 0; padding-left: 24px; }
.chapter li { margin: 8px 0; padding-left: 5px; }
.term-note { color: var(--muted); font-size: 14px; }
code {
  padding: 0.16em 0.4em;
  border-radius: 4px;
  color: #333338;
  background: var(--code);
  font-family: var(--mono);
  font-size: 0.86em;
  overflow-wrap: anywhere;
}
.table-wrap { width: 100%; margin: 38px 0 42px; overflow-x: auto; }
table { width: 100%; min-width: 660px; border-collapse: collapse; font-size: 14px; line-height: 1.62; }
th, td { padding: 16px 14px; border-bottom: 1px solid var(--line); text-align: left; vertical-align: top; }
th { border-top: 1px solid var(--ink); color: var(--ink); font-weight: 600; white-space: nowrap; }
td:first-child { width: 20%; }
.faq-list details { border-top: 1px solid var(--line); }
.faq-list details:last-child { border-bottom: 1px solid var(--line); }
.faq-list summary {
  position: relative;
  padding: 24px 42px 24px 0;
  cursor: pointer;
  color: var(--ink);
  font-size: 16px;
  font-weight: 600;
  list-style: none;
}
.faq-list summary::-webkit-details-marker { display: none; }
.faq-list summary::after { position: absolute; top: 22px; right: 0; color: var(--muted); content: "+"; font-size: 22px; font-weight: 300; line-height: 1; }
.faq-list details[open] summary::after { content: "−"; }
.faq-list details p { max-width: 760px; margin: -2px 0 28px; color: var(--muted); }
footer { display: flex; justify-content: space-between; padding: 46px 0 66px; border-top: 1px solid var(--line); color: var(--muted); font-size: 12px; }
footer a { color: var(--ink); font-weight: 500; }

@media (max-width: 640px) {
  body { font-size: 15.5px; line-height: 1.78; }
  main { width: min(100% - 36px, 880px); }
  .cover { padding-top: 70px; padding-bottom: 98px; }
  .brand-logo { width: 35px; height: 35px; }
  .cover-brand { gap: 12px; }
  .cover h1 { font-size: 26px; }
  .cover p { margin-top: 10px; margin-left: 47px; font-size: 15px; }
  .chapter { padding: 72px 0; }
  .chapter h2 { margin-bottom: 36px; font-size: 27px; }
  .chapter h3 { margin-top: 56px; font-size: 19px; }
  .table-wrap { margin-right: -18px; width: calc(100% + 18px); }
  th, td { padding: 14px 12px; }
  footer { flex-direction: column; gap: 10px; }
}

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
}