@font-face{font-family:Barlow;src:url('assets/barlow-400.ttf');font-weight:400;font-display:swap}@font-face{font-family:Barlow;src:url('assets/barlow-600.ttf');font-weight:600;font-display:swap}@font-face{font-family:Barlow;src:url('assets/barlow-900.ttf');font-weight:900;font-display:swap}
*{box-sizing:border-box}html{scroll-behavior:smooth}body{margin:0;background:#fff;color:#202729;font-family:Barlow,Arial,sans-serif}.lesson-shell{max-width:1640px;margin:auto;padding:32px clamp(24px,5vw,88px) 70px}.lesson-header{padding-bottom:26px;border-bottom:1px solid #e7e7e7}.lesson-header h1{font-size:clamp(27px,2.4vw,38px);line-height:1.2;font-weight:600;margin:0 0 10px}.lesson-header p{font-size:18px;color:#69757d;line-height:1.5;margin:0}.lesson-meta{display:flex;align-items:center;gap:20px;flex-wrap:wrap;padding:20px 0;font-size:15px;color:#7a858d}.lesson-meta strong{color:#34414a;font-weight:600}.classroom{display:grid;grid-template-columns:minmax(0,2.3fr) minmax(250px,1fr);gap:32px;align-items:start}.classroom main{min-width:0}.hero img{display:block;width:100%;aspect-ratio:16/9;object-fit:cover;border-radius:12px}.hero h2{font-size:28px;font-weight:600;margin:20px 0 8px}.hero p{font-size:16px;color:#6c7881;margin:0}.hero{padding:0 0 42px;border-bottom:1px solid #e9ecef}.slide{padding:50px 0 55px;border-bottom:1px solid #e9ecef;scroll-margin-top:20px}.slide>h2{font-size:clamp(28px,3.5vw,54px);font-weight:900;line-height:1.08;letter-spacing:-.025em;text-align:center;margin:0 0 20px;color:#171717}.panel{width:94%;margin:0 auto;background:#fff;border:1px solid #f0f0f0;border-radius:clamp(15px,1.5vw,24px);box-shadow:0 8px 26px rgba(0,0,0,.1),0 2px 7px rgba(0,0,0,.04)}.illustration,.chart-panel{height:auto;aspect-ratio:16/10;max-height:620px;display:flex;align-items:center;justify-content:center}.placeholder{display:flex;flex-direction:column;text-align:center;gap:8px;color:#abb1b6;font-size:14px}.placeholder strong{font-size:18px;color:#949da4;font-weight:600}.explanation{display:flex;flex-direction:column;justify-content:center;gap:clamp(20px,2.3vw,35px);padding:clamp(25px,3vw,46px);min-height:350px}.reason{max-width:700px;margin:0 auto;text-align:center}.reason h3{font-size:clamp(20px,2.1vw,30px);font-weight:900;line-height:1.1;margin:0 0 7px}.reason p{font-size:clamp(16px,1.4vw,20px);font-weight:600;line-height:1.3;margin:0}.chart-panel{padding:clamp(20px,3vw,40px)}.chart-panel img{width:100%;height:100%;object-fit:contain}.checks{display:flex;justify-content:center;gap:10px 22px;flex-wrap:wrap;margin:20px auto 0;width:100%;text-align:center;font-size:clamp(14px,1.1vw,19px);font-weight:600}.checks span{max-width:230px}.practice iframe{display:block;border:0;width:100%;height:min(74vh,780px);min-height:480px}.lesson-guide{border-left:1px solid #e7e7e7;min-width:0;padding-left:24px}.guide-inner{position:sticky;top:24px}.companion{padding-bottom:22px;margin-bottom:24px;border-bottom:1px solid #e7e7e7}.companion h2,.guide-header h2{font-size:22px;font-weight:600;margin:0 0 14px}.companion img{display:block;width:100%;aspect-ratio:16/9;object-fit:cover;border-radius:10px}.companion p{font-size:13px;color:#929ca3;margin:9px 0 0}.guide-header{display:flex;justify-content:space-between;align-items:baseline;gap:10px}.guide-header span{font-size:13px;color:#89939a}.progress-track{height:3px;background:#edf0f2;margin-bottom:14px}.progress-track div{width:14.3%;height:100%;background:#ce3344;transition:width .2s}.lesson-guide nav{display:flex;flex-direction:column;gap:3px}.lesson-guide nav a{display:block;text-decoration:none;border-radius:5px;padding:10px 12px;color:#62717a;font-size:15px}.lesson-guide nav a:hover{background:#f6f7f8}.lesson-guide nav a.active{background:#fff3f5;color:#ac3447;font-weight:600}@media(max-width:900px){.lesson-shell{padding:24px 20px 50px}.classroom{display:flex;flex-direction:column-reverse;gap:28px}.lesson-guide{width:100%;border:0;border-bottom:1px solid #e7e7e7;padding:0 0 20px}.guide-inner{position:static}.companion{display:grid;grid-template-columns:minmax(0,180px) 1fr;gap:5px 14px;align-items:start}.companion h2{grid-column:1/-1;margin-bottom:5px}.companion p{margin:0}.lesson-guide nav{display:flex;flex-direction:row;gap:7px;overflow-x:auto;padding-bottom:4px}.lesson-guide nav a{flex:none;white-space:nowrap;border:1px solid #edf0f2;padding:9px 12px}.classroom main{width:100%}.slide{padding:38px 0 44px}.slide>h2{font-size:clamp(27px,5.5vw,43px)}.panel{width:96%}.illustration,.chart-panel{aspect-ratio:4/3}.practice iframe{height:65svh;min-height:450px}}
@media(prefers-reduced-motion:reduce){html{scroll-behavior:auto}.progress-track div{transition:none}}
.classroom{align-items:stretch}.guide-inner{max-height:calc(100dvh - 48px);overflow:auto}@media(max-width:900px){.guide-inner{max-height:none;overflow:visible}}
/* One lesson at a time, with regular scrolling for smaller screens. */
[hidden]{display:none!important}.hero{padding-bottom:20px}.slide{padding:8px 0 24px;border-bottom:0}.lesson-navigation{display:flex;align-items:center;justify-content:space-between;gap:16px;margin-top:20px;padding:14px 0;border-top:1px solid #e9ecef}.lesson-navigation button{min-width:100px;min-height:44px;border:1px solid #dce1e5;border-radius:6px;background:#fff;color:#35434c;font:600 15px Barlow,Arial,sans-serif;cursor:pointer}.lesson-navigation button:hover:not(:disabled){background:#f5f6f7}.lesson-navigation button:focus-visible{outline:2px solid #2962ff;outline-offset:2px}.lesson-navigation button:disabled{opacity:.35;cursor:default}.lesson-navigation span{font-size:14px;color:#89939a}.progress-track div{width:12.5%}@media(max-width:900px){.slide{padding:8px 0 20px}.lesson-navigation{margin-top:14px}.lesson-navigation button{min-width:88px}}
/* Compact slide navigation in the lesson header. */
.lesson-meta .lesson-navigation{margin:0 0 0 auto;padding:0;border:0;gap:2px}.lesson-meta .lesson-navigation button{display:inline-flex;align-items:center;justify-content:center;min-width:44px;min-height:44px;width:44px;height:44px;padding:0;border:0;background:transparent;color:#59656e}.lesson-meta .lesson-navigation button svg{width:20px;height:20px;fill:none;stroke:currentColor;stroke-width:1.8;stroke-linecap:round;stroke-linejoin:round}.lesson-meta .lesson-navigation button:hover:not(:disabled){background:#f5f6f7}.sr-only{position:absolute!important;width:1px;height:1px;padding:0;margin:-1px;overflow:hidden;clip:rect(0,0,0,0);white-space:nowrap;border:0}
.lesson-meta .lesson-navigation .chart-count{font:600 12px Barlow,Arial,sans-serif;color:#8a959d;margin-right:4px;white-space:nowrap}