:root{--blue:#2E4BB8;--ink:#1C2230;--muted:#4A5366;--line:#E1E4EA;--bg:#F4F5F8;--ok:#1E8A5A;--err:#B3261E}
*{box-sizing:border-box}
html{-webkit-text-size-adjust:100%}
body{margin:0;background:var(--bg);color:var(--ink);font:16px/1.55 "Be Vietnam Pro",system-ui,sans-serif}
h1{font-size:24px;line-height:1.3;margin:0}
h2{font-size:16px;line-height:1.4;margin:0 0 8px}
a{color:var(--blue)}
.brand{font-weight:700;color:var(--ink);text-decoration:none}
.btn{display:inline-flex;align-items:center;justify-content:center;min-height:48px;padding:0 24px;border:0;border-radius:10px;background:var(--blue);color:#fff;font-weight:600;font-size:16px;line-height:1;font-family:inherit;text-decoration:none;cursor:pointer}
.btn:disabled{opacity:.6;cursor:default}
.link{min-height:44px;padding:0;border:0;background:none;color:var(--blue);font-weight:500;font-size:14px;line-height:1.3;font-family:inherit;cursor:pointer}
.link:disabled{color:#6B7385;cursor:default}
.sub{margin:0;color:var(--muted)}
.msg{margin:0;min-height:22px;font-size:14px;color:var(--err)}
.card{background:#fff;border:1px solid var(--line);border-radius:16px;padding:32px;display:flex;flex-direction:column;gap:14px}
.card[hidden]{display:none}
.login{max-width:440px;margin:0 auto;padding:64px 16px}
.login label{font-size:14px}
.login input{min-height:48px;width:100%;padding:0 14px;border:1px solid #C8CDD6;border-radius:10px;font-size:16px;font-family:inherit;color:var(--ink)}
.login input:focus{outline:2px solid var(--blue);outline-offset:1px}
#code{font-size:22px;letter-spacing:.3em;text-align:center}
.row{display:flex;flex-wrap:wrap;justify-content:space-between;gap:4px 16px}
.help{text-align:center;font-size:14px;color:var(--muted)}
.top{display:flex;flex-wrap:wrap;align-items:center;justify-content:space-between;gap:4px 24px;padding:8px 16px;background:#fff;border-bottom:1px solid var(--line)}
.who{display:flex;align-items:center;gap:16px;font-size:14px;min-width:0}
.who span{overflow-wrap:anywhere}
.wrap{max-width:860px;margin:0 auto;padding:32px 16px;display:flex;flex-direction:column;gap:16px}
.hero{display:flex;flex-wrap:wrap;gap:16px 32px;align-items:center;justify-content:space-between;background:#fff;border:1px solid var(--line);border-radius:16px;padding:24px}
.hero>div{flex:1 1 280px;min-width:0}
.bar{height:8px;margin-top:10px;border-radius:8px;background:var(--line);overflow:hidden}
.bar i{display:block;height:100%;background:var(--ok)}
.chapter{background:#fff;border:1px solid var(--line);border-radius:12px;padding:16px}
.chapter ol{list-style:none;margin:0;padding:0}
.chapter li a{display:flex;gap:12px;align-items:flex-start;padding:10px 8px;min-height:44px;border-radius:8px;color:var(--ink);text-decoration:none}
.chapter li a:hover{background:var(--bg)}
.chapter li.on a{background:#E7ECFA;font-weight:600}
.dot{flex:none;width:18px;height:18px;margin-top:3px;border-radius:50%;border:2px solid #C8CDD6}
.dot.ok{background:var(--ok);border-color:var(--ok)}
.study{max-width:1240px;margin:0 auto;padding:24px 16px;display:grid;grid-template-columns:minmax(0,1fr) 360px;gap:24px;align-items:start}
.study h1{margin:0 0 16px;font-size:22px}
.study aside{display:flex;flex-direction:column;gap:12px}
.crumb{margin:14px 0 4px;font-size:14px;color:var(--muted)}
.player{position:relative;background:#000;border-radius:12px;overflow:hidden;aspect-ratio:16/9}
.player video{display:block;width:100%;height:100%}
.mark{position:absolute;left:8%;top:12%;color:rgba(255,255,255,.55);font-size:14px;font-weight:600;text-shadow:0 1px 2px rgba(0,0,0,.6);pointer-events:none;user-select:none;transition:left 1s,top 1s;white-space:nowrap}
.fs{position:absolute;top:8px;right:8px;width:44px;height:44px;border:0;border-radius:8px;background:rgba(0,0,0,.5);color:#fff;font-size:20px;cursor:pointer}
@media(max-width:900px){.study{grid-template-columns:minmax(0,1fr)}}
@media(max-width:480px){.card{padding:24px 20px}.login{padding-top:32px}.hero .btn{width:100%}}