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/* Short, step-specific scaffolding. The graphic remains the main working surface. */
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#limit-response.needs-retry{color:var(--orange)}

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/* Reserve formula space within a lesson; keep the step controls in one position. */
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main::after{content:'';display:block;height:var(--step-tail-space,0px)}
html{overflow-anchor:none}

/* Purpose, invitation and a concrete question before the first experiment. */
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#intro-title:focus{outline:none}

/* Page zero: a live, mathematically linked slope-and-area cover. */
.cover-mode .shell{display:block;max-width:none}.cover-mode aside,.cover-mode #lesson-view,.cover-mode #cover-toggle,.cover-mode #intro-toggle,.cover-mode #map-toggle,.cover-mode #help,.cover-mode #progress{display:none!important}.cover-mode main{max-width:none;padding:0}.cover-mode .topbar{background:#091729;color:#edf9ff;border-color:#173b53}.cover-mode .brand{color:#edf9ff}.cover-mode .brand-mark{background:#46dce6;color:#091729}
.cover{min-height:calc(100dvh - 76px);display:grid;grid-template-columns:minmax(300px,.78fr) minmax(530px,1.22fr);align-items:center;gap:clamp(26px,4vw,70px);position:relative;isolation:isolate;overflow:hidden;padding:clamp(38px,6vw,92px);background:radial-gradient(circle at 17% 23%,#164362 0,transparent 34%),radial-gradient(circle at 86% 82%,#163d4c 0,transparent 36%),#081728;color:#edf9ff}.cover::before{content:'';position:absolute;inset:0;z-index:-2;background-image:linear-gradient(#8ce8f008 1px,transparent 1px),linear-gradient(90deg,#8ce8f008 1px,transparent 1px);background-size:48px 48px;mask-image:linear-gradient(90deg,transparent,#000 32%,#000 83%,transparent)}.cover-glow{position:absolute;border-radius:50%;filter:blur(45px);pointer-events:none;z-index:-1}.cover-glow-a{width:270px;height:270px;background:#08b8d52d;top:-80px;right:25%}.cover-glow-b{width:220px;height:220px;background:#806bff24;bottom:-70px;left:13%}.cover-copy{position:relative;z-index:1}.cover-kicker{font-size:12px;letter-spacing:.18em;color:#8de5f2;text-transform:uppercase;display:flex;align-items:center;gap:11px}.cover-pulse{width:8px;height:8px;border-radius:50%;background:#66edcf;box-shadow:0 0 0 6px #66edcf22,0 0 22px #66edcf}.cover h1{font-size:clamp(56px,6vw,104px);line-height:1.13;letter-spacing:-.065em;font-weight:750;margin:27px 0 25px}.cover h1 em{font-style:normal;color:#72edec;text-shadow:0 0 55px #46ddde57}.cover-copy>p{color:#c1d3e2;font-size:clamp(16px,1.35vw,20px);line-height:1.85;margin:0;max-width:540px}.cover-actions{display:flex;flex-wrap:wrap;gap:12px;margin:35px 0 20px}.cover-actions button{border-color:#416577;background:#153047;color:#e5f7fc;font-size:15px;padding:13px 19px;min-height:50px}.cover-actions button:hover{border-color:#8de5f2;color:white}.cover-actions .cover-primary{background:linear-gradient(105deg,#63e7e8,#9afbca);border:0;color:#07233a;font-weight:700;box-shadow:0 10px 40px #30d2db35}.cover-primary span{font-size:18px;margin-left:8px}.cover-footnote{font-size:12px;letter-spacing:.09em;color:#86aabb;margin-top:45px}.cover-demo{border:1px solid #4e779783;border-radius:22px;padding:20px 20px 15px;background:linear-gradient(145deg,#102a42f5,#071b2ff5);box-shadow:0 26px 80px #020f1d9c,0 0 45px #3ce7de1d;min-width:0;position:relative}.cover-demo::before{content:'';position:absolute;inset:0;border-radius:22px;pointer-events:none;box-shadow:inset 0 1px #a7f9ff2c}.cover-demo-head{display:flex;align-items:center;gap:13px;color:#86aebe;font-size:12px;letter-spacing:.1em;padding:0 5px}.cover-demo-head strong{color:#e7fcff;letter-spacing:0;font-size:14px;font-weight:600;margin-left:auto}.cover-live{font-size:10px;border:1px solid #5de3c085;border-radius:30px;color:#7af3d2;padding:4px 7px;box-shadow:0 0 16px #5de3c038}.cover-demo svg{display:block;width:100%;height:auto;overflow:visible;margin:15px 0 0}.cover-plot-bg{fill:#0a1d33;stroke:#28506a;stroke-width:1.5}.cover-grid{stroke:#45637c;stroke-width:1;opacity:.3}.cover-axis{stroke:#76a8bf;stroke-width:1.5;fill:none;opacity:.8}.cover-plot-label{font:600 18px "Segoe UI","Microsoft YaHei",sans-serif;fill:#bcdbeb}.cover-curve-glow{fill:none;stroke:#53e4ee;stroke-width:10;opacity:.55}.cover-position-curve{fill:none;stroke:url(#cover-curve);stroke-width:5;stroke-linecap:round}.cover-speed-curve{fill:none;stroke:#9ef3cc;stroke-width:4;stroke-linecap:round}.cover-tangent{stroke:#ffba77;stroke-width:4;stroke-linecap:round;filter:drop-shadow(0 0 8px #ff9b55)}.cover-sweep{stroke:#a8dfec;stroke-width:1.5;stroke-dasharray:7 6;opacity:.8}.cover-halo{fill:#48e4e4;opacity:.14}.cover-left-point{fill:#e9ffff;stroke:#51e3ed;stroke-width:5;filter:drop-shadow(0 0 8px #55f0f0)}.cover-right-point{fill:#d9ffed;stroke:#61eabc;stroke-width:5;filter:drop-shadow(0 0 8px #66eeb8)}.cover-tag{font:14px "Segoe UI","Microsoft YaHei",sans-serif;fill:#a2d3df}.cover-bridge{fill:none;stroke:#5ce5dd;stroke-width:2}.cover-bridge-text{font:12px "Segoe UI","Microsoft YaHei",sans-serif;fill:#79d7dc}.cover-metrics{display:grid;grid-template-columns:1fr auto 1fr;gap:10px;align-items:center;margin:2px 9px 19px}.cover-metrics div:not(.cover-equals){background:#142e42;border:1px solid #2c5367;border-radius:10px;padding:12px 15px}.cover-metrics small{display:block;color:#9cbfcd;font-size:11px;margin-bottom:5px}.cover-metrics strong{font:600 25px "Cambria Math",Georgia,serif;color:#98fae0}.cover-metrics div:first-child strong{color:#82ecf3}.cover-equals{color:#84ced5;font-size:20px}.cover-control{display:flex;align-items:center;gap:12px;padding:0 8px}.cover-control button{background:#1e465b;color:#a1f7eb;border-color:#3a6577;border-radius:50%;width:38px;height:38px;padding:3px;min-height:38px}.cover-control label{display:flex;justify-content:space-between;min-width:115px;gap:8px;color:#b8d6e3;font-size:12px}.cover-control output{color:#83ebec;font-variant-numeric:tabular-nums}.cover-control input{accent-color:#67ebdb;min-width:0;flex:1}.cover-formula{color:#7eabbc;font:15px/1.5 "Cambria Math",Georgia,serif;text-align:center;margin:17px 0 0}.cover-formula span{margin:0 11px;color:#5e8b9b}
@media(max-width:1080px){.cover{grid-template-columns:1fr;padding:45px clamp(20px,5vw,62px);gap:26px}.cover h1{font-size:clamp(54px,9vw,88px);margin:20px 0}.cover-footnote{margin-top:20px}.cover-demo{max-width:900px;width:100%}}
@media(max-width:760px){.cover{min-height:calc(100dvh - 65px);padding:34px 15px 48px}.cover h1{font-size:clamp(48px,13vw,74px)}.cover-copy>p{font-size:15px}.cover-actions{margin:24px 0 12px}.cover-actions button{font-size:13px;padding:11px 13px;min-height:44px}.cover-footnote{font-size:11px}.cover-demo{padding:14px 9px 12px;border-radius:15px}.cover-demo-head{font-size:10px}.cover-demo-head strong{font-size:12px}.cover-live{font-size:9px}.cover-metrics{margin:0 2px 13px;gap:5px}.cover-metrics div:not(.cover-equals){padding:8px}.cover-metrics small{font-size:10px}.cover-metrics strong{font-size:20px}.cover-control{gap:7px;padding:0 3px}.cover-control label{min-width:91px;font-size:11px}.cover-formula{font-size:12px}}

/* Keep one chapter question; place transient instructions beside the thing to use. */
.lesson-hook{font-size:15px;line-height:1.7;color:#526780;max-width:760px;margin:8px 0 0}.lesson-heading{align-items:flex-start;margin-bottom:20px}.lab-actions{display:flex;align-items:center;gap:13px}#hint-toggle{font-size:12px;min-height:27px;padding:3px 8px;color:var(--blue);background:#f5f8ff;border-color:#d6e1f5;white-space:nowrap}.slider-field{position:relative}.inline-coach{z-index:3;background:#112e4d;color:#f5fbff;border:1px solid #3a709a;border-radius:10px;box-shadow:0 12px 30px #091a3652;padding:10px 13px;font:13px/1.55 "Segoe UI","Microsoft YaHei",sans-serif;max-width:300px}.inline-coach::before{content:'试一试';display:block;color:#91ecf0;font-size:11px;font-weight:650;margin-bottom:3px}.inline-coach.observed{background:#103d3e;border-color:#5dcbb5}.inline-coach.observed::before{content:'留意这里';color:#a3f6d6}.slider-field>.inline-coach{position:absolute;bottom:calc(100% + 11px);left:0;width:min(300px,calc(100vw - 48px))}.slider-field:last-child>.inline-coach{left:auto;right:0}.slider-field>.inline-coach::after{content:'';position:absolute;bottom:-7px;left:21px;width:12px;height:12px;transform:rotate(45deg);background:inherit;border-right:1px solid #3a709a;border-bottom:1px solid #3a709a}.slider-field:last-child>.inline-coach::after{left:auto;right:21px}.limit-task>.inline-coach,.challenge>.inline-coach{margin:0 0 12px;box-shadow:none;max-width:100%}.step-why{font:13px/1.7 "Segoe UI","Microsoft YaHei",sans-serif;color:#64778e;margin:5px 0 15px}.step-why summary{cursor:pointer;width:fit-content;color:var(--blue)}.step-why p{margin:8px 0 0;color:var(--ink);max-width:780px}.derivation .step-footer{margin-top:auto}.step-explanation:not(:empty)::before{content:'读懂这一步'}
@media(max-width:760px){.lesson-hook{font-size:14px}.lab-actions{gap:7px}.lab-actions .legend{gap:5px}.lab-actions #hint-toggle{font-size:11px;min-height:25px;padding:2px 6px}.slider-field>.inline-coach{max-width:265px}.step-why{margin-bottom:12px}}
.lab{position:relative}.graphic-coach{position:absolute;width:min(300px,calc(100% - 24px));pointer-events:none}.graphic-coach::after{content:'';position:absolute;width:12px;height:12px;left:calc(50% - 6px);bottom:-7px;transform:rotate(45deg);background:inherit;border-right:1px solid #3a709a;border-bottom:1px solid #3a709a}.graphic-coach[data-placement="below"]::after{top:-7px;bottom:auto;transform:rotate(225deg)}
.fallback-coach{position:absolute;width:min(300px,calc(100% - 24px));pointer-events:none}.slider-field.guide-target{outline:2px solid #80a1ff;outline-offset:5px;background:#ebf1ff}.fallback-coach::after{content:'';position:absolute;bottom:-7px;left:calc(50% - 6px);width:12px;height:12px;transform:rotate(45deg);background:inherit;border-right:1px solid #3a709a;border-bottom:1px solid #3a709a}

/* Mathematical manuscript: quiet paper, precise figures, restrained interaction. */
.intro-mode .shell{display:block;max-width:none}
.intro-mode aside,.intro-mode #map-toggle,.intro-mode #intro-toggle,.intro-mode #help,.intro-mode #progress{display:none!important}
.intro-mode main{max-width:1120px;padding:clamp(32px,5vw,68px) 36px}
.intro-mode .course-intro{margin:0}
body{background:var(--bg);line-height:1.65}
button{background:transparent;border-color:#cbd0d2;border-radius:4px;box-shadow:none;transition:background .15s ease,border-color .15s ease,color .15s ease}
button:hover:not(:disabled){background:#edf0ef;border-color:#86959f;color:var(--blue);transform:none}
button.primary{background:var(--blue);border-color:var(--blue);box-shadow:none}
button.primary:hover:not(:disabled){background:#213c54;border-color:#213c54;color:#fff}
button:focus-visible,a:focus-visible,input:focus-visible{outline:2px solid #567f91;outline-offset:3px}
.topbar,.cover-mode .topbar{background:#fafaf7f5;border-bottom:1px solid var(--border);color:var(--ink);box-shadow:none;backdrop-filter:blur(8px)}
.brand,.cover-mode .brand{color:var(--ink);font-weight:600;letter-spacing:.015em}
.brand-mark,.cover-mode .brand-mark{background:transparent;color:var(--blue);border:1px solid #c8d0d4;border-radius:3px;box-shadow:none}
.brand-dot{color:#8a9397}
.top-actions>span{color:var(--muted);font-family:Consolas,"Cascadia Mono",monospace;font-size:12px}
.top-actions button,#help{background:transparent;border-color:var(--border);color:#566168}
.top-actions button:hover:not(:disabled),#help:hover:not(:disabled){background:#edf0ef;border-color:#98a5aa;color:var(--blue)}
aside{background:#f4f5f1;border-right-color:var(--border);scrollbar-color:#bfc7c5 transparent}
.rail-heading{color:var(--ink);font-weight:600;letter-spacing:.04em}
.rail-heading span,.rail-foot{color:var(--muted)}
.nav-group{color:#6c777b;border-top:1px solid #dfe3df;margin-top:14px;padding-top:19px;font-size:11px;letter-spacing:.08em}
.chapter{color:#4f5a5f;border:0;border-radius:2px}
.chapter:hover:not(:disabled){background:#e9edeb;color:var(--blue);transform:none}
.chapter .num{color:#7b8589;font-family:Consolas,monospace;font-size:11px}
.chapter.active{background:#e8eeee;color:var(--blue);box-shadow:inset 2px 0 var(--blue);font-weight:600}
.chapter.active .num{color:var(--blue)}
.chapter.done .num{color:var(--green)}
.key{border-color:#cdd4d1;border-radius:2px}
main{padding-top:38px}
.eyebrow{color:#68787f;font-size:11px;font-weight:500;letter-spacing:.13em}
.lesson-heading{margin-bottom:30px}
.lesson-heading h1{font-size:clamp(29px,2.5vw,36px);font-weight:600;letter-spacing:.015em;line-height:1.45;margin-top:10px}
#lesson-title{scroll-margin-top:106px}
.lesson-hook{color:#606b70;font-size:15px;line-height:1.9;margin-top:10px}
.concept-mark{color:#8c999e;font-size:43px}
.lab{background:#fff;border:1px solid #d9dfe0;border-radius:4px;box-shadow:none}
.lab-top{padding-top:20px;padding-bottom:12px}
#scene-label{color:#3e4b51;font-size:13px;font-weight:600;letter-spacing:.025em}
.legend{color:#657078}
.legend i{width:6px;height:6px}
.graphic{background:#fff}
.graphic text{font-variant-numeric:tabular-nums}
.graphic .grid{stroke:#ecefed;stroke-width:.8}
.graphic .axis{stroke:#aab4b8;stroke-width:1}
.graphic .curve{stroke-width:2.2}
.graphic .handle{filter:none}
.readouts{padding:10px 18px 21px}
.readout small{color:#69757a;font-size:11px;letter-spacing:.03em}
.readout strong{font:400 21px/1.5 Consolas,"Cascadia Mono",monospace;font-variant-numeric:tabular-nums;color:var(--ink)}
.controls{background:#f6f7f4;border-top-color:#e0e4e1;padding-top:17px;padding-bottom:17px}
.slider-caption{color:#4e5b61;font-size:13px}
.slider-caption output{font-family:Consolas,"Cascadia Mono",monospace;font-size:12px;color:var(--blue)}
input[type=range]{accent-color:var(--blue)}
.play-controls button{background:#fff;border-radius:3px}
.derivation{background:transparent;border:0;border-top:1px solid #cbd3d5;border-bottom:1px solid #dfe3e1;border-radius:0;box-shadow:none;margin-top:30px;padding:24px 0}
.step-head{color:#6a777c}
#step-count{font-family:Consolas,monospace}
.step-dot{height:3px;min-height:3px;border-radius:0;background:#dfe5e4}
.step-dot.past{background:#9eb5b9}
.step-dot.current{background:var(--blue)}
.previous-formula{color:#7b878c}
.formula{font-family:"Cambria Math","STIX Two Math",Georgia,serif}
.step-explanation{color:#49585e}
.step-explanation:not(:empty){background:transparent;border-left:1px solid #a3b8bd;border-radius:0;padding:12px 19px}
.step-explanation:not(:empty)::before{color:#52737d;font-weight:500;letter-spacing:.1em}
.step-why summary{color:#52737d}
.challenge{background:transparent;border:0;border-top:1px solid #cbd3d5;border-radius:0;box-shadow:none;padding:26px 0;margin-top:26px}
.answer{background:#fff;border-color:#cdd6d7;border-radius:3px}
.answer.wrong{background:#fbf0ed;border-color:var(--red)}
.answer.correct{background:#edf5f1;color:var(--green);border-color:var(--green)}
.learning-bridge{border-color:#d5dedb}
.page-foot{color:#778187;font-size:11px}
.course-intro{background:transparent;border:0;border-radius:0;box-shadow:none;padding:0}
.course-intro .eyebrow{color:#60727c;padding-bottom:20px;border-bottom:1px solid #c9d1d3}
.course-intro h2{font-size:clamp(28px,3vw,40px);font-weight:600;letter-spacing:.02em;line-height:1.6;color:var(--ink);margin:30px 0 21px}
.intro-lead{color:#4f5d63;font-size:16px;line-height:2.05;max-width:820px}
.intro-lead strong{font-weight:600;color:#303c44}
.intro-actions{margin-top:27px}
.intro-outcomes{gap:0;margin:40px 0 34px;padding:28px 0;border-top:1px solid #d3dada;border-bottom:1px solid #d3dada}
.intro-outcomes article{background:transparent;border:0;border-right:1px solid #dfe4e1;border-radius:0;box-shadow:none;padding:0 24px}
.intro-outcomes article:first-child{padding-left:0}
.intro-outcomes article:last-child{padding-right:0;border-right:0}
.intro-outcomes span{color:var(--blue);font-size:11px;font-weight:500;letter-spacing:.07em}
.intro-outcomes h3{font-size:17px;font-weight:600;margin:12px 0 9px}
.intro-outcomes p,.intro-route p{color:#677277;font-size:14px;line-height:1.95}
.intro-puzzle{background:transparent;border-left:2px solid #ae795a;padding:0 0 0 23px;border-radius:0;margin:34px 0}
.intro-puzzle h3{font-size:16px;font-weight:600}
.intro-route{padding-top:25px;border-top:1px solid #d3dada}
#intro-choices button[aria-pressed=true],#limit-options button[aria-pressed=true]{background:#edf2f3;border-color:#6c8b99}
.limit-task,.limit-advanced{background:transparent;border:0;border-top:1px solid #d3dada;border-radius:0;box-shadow:none;padding:22px 0;margin:20px 0}
#limit-options button{background:#fff;border-radius:3px}
#hint-toggle{background:transparent;border-color:#d3dddd;color:#52737d;border-radius:3px}
.slider-field.guide-target{outline:1px solid #7a9ba5;outline-offset:5px;background:#eaf0f0;border-radius:2px}
.inline-coach{background:#fffffc;color:#354650;border:1px solid #9eafb5;border-radius:3px;box-shadow:0 3px 12px #22344212;font-size:13px}
.inline-coach::before{color:#496e80;font-weight:600}
.inline-coach.observed{background:#f3f8f5;border-color:#9bb8ab}
.inline-coach.observed::before{color:#3e7869}
.graphic-coach::after,.fallback-coach::after,.slider-field>.inline-coach::after{border-color:#9eafb5}
dialog{background:var(--paper);border-radius:5px;box-shadow:0 10px 40px #27373c20}
dialog::backdrop{background:#26343b45;backdrop-filter:blur(2px)}
.cover{background:var(--bg);color:var(--ink)}
.cover::before,.cover-glow,.cover-demo::before{display:none}
.cover-kicker{color:#60727c;font-weight:500;font-size:11px;letter-spacing:.15em}
.cover-pulse{width:5px;height:5px;background:#607d89;box-shadow:none}
.cover h1{font-weight:600;letter-spacing:.01em;line-height:1.3}
.cover h1 em{color:var(--blue);text-shadow:none}
.cover-copy>p{color:#59686e;line-height:2;font-size:16px}
.cover-actions button{background:transparent;border-color:#acb8bc;color:var(--blue);border-radius:3px}
.cover-actions button:hover:not(:disabled){background:#edf0ef;border-color:#647c89;color:var(--blue)}
.cover-actions .cover-primary{background:var(--blue);color:#fff;border:1px solid var(--blue);box-shadow:none;font-weight:500}
.cover-actions .cover-primary:hover:not(:disabled){background:#213c54;border-color:#213c54;color:#fff}
.cover-footnote{color:#737f83;font-size:11px;letter-spacing:.04em}
.cover-demo{background:transparent;border:0;border-left:1px solid #cbd3d5;border-radius:0;padding:20px 0 20px 26px;box-shadow:none}
.cover-demo-head{color:#687a83;letter-spacing:.08em}
.cover-demo-head strong{color:#3b4d56;font-size:13px;font-weight:500}
.cover-live{color:#58766e;border-color:#b4c5bc;border-radius:2px;box-shadow:none}
.cover-plot-bg{fill:transparent;stroke:none}
.cover-grid,.cover-curve-glow{display:none}
.cover-axis{stroke:#9da9ae;stroke-width:1.2;opacity:1}
.cover-plot-label{fill:#465860;font-weight:500}
.cover-position-curve{stroke:var(--blue);stroke-width:2.8}
.cover-speed-curve{stroke:var(--green);stroke-width:2.8}
.cover-tangent{stroke:var(--orange);stroke-width:2.3;filter:none}
.cover-sweep{stroke:#9da9ae;stroke-width:1;opacity:1}
.cover-halo{fill:#537b86;opacity:.07}
.cover-left-point{fill:#fff;stroke:var(--blue);stroke-width:3;filter:none}
.cover-right-point{fill:#fff;stroke:var(--green);stroke-width:3;filter:none}
.cover-tag,.cover-bridge-text{fill:#62777f}
.cover-bridge{stroke:#7b9299;stroke-width:1.5}
.cover-metrics div:not(.cover-equals){background:transparent;border:0;border-top:1px solid #d3dada;border-radius:0;padding:13px 0}
.cover-metrics small{color:#65757c;font-size:11px}
.cover-metrics strong,.cover-metrics div:first-child strong{color:var(--ink);font:400 24px/1.4 Consolas,"Cascadia Mono",monospace}
.cover-equals{color:#8b979c;font-size:18px}
.cover-control button{background:transparent;color:var(--blue);border-color:#bac8cb;box-shadow:none}
.cover-control label{color:#5f6f76}
.cover-control output{color:#365466;font-family:Consolas,monospace}
.cover-control input{accent-color:var(--blue)}
.cover-formula{color:#5c6b73;font-family:"Cambria Math",Georgia,serif}
.cover-formula span{color:#98a2a5}
@media(max-width:1080px){.cover-demo{border-left:0;border-top:1px solid #cbd3d5;padding:25px 0 0}}
@media(max-width:760px){
 .intro-mode main{padding:30px 22px 38px}
 .topbar{padding:0 15px}
 .brand{white-space:nowrap;flex-shrink:0;font-size:16px;gap:7px}
 .top-actions{gap:5px}
 .top-actions>span{display:none}
 .top-actions button,#intro-toggle,#map-toggle{white-space:nowrap;font-size:11px;padding:5px 7px;min-height:33px}
 aside.open{background:#f4f5f1}
 main{padding:26px 15px 18px}
 .lesson-heading h1{font-size:26px}
 #lesson-title{scroll-margin-top:89px}
 .course-intro{padding:0}
 .course-intro h2{font-size:27px;line-height:1.6;margin-top:24px}
 .course-intro .eyebrow{line-height:1.8;padding-bottom:16px}
 .intro-lead{font-size:15px;line-height:2}
 .intro-outcomes{margin:29px 0;padding:0}
 .intro-outcomes article,.intro-outcomes article:first-child,.intro-outcomes article:last-child{padding:23px 0;border-right:0;border-bottom:1px solid #dfe4e1}
 .intro-outcomes article:last-child{border-bottom:0}
 .intro-puzzle{padding-left:16px}
 .derivation{padding:21px 0}
 .lab-top{padding-top:16px}
 .readout strong{font-size:19px}
 .cover{padding:35px 22px 42px}
 .cover-demo{border-radius:0;padding:22px 0 0}
 .cover-metrics strong,.cover-metrics div:first-child strong{font-size:20px}
}
@media(prefers-reduced-motion:reduce){button{transition:none}}

/* Keep the lesson still while the chapter directory opens above it. */
.shell{display:block}
.top-start{display:flex;align-items:center;gap:14px;min-width:0}
#map-toggle{display:grid;place-items:center;flex-shrink:0;width:44px;height:44px;min-width:44px;padding:10px;background:transparent;border-color:transparent;color:var(--ink)}
#map-toggle svg{display:block;fill:none;stroke:currentColor;stroke-width:1.5;stroke-linecap:round}
#map-toggle:hover,#map-toggle[aria-expanded="true"]{background:#e8eeee;border-color:var(--border);color:var(--blue)}
#chapter-nav{position:fixed;top:76px;bottom:0;left:0;width:240px;max-width:calc(100vw - 36px);height:calc(100dvh - 76px);z-index:4;padding:24px 17px 20px 22px;background:#f4f5f1;border-right:1px solid var(--border);box-shadow:6px 0 18px #30373d0a;overscroll-behavior:contain}
#chapter-nav.open{display:block}
@media(max-width:760px){#chapter-nav{top:65px;height:calc(100dvh - 65px)}.top-start{gap:7px}#map-toggle{min-width:44px;padding:10px}}