#tetrahedron-lesson{--ink:#29343a;--muted:#687175;--accent:#385d70;--line:#d7dcdf;--paper:#f7f8f8;box-sizing:border-box;max-width:1280px;padding:32px 40px 55px;margin:auto;background:#fff;color:var(--ink);font:15px/1.65 Arial,Helvetica,sans-serif;container-type:inline-size;container-name:shape-page}
#tetrahedron-lesson *,#tetrahedron-lesson *::before,#tetrahedron-lesson *::after{box-sizing:border-box}
#tetrahedron-lesson [hidden]{display:none!important}
#tetrahedron-lesson h1,#tetrahedron-lesson h2,#tetrahedron-lesson h3,#tetrahedron-lesson p,#tetrahedron-lesson figure,#tetrahedron-lesson dl{margin:0}
#tetrahedron-lesson h1,#tetrahedron-lesson h2,#tetrahedron-lesson h3{line-height:1.2}
#tetrahedron-lesson h1{font-size:56px;letter-spacing:-2px;font-weight:500}
#tetrahedron-lesson h2{font-size:29px;font-weight:500;letter-spacing:-.6px}
#tetrahedron-lesson h3{font-size:14px}
#tetrahedron-lesson a{color:var(--accent);text-underline-offset:4px}
#tetrahedron-lesson button,#tetrahedron-lesson input,#tetrahedron-lesson select{font:inherit;color:inherit;border-radius:0}
#tetrahedron-lesson button{cursor:pointer}
#tetrahedron-lesson button:disabled{cursor:default;opacity:.6}
#tetrahedron-lesson button:focus-visible,#tetrahedron-lesson input:focus-visible,#tetrahedron-lesson select:focus-visible,#tetrahedron-lesson a:focus-visible,#tetrahedron-lesson summary:focus-visible,#tetrahedron-lesson .viewport:focus-visible{outline:3px solid var(--accent);outline-offset:3px}
#tetrahedron-lesson section[id]{scroll-margin-top:22px}
#tetrahedron-lesson .kicker{font-size:10px;line-height:1.5;letter-spacing:1.8px;text-transform:uppercase;font-weight:700;color:var(--accent);margin-bottom:12px}
#tetrahedron-lesson .dimension-field{margin-top:17px;min-width:0}
#tetrahedron-lesson .dimension-field label{display:block;font-size:12px;line-height:1.5;margin-bottom:7px;font-weight:600}
#tetrahedron-lesson .input-line{display:grid;grid-template-columns:85px minmax(0,1fr);align-items:center;gap:13px}
#tetrahedron-lesson .input-line input[type=number]{width:100%;height:42px;min-width:0;padding:7px 9px;background:#fff;border:1px solid #b8c0c4;font-size:16px}
#tetrahedron-lesson input[type=range]{width:100%;min-width:0;min-height:38px;margin:0;accent-color:var(--accent);cursor:pointer}
#tetrahedron-lesson .unit-control{display:flex;gap:13px;align-items:center;margin-top:15px;font-size:12px}
#tetrahedron-lesson .unit-control select{height:38px;min-width:0;max-width:100%;padding:5px 9px;border:1px solid #bdc5c9;background:#fff;font-size:12px}
#tetrahedron-lesson .input-note,#tetrahedron-lesson .fine-print{font-size:11px;line-height:1.75;color:var(--muted);margin-top:13px}
#tetrahedron-lesson .input-error{font-size:12px;color:#943c2d;margin-top:12px;line-height:1.6}
#tetrahedron-lesson input[aria-invalid=true]{outline:1px solid #a74633;border-color:#a74633!important}
#tetrahedron-lesson output{overflow-wrap:anywhere}
#tetrahedron-lesson .unit{font:11px/1.5 Arial,sans-serif;letter-spacing:0;white-space:nowrap;margin-left:6px;font-weight:400}
#tetrahedron-lesson .long-value{font-size:.67em!important;letter-spacing:-.3px!important}
#tetrahedron-lesson .viewer-shell{min-width:0;background:var(--paper);border:1px solid var(--line)}
#tetrahedron-lesson .viewer-bar{display:flex;align-items:center;justify-content:space-between;gap:8px;padding:9px 13px;border-bottom:1px solid var(--line);background:#ffffffda}
#tetrahedron-lesson .mode-buttons{display:flex;flex-wrap:wrap;gap:3px}
#tetrahedron-lesson .mode-buttons button{padding:7px 10px;min-height:36px;font-size:11px;border:1px solid transparent;background:none;white-space:nowrap}
#tetrahedron-lesson .mode-buttons button[aria-pressed=true]{background:var(--accent);border-color:var(--accent);color:#fff}
#tetrahedron-lesson .mode-buttons button:hover{border-color:var(--accent)}
#tetrahedron-lesson .fullscreen{display:flex;align-items:center;justify-content:center;gap:6px;min-width:40px;min-height:40px;border:0;background:none;font-size:10px;white-space:nowrap;padding:5px}
#tetrahedron-lesson .fullscreen svg{width:17px;height:17px;fill:none;stroke:currentColor;stroke-width:1.5}
#tetrahedron-lesson .view-stage{position:relative;height:520px;overflow:hidden;min-width:0}
#tetrahedron-lesson .viewport{position:absolute;inset:0;touch-action:none;overflow:hidden}
#tetrahedron-lesson .viewport canvas{position:absolute;inset:0;display:block;width:100%;height:100%;touch-action:none}
#tetrahedron-lesson [data-annotations]{position:absolute;inset:0;width:100%;height:100%;pointer-events:none;z-index:2}
#tetrahedron-lesson [data-annotations] rect{fill:#fffffff2;stroke:var(--line);stroke-width:.8}
#tetrahedron-lesson [data-annotations] text{font:12px Arial,sans-serif;text-anchor:middle;fill:var(--ink)}
#tetrahedron-lesson .camera-bar{position:absolute;z-index:3;bottom:12px;left:16px;right:13px;display:flex;align-items:center;justify-content:space-between;gap:8px;pointer-events:none}
#tetrahedron-lesson .camera-bar>span{font-size:10px;color:var(--muted)}
#tetrahedron-lesson .camera-bar>div{display:flex;gap:4px;pointer-events:auto}
#tetrahedron-lesson .camera-bar button{width:35px;height:35px;border:1px solid var(--line);background:#fffffff2;font-size:20px;line-height:1}
#tetrahedron-lesson .camera-bar button:hover{background:#fff}
#tetrahedron-lesson .model-tools{display:flex;gap:15px;align-items:center;padding:6px 15px;border-top:1px solid var(--line)}
#tetrahedron-lesson .model-tools button{border:0;background:none;padding:6px 0;font-size:11px;min-height:33px;color:var(--muted)}
#tetrahedron-lesson .model-tools button[aria-pressed=true]{color:var(--accent);text-decoration:underline;text-underline-offset:4px}
#tetrahedron-lesson .model-status{margin-left:auto;font-size:10px;color:var(--muted)}
#tetrahedron-lesson .model-context{padding:10px 16px 12px;border-top:1px solid var(--line);font-size:11px;line-height:1.7;color:var(--muted);background:#ffffff6b;min-height:41px}
#tetrahedron-lesson .model-slider{display:flex;align-items:center;justify-content:center;gap:12px;border-top:1px solid var(--line);padding:7px 16px;font-size:11px;background:#ffffffb3}
#tetrahedron-lesson .model-slider input{max-width:300px}
#tetrahedron-lesson .fallback{position:absolute;inset:20px 25px 55px;display:flex;flex-direction:column;align-items:center;justify-content:center}
#tetrahedron-lesson .fallback img{width:100%;height:calc(100% - 30px);object-fit:contain}
#tetrahedron-lesson .fallback p{font-size:11px;text-align:center;line-height:1.6;color:var(--muted)}
#tetrahedron-lesson .action-link{display:inline-flex;align-items:center;border:0;border-bottom:1px solid var(--accent);padding:8px 0;margin-top:10px;min-height:44px;color:var(--accent);background:none;font-size:12px;line-height:1.6;text-align:left}
#tetrahedron-lesson figure img{display:block;max-width:100%;height:auto}
#tetrahedron-lesson figcaption{font-size:11px;color:var(--muted);line-height:1.7;margin-top:11px}
#tetrahedron-lesson picture,#tetrahedron-lesson picture img{display:block;width:100%}
#tetrahedron-lesson details{border-bottom:1px solid var(--line);padding:14px 0}
#tetrahedron-lesson summary{cursor:pointer;font-size:14px;line-height:1.6;font-weight:600;color:var(--ink)}
#tetrahedron-lesson details p{font-size:13px;line-height:1.8;color:var(--muted);margin-top:12px}
#tetrahedron-lesson .source{font-size:11px;color:var(--muted);margin-top:23px}
#tetrahedron-lesson .sr-only{position:absolute;width:1px;height:1px;padding:0;overflow:hidden;clip:rect(0,0,0,0);white-space:nowrap;border:0}
#tetrahedron-lesson .no-script{font-size:12px;border-top:1px solid var(--line);padding-top:20px;margin-top:25px}
#tetrahedron-lesson .model-dialog{position:fixed;inset:0;width:100vw;height:100dvh;max-width:none;max-height:none;margin:0;padding:12px;border:0;background:#fff;overflow:auto;color:var(--ink)}
#tetrahedron-lesson .model-dialog::backdrop{background:#fff}
#tetrahedron-lesson .model-dialog .viewer-shell{min-height:100%;display:flex;flex-direction:column}
#tetrahedron-lesson .model-dialog .view-stage{height:auto;flex:1;min-height:300px}
#tetrahedron-lesson .viewer-shell:fullscreen{width:100vw;height:100dvh;padding:12px;overflow:auto;display:flex;flex-direction:column}
@container shape-page (max-width:900px){#tetrahedron-lesson .fullscreen span{display:none}#tetrahedron-lesson .viewer-bar{padding:8px}#tetrahedron-lesson .mode-buttons button{padding:7px 8px}}
@container shape-page (max-width:600px){#tetrahedron-lesson h1{font-size:43px}#tetrahedron-lesson h2{font-size:26px}#tetrahedron-lesson .view-stage{height:420px}#tetrahedron-lesson .mode-buttons button{font-size:10px;padding:6px 7px}#tetrahedron-lesson .viewer-bar{gap:4px}#tetrahedron-lesson .fullscreen{min-width:35px}#tetrahedron-lesson .model-tools{gap:13px;padding:6px 12px}#tetrahedron-lesson .model-status{display:none}#tetrahedron-lesson .camera-bar{left:11px;right:10px}#tetrahedron-lesson .camera-bar>span{font-size:9px;max-width:135px}#tetrahedron-lesson .camera-bar button{width:32px}#tetrahedron-lesson .model-context{padding:9px 12px;font-size:10px}#tetrahedron-lesson .model-slider{font-size:10px;gap:8px;padding:6px 12px}#tetrahedron-lesson .model-slider input{max-width:190px}#tetrahedron-lesson .unit{font-size:10px}}
@media(max-width:600px){#tetrahedron-lesson{padding:24px 18px 35px}#tetrahedron-lesson .model-dialog{padding:5px}#tetrahedron-lesson .model-dialog .view-stage{min-height:330px}}

#tetrahedron-lesson{--ink:#3c3029;--muted:#776b61;--accent:#9a503b;--line:#ded4c8;--paper:#fbf7ef}
#tetrahedron-lesson .tet-title{display:block;padding:0 0 24px}
#tetrahedron-lesson .tet-title h1{font-weight:700;font-size:57px;letter-spacing:-2.5px}
#tetrahedron-lesson .subtitle{font:italic 27px/1.4 Georgia,serif;color:var(--accent);margin-top:6px}
#tetrahedron-lesson .introduction{max-width:none;font-size:16px;line-height:1.85;color:var(--muted);padding-bottom:3px}
#tetrahedron-lesson .chapter-links{display:flex;gap:32px;border-top:1px solid var(--line);border-bottom:1px solid var(--line);margin-bottom:29px;flex-wrap:wrap}
#tetrahedron-lesson .chapter-links a{font-size:12px;text-decoration:none;min-height:48px;display:flex;align-items:center;color:var(--ink)}
#tetrahedron-lesson .chapter-links a:hover{text-decoration:underline;color:var(--accent)}
#tetrahedron-lesson .tet-study{display:grid;grid-template-columns:minmax(0,2.1fr) minmax(265px,1fr);gap:35px}
#tetrahedron-lesson .tet-model{min-width:0}
#tetrahedron-lesson .count-strip{display:flex;justify-content:space-between;gap:15px;padding:0 8px 17px;font-size:11px;color:var(--muted)}
#tetrahedron-lesson .count-strip b{font:30px/1 Georgia,serif;color:var(--accent);margin-right:6px}
#tetrahedron-lesson .tet-calculations{padding:22px 25px;border:1px solid var(--line);background:#fffefa}
#tetrahedron-lesson .tet-calculations h2{font:27px/1.17 Georgia,serif;letter-spacing:-.4px}
#tetrahedron-lesson .tet-calculations .kicker{margin-bottom:8px}
#tetrahedron-lesson .tet-calculations .dimension-field{margin-top:22px}
#tetrahedron-lesson .major-answer{border-top:1px solid var(--line);padding-top:18px;margin-top:21px}
#tetrahedron-lesson .major-answer h3{font-size:12px;letter-spacing:.2px}
#tetrahedron-lesson .major-answer p{font:42px/1.2 Georgia,serif;letter-spacing:-1.4px;color:var(--accent);margin:8px 0 4px;overflow-wrap:anywhere}
#tetrahedron-lesson .major-answer small{font:13px Georgia,serif;color:var(--muted)}
#tetrahedron-lesson .height-answer{display:flex;justify-content:space-between;align-items:center;gap:15px;border-top:1px solid var(--line);padding-top:15px;margin-top:20px}
#tetrahedron-lesson .height-answer>span{max-width:100px;font-size:11px;color:var(--muted)}
#tetrahedron-lesson .height-answer p{font:21px Georgia,serif;text-align:right}
#tetrahedron-lesson .definition{display:grid;grid-template-columns:minmax(220px,.7fr) 2fr;gap:40px;padding:30px 0 33px;border-bottom:1px solid var(--line)}
#tetrahedron-lesson .definition h2{font:23px/1.3 Georgia,serif}
#tetrahedron-lesson .definition p{font-size:14px;line-height:1.85;color:var(--muted)}
#tetrahedron-lesson .unfold-study{display:grid;grid-template-columns:minmax(0,1.2fr) minmax(0,1fr);gap:55px;align-items:center;padding:42px 0}
#tetrahedron-lesson .net-paper{background:var(--paper);border:1px solid var(--line);padding:12px 24px 20px}
#tetrahedron-lesson .net-paper img{width:100%}
#tetrahedron-lesson .net-paper p{text-align:center;font-size:11px;color:var(--muted)}
#tetrahedron-lesson .unfold-copy h2{font:36px/1.18 Georgia,serif;margin-bottom:20px}
#tetrahedron-lesson .unfold-copy>p:not(.kicker){font-size:14px;line-height:1.85;color:var(--muted)}
#tetrahedron-lesson .angle-fact{display:flex;align-items:center;gap:20px;border-top:1px solid var(--line);margin-top:29px;padding-top:22px}
#tetrahedron-lesson .angle-fact strong{font:45px/1 Georgia,serif;color:var(--accent)}
#tetrahedron-lesson .angle-fact span{font-size:11px;line-height:1.8;color:var(--muted)}
#tetrahedron-lesson .height-study{display:grid;grid-template-columns:minmax(0,1fr) minmax(0,1.1fr);gap:45px;background:#f6f1e8;padding:35px;border-top:1px solid var(--line);border-bottom:1px solid var(--line);align-items:center}
#tetrahedron-lesson .height-study h2{font:31px/1.2 Georgia,serif;margin-bottom:19px}
#tetrahedron-lesson .height-study p:not(.kicker){font-size:14px;color:var(--muted);margin-bottom:14px;line-height:1.85}
#tetrahedron-lesson .height-plate img{width:100%}
#tetrahedron-lesson .height-plate figcaption{text-align:center}
#tetrahedron-lesson .tet-formulas{display:grid;grid-template-columns:minmax(0,.85fr) minmax(0,1.8fr);gap:45px;margin-top:44px}
#tetrahedron-lesson .tet-formulas h2{font:29px/1.2 Georgia,serif}
#tetrahedron-lesson .tet-formulas>div>p:not(.kicker){font-size:12px;color:var(--muted);margin-top:17px}
#tetrahedron-lesson .table-wrap{min-width:0}
#tetrahedron-lesson table{width:100%;border-collapse:collapse;border-top:2px solid var(--accent);font-size:12px}
#tetrahedron-lesson thead th{font-size:10px;text-transform:uppercase;letter-spacing:1px;color:var(--muted);padding:12px 7px;text-align:left;font-weight:500;border-bottom:1px solid var(--line)}
#tetrahedron-lesson tbody th,#tetrahedron-lesson tbody td{padding:16px 7px;border-bottom:1px solid var(--line);text-align:left;font-weight:400}
#tetrahedron-lesson thead th:last-child,#tetrahedron-lesson tbody td:last-child{text-align:right}
#tetrahedron-lesson tbody td:last-child{font:21px Georgia,serif;color:var(--accent)}
#tetrahedron-lesson tbody td:last-child .unit{display:block}
#tetrahedron-lesson .math{font:15px Georgia,serif;white-space:nowrap}
#tetrahedron-lesson .tet-reference{margin-top:43px;border-top:1px solid var(--line);padding-top:35px}
#tetrahedron-lesson .tet-reference h2{font:33px Georgia,serif;margin-bottom:24px}
#tetrahedron-lesson .tet-reference picture{border:1px solid var(--line)}
#tetrahedron-lesson .tet-questions{display:grid;grid-template-columns:minmax(0,.85fr) minmax(0,1.8fr);gap:45px;margin-top:38px;border-top:1px solid var(--line);padding-top:30px}
#tetrahedron-lesson .tet-questions>h2{font:28px/1.3 Georgia,serif}
@container shape-page (max-width:950px){#tetrahedron-lesson .tet-study{grid-template-columns:minmax(0,1fr) 265px;gap:22px}#tetrahedron-lesson .tet-calculations{padding:20px}#tetrahedron-lesson .tet-title h1{font-size:50px}#tetrahedron-lesson .introduction{max-width:none}#tetrahedron-lesson .count-strip{font-size:10px;gap:8px;padding-left:0;padding-right:0}#tetrahedron-lesson .count-strip b{font-size:26px}#tetrahedron-lesson .unfold-study{gap:30px}#tetrahedron-lesson .height-study{gap:25px;padding:28px}#tetrahedron-lesson .tet-formulas,#tetrahedron-lesson .tet-questions{gap:28px;grid-template-columns:minmax(0,.7fr) minmax(0,1.8fr)}}
@container shape-page (max-width:740px){#tetrahedron-lesson .tet-title{display:block}#tetrahedron-lesson .introduction{margin-top:18px;max-width:100%}#tetrahedron-lesson .chapter-links{gap:24px}#tetrahedron-lesson .tet-study{display:block}#tetrahedron-lesson .tet-calculations{margin-top:22px;display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:0 25px}#tetrahedron-lesson .tet-calculations>.kicker,#tetrahedron-lesson .tet-calculations>h2,#tetrahedron-lesson .tet-calculations>.dimension-field,#tetrahedron-lesson .tet-calculations>.unit-control,#tetrahedron-lesson .tet-calculations>.input-note,#tetrahedron-lesson .tet-calculations>.input-error{grid-column:1/-1}#tetrahedron-lesson .tet-calculations h2 br{display:none}#tetrahedron-lesson .height-answer{grid-column:1/-1}#tetrahedron-lesson .height-answer>span{max-width:none}#tetrahedron-lesson .definition{display:block}#tetrahedron-lesson .definition h2{margin-bottom:13px}#tetrahedron-lesson .unfold-study{gap:25px;grid-template-columns:1fr}#tetrahedron-lesson .net-paper{max-width:550px;width:100%;margin:auto}#tetrahedron-lesson .unfold-copy h2 br{display:none}#tetrahedron-lesson .height-study{grid-template-columns:1fr}#tetrahedron-lesson .height-plate{max-width:550px;margin:auto}#tetrahedron-lesson .tet-formulas,#tetrahedron-lesson .tet-questions{display:block}#tetrahedron-lesson .table-wrap{margin-top:24px}#tetrahedron-lesson .tet-questions>h2{margin-bottom:13px}}
@container shape-page (max-width:430px){#tetrahedron-lesson .tet-title h1{font-size:42px}#tetrahedron-lesson .subtitle{font-size:25px}#tetrahedron-lesson .chapter-links{gap:0;justify-content:space-between}#tetrahedron-lesson .chapter-links a{font-size:11px}#tetrahedron-lesson .count-strip span{max-width:80px;line-height:1.5}#tetrahedron-lesson .count-strip b{display:block;font-size:30px;margin-bottom:5px}#tetrahedron-lesson .major-answer p{font-size:34px}#tetrahedron-lesson .tet-calculations{padding:20px 16px;column-gap:17px}#tetrahedron-lesson .height-study{padding:24px 19px}#tetrahedron-lesson .unfold-copy h2{font-size:30px}#tetrahedron-lesson .angle-fact{gap:15px}#tetrahedron-lesson tbody th,#tetrahedron-lesson tbody td{padding:14px 3px;font-size:11px}#tetrahedron-lesson .math{font-size:13px}#tetrahedron-lesson tbody td:last-child{font-size:18px}#tetrahedron-lesson .mode-buttons{gap:0}#tetrahedron-lesson .mode-buttons button{padding:6px 5px;font-size:9px}}

#tetrahedron-lesson .introduction{margin-top:18px;width:100%;max-width:none}

#tetrahedron-lesson .tet-angle-study{display:grid;grid-template-columns:1.4fr 1fr;gap:38px;padding:30px 0;margin:30px 0;border-block:1px solid var(--line)}
#tetrahedron-lesson .tet-angle-study p{max-width:none}#tetrahedron-lesson .tet-angle-study p+p{margin-top:15px}#tetrahedron-lesson .tet-proof-line{font:24px/1.7 Georgia,serif;padding:20px 0;color:var(--accent)}
#tetrahedron-lesson .tet-angle-study aside{padding:24px;background:#eee5d7;align-self:start}#tetrahedron-lesson .tet-angle-study h3{margin:0 0 14px}
@container shape-page (max-width:700px){#tetrahedron-lesson .tet-angle-study{grid-template-columns:1fr;gap:24px}}

/* Shared reading and navigation behavior; each lesson keeps its composition. */
#tetrahedron-lesson .mobile-contents { display: none; }
#tetrahedron-lesson .mobile-contents summary { list-style: none; }
#tetrahedron-lesson .mobile-contents summary::-webkit-details-marker { display: none; }

@media (max-width: 760px) {
  #tetrahedron-lesson [data-desktop-contents] { display: none !important; }
  #tetrahedron-lesson .mobile-contents { display: block; }
}
@container shape-page (max-width: 760px) {
  #tetrahedron-lesson [data-desktop-contents] { display: none !important; }
  #tetrahedron-lesson .mobile-contents { display: block; }
}
#tetrahedron-lesson .mobile-contents {
  margin: 20px 0 24px;
  padding: 0;
  max-width: none;
  border: 1px solid var(--line, #ccd6dc);
  border-radius: 3px;
  background: #fff;
  color: var(--ink, #294e69);
  font: 15px/1.45 Arial, Helvetica, sans-serif;
}
#tetrahedron-lesson .mobile-contents > summary {
  display: flex;
  align-items: center;
  gap: 12px;
  min-height: 56px;
  padding: 12px 16px;
  cursor: pointer;
  background: var(--tint, var(--paper, #f4f7f9));
  color: inherit;
}
#tetrahedron-lesson .mobile-contents .contents-title { font-weight: 700; }
#tetrahedron-lesson .mobile-contents .contents-count {
  margin-left: auto;
  font-size: 12px;
  font-weight: 400;
  color: var(--muted, #617582);
  white-space: nowrap;
}
#tetrahedron-lesson .mobile-contents > summary::after {
  content: '';
  width: 7px;
  height: 7px;
  flex: none;
  margin-left: 3px;
  border-right: 1.5px solid currentColor;
  border-bottom: 1.5px solid currentColor;
  transform: translateY(-2px) rotate(45deg);
}
#tetrahedron-lesson .mobile-contents[open] > summary { border-bottom: 1px solid var(--line, #ccd6dc); }
#tetrahedron-lesson .mobile-contents[open] > summary::after { transform: translateY(2px) rotate(225deg); }
#tetrahedron-lesson .mobile-contents nav { display: block; padding: 0 16px; margin: 0; border: 0; }
#tetrahedron-lesson .mobile-contents ol { list-style: none; padding: 0; margin: 0; }
#tetrahedron-lesson .mobile-contents li { padding: 0; margin: 0; border-bottom: 1px solid var(--line, #dce3e7); }
#tetrahedron-lesson .mobile-contents li:last-child { border-bottom: 0; }
#tetrahedron-lesson .mobile-contents a {
  display: grid;
  grid-template-columns: 22px minmax(0, 1fr) 18px;
  align-items: center;
  gap: 10px;
  min-height: 50px;
  padding: 11px 0;
  color: inherit;
  font-size: 14px;
  line-height: 1.45;
  text-decoration: none;
  overflow-wrap: break-word;
}
#tetrahedron-lesson .mobile-contents .contents-number { font-size: 11px; color: var(--muted, #617582); font-variant-numeric: tabular-nums; }
#tetrahedron-lesson .mobile-contents .contents-arrow { color: var(--accent, var(--green, #315f81)); }
#tetrahedron-lesson .mobile-contents a:hover { color: var(--accent, var(--green, #315f81)); text-decoration: underline; text-underline-offset: 3px; }
#tetrahedron-lesson .mobile-contents a:focus-visible, #tetrahedron-lesson .mobile-contents summary:focus-visible { outline: 2px solid var(--accent, var(--green, #315f81)); outline-offset: -3px; }

/* Keep the mode labels readable and separate them from the full-screen action. */
@media (max-width: 760px) {
  #tetrahedron-lesson .model-toolbar, #tetrahedron-lesson .viewer-bar { flex-wrap: wrap; gap: 10px; }
  #tetrahedron-lesson .mode-tabs, #tetrahedron-lesson .mode-buttons { flex: 1 0 100%; display: flex; flex-wrap: wrap; gap: 6px; }
  #tetrahedron-lesson .mode-tabs button, #tetrahedron-lesson .mode-buttons button { flex: 1 1 auto; min-height: 44px; padding: 9px 12px; font-size: 13px; white-space: normal; }
  #tetrahedron-lesson .model-toolbar .fullscreen, #tetrahedron-lesson .viewer-bar .fullscreen { margin-left: auto; min-height: 44px; padding: 9px 12px; font-size: 13px; }
  #tetrahedron-lesson .viewer-bar .fullscreen span { display: inline; }
  #tetrahedron-lesson .camera-controls button { min-width: 44px; min-height: 44px; font-size: 13px; }
  #tetrahedron-lesson .stage-top .view-switch { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); width: 100%; gap: 5px; border: 0; background: transparent; }
  #tetrahedron-lesson .stage-top .view-switch button { min-height: 44px; padding: 9px 8px; font-size: 13px; border: 1px solid var(--line, #ccd6dc); background: #fff; white-space: normal; }
  #tetrahedron-lesson .stage-top .view-switch button[aria-pressed=true] { background: var(--green, #315f81); color: #fff; }
  #tetrahedron-lesson .stage > .viewport { top: 112px; }
  #tetrahedron-lesson .stage { min-height: 480px; }
  #tetrahedron-lesson .camera-buttons button, #tetrahedron-lesson .camera-bar button { min-width: 44px; min-height: 44px; }
}
@container shape-page (max-width: 760px) {
  #tetrahedron-lesson .model-toolbar, #tetrahedron-lesson .viewer-bar { flex-wrap: wrap; gap: 10px; }
  #tetrahedron-lesson .mode-tabs, #tetrahedron-lesson .mode-buttons { flex: 1 0 100%; display: flex; flex-wrap: wrap; gap: 6px; }
  #tetrahedron-lesson .mode-tabs button, #tetrahedron-lesson .mode-buttons button { flex: 1 1 auto; min-height: 44px; padding: 9px 12px; font-size: 13px; white-space: normal; }
  #tetrahedron-lesson .model-toolbar .fullscreen, #tetrahedron-lesson .viewer-bar .fullscreen { margin-left: auto; min-height: 44px; padding: 9px 12px; font-size: 13px; }
  #tetrahedron-lesson .viewer-bar .fullscreen span { display: inline; }
  #tetrahedron-lesson .camera-controls button { min-width: 44px; min-height: 44px; font-size: 13px; }
  #tetrahedron-lesson .stage-top .view-switch { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); width: 100%; gap: 5px; border: 0; background: transparent; }
  #tetrahedron-lesson .stage-top .view-switch button { min-height: 44px; padding: 9px 8px; font-size: 13px; border: 1px solid var(--line, #ccd6dc); background: #fff; white-space: normal; }
  #tetrahedron-lesson .stage-top .view-switch button[aria-pressed=true] { background: var(--green, #315f81); color: #fff; }
  #tetrahedron-lesson .stage > .viewport { top: 112px; }
  #tetrahedron-lesson .stage { min-height: 480px; }
  #tetrahedron-lesson .camera-buttons button, #tetrahedron-lesson .camera-bar button { min-width: 44px; min-height: 44px; }
}
#tetrahedron-lesson .featured-pin {
  --pin-ink: #223b48;
  --pin-muted: #536672;
  --pin-rule: #cbd3d4;
  margin-top: 56px;
  color: var(--pin-ink);
  container-type: inline-size;
  container-name: pin-reference;
}
#tetrahedron-lesson .featured-pin * { box-sizing: border-box; }
#tetrahedron-lesson .featured-pin h2 { margin: 0; font: 42px/1.12 Georgia, serif; letter-spacing: -1.1px; color: inherit; }
#tetrahedron-lesson .featured-pin h3 { margin: 0; color: inherit; }
#tetrahedron-lesson .featured-pin p { margin: 0; max-width: none; }
#tetrahedron-lesson .featured-pin .pin-intro { margin: 18px 0 26px; font-size: 15px; line-height: 1.75; color: var(--pin-muted); }
#tetrahedron-lesson .featured-pin .pin-art { width: 100%; max-width: 420px; margin: 0; justify-self: center; min-width: 0; }
#tetrahedron-lesson .featured-pin .pin-preview { display: block; line-height: 0; background: #fff; box-shadow: 0 8px 24px #132e3914; }
#tetrahedron-lesson .featured-pin .pin-preview img { display: block; width: 100%; max-width: 100%; height: auto; aspect-ratio: 2 / 3; border: 1px solid #17344326; }
#tetrahedron-lesson .featured-pin .pin-preview:hover { outline: 2px solid var(--pin-ink); outline-offset: 4px; }
#tetrahedron-lesson .featured-pin figcaption { margin: 14px 0 0; padding: 0; text-align: center; font: 12px/1.5 Arial, sans-serif; color: var(--pin-muted); }
#tetrahedron-lesson .featured-pin .pin-actions { display: flex; flex-wrap: wrap; align-items: stretch; gap: 10px; margin-top: 26px; }
#tetrahedron-lesson .featured-pin .pin-actions :is(a, button) { display: inline-flex; justify-content: center; align-items: center; gap: 10px; min-height: 46px; padding: 10px 18px; font: 600 14px/1.4 Arial, sans-serif; text-decoration: none; border: 1px solid var(--pin-ink); border-radius: 0; cursor: pointer; }
#tetrahedron-lesson .featured-pin .pin-actions .pin-pinterest { flex: 0 0 auto; padding-inline: 14px; background: #bd081b; border-color: #bd081b; color: #fff; }
#tetrahedron-lesson .featured-pin .pin-actions .pin-pinterest:hover { background: #9b0716; border-color: #9b0716; }
#tetrahedron-lesson .featured-pin .pin-save { background: transparent; color: var(--pin-ink); }
#tetrahedron-lesson .featured-pin .pin-enlarge { background: transparent; color: var(--pin-ink); }
#tetrahedron-lesson .featured-pin .pin-actions a:hover { text-decoration: underline; text-underline-offset: 4px; }
#tetrahedron-lesson .featured-pin :is(a, button):focus-visible { outline: 3px solid var(--pin-ink); outline-offset: 4px; }
#tetrahedron-lesson .featured-pin .pin-actions svg { width: 18px; height: 18px; fill: none; stroke: currentColor; stroke-width: 1.6; }
#tetrahedron-lesson .featured-pin .pin-return { display: inline-flex; align-items: center; gap: 10px; min-height: 44px; margin-top: 16px; font-size: 13px; color: var(--pin-ink); text-underline-offset: 4px; }
#tetrahedron-lesson .featured-pin .pin-sr { position: absolute; width: 1px; height: 1px; overflow: hidden; clip-path: inset(50%); white-space: nowrap; }
@container pin-reference (max-width: 700px) {
  #tetrahedron-lesson .featured-pin h2 { font-size: 34px; letter-spacing: -.7px; }
  #tetrahedron-lesson .featured-pin .pin-art { max-width: 420px; }
  #tetrahedron-lesson .featured-pin .pin-intro { font-size: 15px; margin: 15px 0 22px; }
}
@media (max-width: 600px) {
  #tetrahedron-lesson .featured-pin { margin-top: 36px; }
}

#tetrahedron-lesson .featured-pin { --pin-ink: #6c382d; --pin-muted: #596773; --pin-rule: #ccd4d4; }
#tetrahedron-lesson .topic-pin-tetrahedron { display: grid; grid-template-columns: minmax(0, 1fr) minmax(0, 1fr); grid-template-rows: auto 1fr; gap: 0 48px; border-top: 3px solid #6c382d; padding: 34px 28px; background: #fbf4e9; }
#tetrahedron-lesson .topic-pin-tetrahedron .topic-pin-heading { grid-column: 2; grid-row: 1; }
#tetrahedron-lesson .topic-pin-tetrahedron .pin-art { grid-column: 1; grid-row: 1 / 3; }
#tetrahedron-lesson .topic-pin-tetrahedron .topic-pin-details { grid-column: 2; grid-row: 2; align-self: start; }
#tetrahedron-lesson .topic-pin-tetrahedron .topic-pin-facts { margin: 0; }
#tetrahedron-lesson .topic-pin-tetrahedron .topic-pin-facts > div { padding: 18px 0; border-top: 1px solid var(--pin-rule); }
#tetrahedron-lesson .topic-pin-tetrahedron .topic-pin-facts dt { font: 22px/1.3 Georgia, serif; }
#tetrahedron-lesson .topic-pin-tetrahedron .topic-pin-facts dd { margin: 8px 0 0; color: var(--pin-muted); font-size: 14px; line-height: 1.7; }
@container pin-reference (max-width: 850px) { #tetrahedron-lesson .topic-pin-tetrahedron { padding: 28px 20px; column-gap: 28px; } #tetrahedron-lesson .topic-pin-tetrahedron h2 { font-size: 34px; } }
@container pin-reference (max-width: 700px) {
  #tetrahedron-lesson .topic-pin-tetrahedron { grid-template-columns: minmax(0, 1fr); padding: 25px 18px; gap: 0; }
  #tetrahedron-lesson .topic-pin-tetrahedron .topic-pin-heading { grid-column: 1; grid-row: 1; display: block; text-align: left; padding-bottom: 0; }
  #tetrahedron-lesson .topic-pin-tetrahedron .topic-pin-heading .pin-intro { margin-bottom: 22px; }
  #tetrahedron-lesson .topic-pin-tetrahedron .pin-art { grid-column: 1; grid-row: 2; margin: 0 0 26px; }
  #tetrahedron-lesson .topic-pin-tetrahedron .topic-pin-details { grid-column: 1; grid-row: 3; }
}