/* InkTeach 官网样式：单页、浅色、红色点缀（与产品工具带同色） */
:root {
  --red: #e23b3b;
  --ink: #1f2328;
  --gray: #57606a;
  --line: #e6e8ec;
  --bg: #f3f6fb;
  --card: #ffffff;
}
* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  font-family: "Microsoft YaHei", "PingFang SC", "Segoe UI", system-ui, sans-serif;
  color: var(--ink);
  background: linear-gradient(#f3f6fb, #ffffff 360px);
  line-height: 1.7;
}
.wrap { max-width: 1080px; margin: 0 auto; padding: 0 20px; }
a { color: inherit; }

/* 顶栏 */
.topbar {
  position: sticky; top: 0; z-index: 10;
  background: rgba(255,255,255,.92);
  backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--line);
}
.topbar .wrap { display: flex; align-items: center; justify-content: space-between; height: 56px; }
.brand { font-weight: 800; font-size: 20px; text-decoration: none; letter-spacing: .2px; }
.brand img { width: 24px; height: 24px; border-radius: 7px; vertical-align: -5px; margin-right: 8px;
  box-shadow: 0 1px 3px rgba(31,35,40,.18); }
.topbar nav a { margin-left: 18px; text-decoration: none; color: var(--gray); font-size: 14px; }
.topbar nav a:hover { color: var(--ink); }

/* 首屏 */
.hero { padding: 56px 0 24px; text-align: center; }
.hero-icon { display: block; width: 84px; height: 84px; margin: 0 auto 16px; border-radius: 19px;
  box-shadow: 0 12px 30px rgba(31,35,40,.14); }
.hero h1 { font-size: 42px; margin: 0 0 14px; letter-spacing: 1px; }
.pill { display: inline-block; margin: 0 auto 12px; padding: 4px 13px; border-radius: 999px;
  background: #fff0f0; border: 1px solid #ffd4d4; color: #b4232b; font-size: 13px; font-weight: 600; }
.hero .lead { font-size: 17px; color: var(--gray); max-width: 760px; margin: 0 auto 22px; }
.hero .lead b { color: var(--ink); }
.hero .btns { margin: 6px 0 10px; }
.hero-shot { margin: 30px auto 6px; max-width: 860px; }
.hero-shot img { width: 100%; height: auto; border-radius: 14px;
  box-shadow: 0 18px 50px rgba(31,35,40,.14), 0 0 0 1px var(--line); background: #fff; }
.caption { color: var(--gray); font-size: 13px; }
.note { color: var(--gray); font-size: 13px; }
.center { text-align: center; }

/* 按钮 */
.btn { display: inline-block; padding: 10px 20px; margin: 4px 6px 4px 0; border-radius: 999px;
  border: 1px solid var(--line); background: #fff; color: var(--ink);
  text-decoration: none; font-size: 15px; box-shadow: 0 1px 2px rgba(0,0,0,.04); }
.btn:hover { border-color: #cfd4da; }
.btn.primary { background: var(--red); border-color: var(--red); color: #fff; font-weight: 600; }
.btn.primary:hover { filter: brightness(1.05); }
.btn.ghost { background: transparent; box-shadow: none; }

/* 区块 */
.section { padding: 54px 0; }
.section.alt { background: #f7f9fc; border-top: 1px solid var(--line); border-bottom: 1px solid var(--line); }
.section h2 { font-size: 26px; margin: 0 0 8px; }
.section h3.h3 { margin-top: 28px; }
.lead.small { font-size: 15px; color: var(--gray); }

/* 卡片 */
.grid { display: grid; grid-template-columns: repeat(3, 1fr); gap: 16px; margin-top: 24px; }
.grid.two { grid-template-columns: repeat(2, 1fr); }
.card { background: var(--card); border: 1px solid var(--line); border-radius: 14px; padding: 18px 20px;
  box-shadow: 0 4px 14px rgba(31,35,40,.05); }
.card h3 { margin: 0 0 8px; font-size: 17px; }
.card p { margin: 0 0 10px; color: var(--gray); font-size: 14px; }
.card code { background: #eef1f5; padding: 1px 6px; border-radius: 6px; font-size: 13px; }

/* 数学图形 */
.math-grid { display: grid; grid-template-columns: repeat(2, 1fr); gap: 16px; margin-top: 24px; }
.math-grid figure:nth-child(3) { grid-column: 1 / -1; }
.math-grid figure { margin: 0; background: #fff; border: 1px solid var(--line); border-radius: 14px;
  padding: 12px; box-shadow: 0 4px 14px rgba(31,35,40,.05); }
.math-grid img { width: 100%; display: block; border-radius: 10px; border: 1px solid #edf0f4; }
.math-grid figcaption { margin-top: 10px; color: var(--gray); font-size: 13px; line-height: 1.65; }
.math-grid figcaption b { color: var(--ink); }

/* 截图 */
.shots { display: grid; gap: 18px; margin-top: 22px; }
.shots.two { grid-template-columns: repeat(2, 1fr); }
.shots figure { margin: 0; }
.shots img { width: 100%; border-radius: 12px; border: 1px solid var(--line); background: #fff;
  box-shadow: 0 8px 24px rgba(31,35,40,.08); }
.shots figcaption { text-align: center; color: var(--gray); font-size: 13px; margin-top: 8px; }
img.dark { max-width: 860px; width: 100%; border-radius: 12px; border: 1px solid var(--line); }

/* 表格 */
table { width: 100%; border-collapse: collapse; margin-top: 18px; background: #fff;
  border: 1px solid var(--line); border-radius: 12px; overflow: hidden; }
th, td { text-align: left; padding: 11px 16px; border-bottom: 1px solid var(--line); font-size: 14px; }
th { background: #f3f6fb; font-weight: 600; }
tr:last-child td { border-bottom: 0; }
table.keys td:first-child { width: 320px; white-space: nowrap; }
kbd { background: #eef1f5; border: 1px solid #d7dce3; border-bottom-width: 2px;
  border-radius: 6px; padding: 1px 7px; font-size: 12px; font-family: Consolas, monospace; }

/* 页脚 */
footer { border-top: 1px solid var(--line); padding: 26px 0 40px; color: var(--gray); font-size: 13px; text-align: center; }
footer a { color: var(--gray); }
footer .en { color: #8a939d; }

/* ---- 特色条目（编号 + 图 + 说明）---- */
.feat { display: grid; grid-template-columns: repeat(2, 1fr); gap: 18px; margin-top: 24px; }
.feat article { background: var(--card); border: 1px solid var(--line); border-radius: 14px;
  padding: 18px 20px; box-shadow: 0 4px 14px rgba(31,35,40,.05); }
.feat h3 { margin: 0 0 8px; font-size: 17px; display: flex; align-items: baseline; gap: 10px; }
.feat .num { font-size: 13px; font-weight: 700; color: var(--red);
  background: rgba(226,59,59,.10); border-radius: 999px; padding: 2px 9px; letter-spacing: .5px; }
.feat p { margin: 0 0 8px; color: var(--gray); font-size: 14px; }
.feat p b { color: var(--ink); }
.feat .wide { grid-column: 1 / -1; }
.feat .shot { margin-top: 12px; }
.feat .shot img { width: 100%; border-radius: 10px; border: 1px solid var(--line);
  background: #fff; box-shadow: 0 6px 18px rgba(31,35,40,.07); }
.feat .shot.dark img { background: #1c1e21; }
.feat .cap { text-align: center; color: var(--gray); font-size: 12.5px; margin-top: 7px; }

/* 深色展示带 */
.darkband { background: #14161a; border-radius: 16px; padding: 26px 26px 20px; margin-top: 24px; }
.darkband h3 { color: #f2f4f7; margin: 0 0 4px; font-size: 18px; }
.darkband .note { color: #98a1ad; }
.darkband .pair { display: grid; grid-template-columns: repeat(2, 1fr); gap: 16px; margin-top: 16px; }
.darkband .pair.three { grid-template-columns: repeat(3, 1fr); }
.darkband .pair img { width: 100%; border-radius: 10px; display: block; }
.darkband .cap { text-align: center; color: #98a1ad; font-size: 12.5px; margin-top: 8px; }

/* "为什么"小节 */
.why { margin-top: 22px; }
.why h4 { margin: 26px 0 6px; font-size: 17px; }
.why p { color: var(--gray); font-size: 14.5px; margin: 0 0 8px; }
.why p b { color: var(--ink); }
.why code { background: #eef1f5; padding: 1px 6px; border-radius: 6px; font-size: 13px; }
.honest { border-left: 3px solid var(--red); background: #fff; padding: 14px 18px;
  border-radius: 0 12px 12px 0; box-shadow: 0 4px 14px rgba(31,35,40,.05); margin-top: 18px; }
.honest h4 { margin: 0 0 6px; font-size: 15.5px; }
.honest ul { margin: 0; padding-left: 20px; color: var(--gray); font-size: 14px; }
.honest li { margin-bottom: 4px; }

@media (max-width: 820px) {
  .grid, .grid.two, .shots.two, .feat, .math-grid, .darkband .pair { grid-template-columns: 1fr; }
  .hero h1 { font-size: 30px; }
  .topbar nav a { margin-left: 10px; font-size: 13px; }
  table.keys td:first-child { width: auto; }
}