:root{--ink:#192e2a;--muted:#52645e;--green:#173e37;--green-light:#e5eee7;--paper:#fffefa;--canvas:#f3f1e9;--line:#cdd6cf;--blue:#22547b;--orange:#944415;--focus:#9b4517;font-family:"Segoe UI",Arial,sans-serif;color:var(--ink);background:var(--canvas);font-synthesis:none}
*{box-sizing:border-box}body{margin:0}button,select{font:inherit}button,a,select{-webkit-tap-highlight-color:transparent}
button{cursor:pointer;border:1px solid #a5b5ad;border-radius:8px;background:var(--paper);color:var(--green);padding:.7rem 1rem;font-weight:650;min-height:46px;line-height:1.35}
button:hover{background:var(--green-light);border-color:var(--green)}button:disabled{opacity:.55;cursor:default}
button.primary{background:var(--green);color:white;border-color:var(--green)}button.primary:hover{background:#25584c}
button:focus-visible,a:focus-visible,select:focus-visible,input:focus-visible,summary:focus-visible{outline:3px solid var(--focus);outline-offset:4px}
a{color:var(--blue);text-underline-offset:4px}p{line-height:1.55;margin:.6rem 0}h1,h2,h3{line-height:1.18;margin:0 0 1rem;font-weight:650;letter-spacing:-.025em}
h1{font-size:clamp(1.9rem,3vw,2.65rem)}h2{font-size:1.7rem}h3{font-size:1.2rem}
.eyebrow{font-size:.76rem;letter-spacing:.1em;font-weight:750;margin:0 0 .5rem;color:var(--muted)}.quiet{color:var(--muted);font-size:.9rem}
.site-header,nav,main,.lesson-controls,footer{max-width:1248px;margin:0 auto}
.site-header{display:flex;justify-content:space-between;align-items:center;gap:1.5rem;padding:1.3rem 2rem 1.1rem}
.brand{display:flex;gap:.8rem;align-items:center}.brand-mark{display:grid;place-items:center;background:var(--green);color:white;font:2rem Georgia,serif;width:48px;height:48px;border-radius:12px;flex-shrink:0}
.brand-title{margin:0;font-weight:750;font-size:1.15rem}.brand-title span{font-weight:400;margin-left:.35rem}
.utilities{display:flex;gap:.5rem;flex-wrap:wrap}.utilities button{font-size:.9rem;padding:.55rem .7rem}
nav{display:flex;gap:.4rem;padding:0 2rem 1rem;flex-wrap:wrap;border-bottom:1px solid var(--line)}nav button{background:transparent;border-color:transparent;font-size:.92rem;padding:.55rem .8rem}nav button[aria-current=step]{background:var(--green);color:white}
main{padding:2rem 2rem 1.3rem;min-height:440px;container-type:inline-size}main:focus{outline:none}
.section-heading{display:flex;gap:1rem;align-items:flex-start;justify-content:space-between;margin-bottom:1rem}.section-heading h1{margin-bottom:.4rem;max-width:960px}
.step-tag{background:var(--green-light);padding:.45rem .8rem;border-radius:24px;white-space:nowrap;font-size:.9rem;font-weight:650;margin-top:.25rem}
.context-row{display:flex;justify-content:space-between;gap:1rem;align-items:center;border-block:1px solid var(--line);padding:.6rem 0;margin-bottom:1.5rem}.context-row p{font-size:1rem;margin:0}.context-row button{font-size:.9rem;flex-shrink:0}
.start-grid{display:grid;grid-template-columns:1.12fr 1fr;gap:3rem;align-items:start}.start-grid h1{font-size:clamp(2.4rem,4vw,3.4rem);max-width:650px}.lead{font-size:1.2rem;max-width:620px}
.start-visual,.board{background:var(--paper);border:1px solid var(--line);border-radius:14px;padding:1.5rem;min-width:0}
.three-actions{padding:0;list-style:none;margin:1.4rem 0}.three-actions li{display:flex;gap:1rem;padding:.75rem 0;border-bottom:1px solid var(--line);line-height:1.5;font-size:1.03rem}.three-actions b{display:grid;place-items:center;flex-shrink:0;background:var(--green-light);width:30px;height:30px;border-radius:50%;font-size:.95rem}
.paper-ready{font-size:1rem;margin:1rem 0 1.2rem}.parent-start{border-left:3px solid var(--green);padding:.2rem 0 .2rem 1rem;margin-top:1.5rem}.parent-start button,.text-button{border:0;background:transparent;text-decoration:underline;text-underline-offset:4px;padding:.5rem 0;font-size:.95rem}.text-button{margin-top:1rem}.parent-start p{font-size:1.05rem}
.goal{font-size:1.1rem;max-width:850px;margin-bottom:1.3rem}.lesson-grid{display:grid;grid-template-columns:minmax(0,1.22fr) minmax(0,1fr);gap:2rem;align-items:start}
.board-caption,.scale-label,.check-label{font-size:.78rem;letter-spacing:.06em;font-weight:700;color:var(--muted);margin:0 0 1rem;line-height:1.45}
.explain{font-size:1.15rem;min-width:0}.explain p:first-child{margin-top:0}.action-line{font-weight:650;margin:1.2rem 0}
.discovery{background:var(--green);color:#fffefa;border-radius:10px;padding:1.15rem 1.25rem;margin:1.25rem 0}.discovery .eyebrow{color:#e2eee7;font-size:.74rem}.discovery strong{font-size:1.42rem;line-height:1.35;display:block;letter-spacing:-.015em}
.note{border-top:1px solid var(--line);padding-top:1rem;margin-top:1.25rem;font-size:1rem;color:var(--muted)}.step-help{margin-top:1rem;border-top:1px solid var(--line);font-size:1rem}.step-help p{margin:.5rem 0 1rem}
.math-scroll{max-width:100%;overflow-x:auto;padding:.1rem .05rem .35rem}.math-stack{display:table;margin:0 auto;border-collapse:separate;border-spacing:.2rem .55rem;font-family:"Cambria Math",Cambria,Georgia,serif;font-size:clamp(1.6rem,2.7vw,2.25rem);line-height:1.4;white-space:nowrap}
.math-row{display:table-row}.math-row>span{display:table-cell;vertical-align:middle}.math-row .lhs{text-align:right}.math-row .eq{padding:0 .35em;text-align:center}.math-row .rhs{text-align:left}.math-stack.derived .math-row:last-child{font-weight:650}.math-row.last-rule>span{border-top:2px solid var(--ink);padding-top:.45rem}
var{font-family:"Cambria Math",Cambria,Georgia,serif}.math-row var,.addition var{font-weight:inherit}
.frac{display:inline-flex;vertical-align:middle;flex-direction:column;align-items:center;line-height:1.2;padding:.18em .05em;font-weight:400}.frac>span:first-child{border-bottom:1.6px solid currentColor;padding:0 .18em .1em}.frac>span:last-child{padding:.1em .18em 0}
.addition{display:grid;grid-template-columns:.7em 1.2fr 1.3fr .7em 1fr;gap:.65rem .2rem;align-items:center;font-family:"Cambria Math",Cambria,Georgia,serif;font-size:clamp(1.6rem,2.7vw,2.25rem);padding:.5rem 0}
.addition>span{text-align:center;padding:.2rem;white-space:nowrap}.addition .add-label{font-family:"Segoe UI",Arial,sans-serif;font-size:.76rem;color:var(--muted);font-weight:600;padding:0;white-space:normal;line-height:1.4}.addition .total{font-weight:650}.addition .opposites{background:#f8eade;color:#7f3810;border-radius:5px}.addition .zero{background:var(--green-light);color:var(--green);border-radius:5px}
.sum-rule{grid-column:1/-1;border-top:2px solid var(--ink);height:0;padding:0!important;margin:0}
.under-board{font-size:1rem;line-height:1.55;margin:1rem 0 0;color:var(--muted);text-align:center}.under-board var{font-size:1.2em}
.scale-group+.scale-group{border-top:1px solid var(--line);margin-top:1.2rem;padding-top:1.2rem}.scale-group .math-stack{font-size:clamp(1.5rem,2.3vw,2rem)}
.choice-math{font:1.9rem/1.5 Cambria,Georgia,serif;display:grid;gap:.25rem;margin:.8rem 0;text-align:center}.pair-result{margin:1.2rem auto 0;text-align:center;font:2.6rem Cambria,Georgia,serif;color:var(--green)}.pair-result small{display:block;font:1rem "Segoe UI",Arial,sans-serif;margin-top:.4rem}
.checks{display:grid;gap:1.4rem}.checks section+section{padding-top:1.2rem;border-top:1px solid var(--line)}.checks .math-stack{font-size:clamp(1.5rem,2.2vw,2rem)}.check-true{display:block;color:var(--green);font-size:1rem;text-align:center;margin-top:.6rem;font-weight:700}
.lesson-controls{display:flex;justify-content:space-between;align-items:center;gap:1rem;padding:1rem 2rem 1.3rem;border-top:1px solid var(--line)}.progress span{display:block;line-height:1.5;font-size:.9rem}.progress span:first-child{font-weight:700}.progress span+span{color:var(--muted)}.travel{display:flex;gap:.7rem}.travel .primary{max-width:370px;text-align:center}
footer{padding:1rem 2rem 1.5rem}.footer-row,.footer-links{display:flex;gap:1rem;align-items:center;justify-content:space-between;flex-wrap:wrap}.footer-links{justify-content:flex-start}.preferences{display:flex;gap:.7rem;align-items:center;flex-wrap:wrap;font-size:.9rem}.preferences select{background:var(--paper);border:1px solid #839c8f;padding:.55rem;border-radius:6px;color:var(--ink);min-height:44px;font-size:1rem}.preferences button{font-size:.9rem;background:transparent}.optional-tools{margin:1rem 0;border-block:1px solid var(--line);padding:.25rem 0 .5rem}.optional-tools>p{font-size:1rem}
summary{cursor:pointer;font-weight:650;line-height:1.5;min-height:46px;padding:.65rem .1rem}.paper-tabs{display:flex;gap:.6rem;margin:0 0 1.3rem}.paper-tabs button{flex:1}.paper-tabs button[aria-pressed=true]{background:var(--green);color:white}
.guided-prep{background:var(--paper);border:1px solid var(--line);border-radius:14px;padding:1.5rem;margin:1.5rem 0}.guided-prep>p{max-width:850px;font-size:1.1rem}.prep-grid{display:grid;grid-template-columns:minmax(0,1fr) minmax(0,1fr);gap:2rem;margin-top:1.5rem}.prep-grid>section+section{border-left:1px solid var(--line);padding-left:2rem}.prep-grid .math-stack{font-size:1.8rem}
.try-section{padding-top:1.5rem;margin-top:1.4rem;border-top:1px solid var(--line)}.try-section>p{max-width:850px;font-size:1.1rem}.paper-actions{display:flex;gap:.7rem;flex-wrap:wrap;margin-top:1rem}
.hint-box{padding:1rem 1.2rem;border-left:3px solid var(--orange);background:#f9eee4;font-size:1.05rem;margin-top:1rem;line-height:1.6}.hint-box ol{padding-left:1.3rem;margin-bottom:0}.hint-box li+li{margin-top:.7rem}
.paper-check{margin-top:1.5rem;padding-top:1.3rem;border-top:1px solid var(--line)}.paper-check details{margin:1rem 0;border:1px solid var(--line);border-radius:8px;padding:1rem;background:var(--paper)}.paper-check .math-stack{font-size:1.8rem}.paper-check .scale-group{max-width:600px;margin-inline:auto}.paper-check .addition{max-width:600px;margin-inline:auto}.work-chunk{padding:1.5rem 0;border-top:1px solid var(--line);max-width:800px;margin:auto}.work-chunk h3{text-align:center}.route-note{font-size:1rem;color:var(--muted)}
.finish-grid{display:grid;grid-template-columns:1.25fr 1fr;gap:3rem}.finish-list{padding-left:1.5rem;line-height:1.7;font-size:1.1rem}.finish-list li{margin:.8rem 0}
dialog{border:1px solid var(--line);border-radius:14px;background:var(--paper);color:var(--ink);padding:2rem;width:min(720px,calc(100% - 2rem));max-height:90dvh;overflow:auto;box-shadow:0 20px 80px #142e2940}dialog::backdrop{background:#142e2980}dialog p{font-size:1.05rem}.dialog-top{display:flex;gap:1rem;justify-content:space-between;align-items:start;margin-bottom:1rem}.dialog-top .eyebrow{padding-top:.6rem}.dialog-actions{display:flex;gap:.7rem;flex-wrap:wrap}.guide-list{font-size:1.05rem;line-height:1.6;padding-left:1.4rem}.guide-list li{margin:.9rem 0}.glossary{font-size:1rem;line-height:1.6}.glossary dt{font-weight:700;margin-top:.8rem}.glossary dd{margin:0}.resume-banner{padding:1rem;background:var(--green-light);border-left:3px solid var(--green);margin:0 0 1.4rem}
.explorer-dialog{width:min(1120px,calc(100% - 2rem))}.explorer-controls{display:flex;align-items:flex-end;flex-wrap:wrap;gap:1rem;margin:1.5rem 0}.explorer-controls label{display:flex;flex-direction:column;gap:.5rem;font-weight:650;font-size:.95rem}.explorer-controls select{background:var(--paper);border:1px solid #839c8f;color:var(--ink);border-radius:7px;min-height:46px;padding:.6rem;font-size:1.05rem}.explorer-controls label:first-child{flex:1;min-width:220px}.explorer-layout{display:grid;grid-template-columns:minmax(0,.85fr) minmax(0,1.4fr);gap:1.4rem}.explorer-original{padding:1rem .2rem}.explorer-original-row{display:flex;gap:.8rem;align-items:center;margin:.9rem 0}.eq-number{display:grid;place-items:center;flex-shrink:0;border:1px solid var(--line);border-radius:50%;width:28px;height:28px;font:650 .9rem "Segoe UI",Arial,sans-serif}.explorer-equation{display:grid;grid-template-columns:1fr auto auto;align-items:center;gap:.55rem;font:1.65rem Cambria,Georgia,serif;flex:1}.explorer-equation>span:first-child{text-align:right}.explorer-add{display:grid;grid-template-columns:max-content 1fr 1.2fr auto .9fr;gap:.65rem .3rem;align-items:center;font:1.9rem Cambria,Georgia,serif}.explorer-add>span{text-align:center;padding:.2rem .1rem;white-space:nowrap}.explorer-add .explorer-col-label{font:600 .76rem/1.3 "Segoe UI",Arial,sans-serif;white-space:normal;color:var(--muted)}.explorer-add .explorer-row-label{font:600 .8rem/1.4 "Segoe UI",Arial,sans-serif;color:var(--muted);text-align:right;padding-right:.3rem}.explorer-add .cancels{background:var(--green-light);border-radius:4px}.explorer-status{font-size:1.05rem;margin-top:1.2rem;padding-top:1rem;border-top:1px solid var(--line)}.explorer-status.ready{color:var(--green);font-weight:650}
.sr-only{position:absolute;width:1px;height:1px;padding:0;margin:-1px;overflow:hidden;clip:rect(0,0,0,0);white-space:nowrap;border:0}.skip:not(:focus){position:fixed;width:1px;height:1px;overflow:hidden;clip-path:inset(50%);white-space:nowrap}.skip:focus{position:fixed;left:1rem;top:1rem;background:var(--paper);padding:1rem;z-index:20}.noscript{padding:2rem;max-width:800px;margin:auto}[hidden]{display:none!important}
.scene-enter{animation:enter .2s ease-out}@keyframes enter{from{opacity:.7;transform:translateY(4px)}to{opacity:1;transform:none}}body[data-motion=still] .scene-enter{animation:none}
@media(prefers-reduced-motion:reduce){*{animation:none!important;scroll-behavior:auto!important}}
@media(max-width:1000px){.brand-title span{display:block;margin:0}.site-header{align-items:flex-start}.utilities{justify-content:flex-end}.lesson-grid{gap:1.4rem}.board{padding:1.2rem}.explain{font-size:1.1rem}.start-grid{gap:1.8rem}}
@media(max-width:850px){.explorer-layout{grid-template-columns:1fr}.explorer-original{max-width:500px}.explorer-result{padding:1.2rem}.explorer-add{font-size:1.85rem}.context-row{align-items:flex-start}.context-row button{max-width:180px}}
@media(max-width:760px){.site-header{padding:1rem;display:block}.brand-title span{display:inline;margin-left:.3rem}.utilities{justify-content:flex-start;margin-top:.8rem;gap:.4rem}nav{padding:0 1rem .8rem;gap:.2rem}nav button{padding:.5rem .65rem;font-size:.9rem}main{padding:1.5rem 1rem}.start-grid,.lesson-grid,.finish-grid,.prep-grid{grid-template-columns:1fr;gap:1.3rem}.start-grid h1{font-size:2.6rem}.section-heading{display:block}.step-tag{display:inline-block;margin-top:.5rem}.context-row{display:block;margin-bottom:1.2rem}.context-row button{max-width:none;margin-top:.6rem}.board{padding:1.2rem}.math-stack{font-size:1.85rem}.addition{font-size:1.85rem}.scale-group .math-stack{font-size:1.65rem}.checks .math-stack{font-size:1.7rem}.explain{font-size:1.12rem}.guided-prep{padding:1.2rem}.prep-grid>section+section{border-left:0;border-top:1px solid var(--line);padding:1.2rem 0 0}.prep-grid .math-stack{font-size:1.7rem}.lesson-controls{padding:1rem;display:block}.travel{margin-top:.8rem;justify-content:space-between}.travel .primary{flex:1}.progress span+span{display:none}footer{padding:1rem}.footer-row{gap:.4rem}.paper-tabs{flex-wrap:wrap}.paper-tabs button{font-size:.95rem;padding:.65rem;min-width:140px}.paper-check .math-stack{font-size:1.6rem}dialog{padding:1.2rem;width:calc(100% - 1.2rem)}.dialog-top{gap:.5rem}.dialog-top .eyebrow{font-size:.75rem}.explorer-dialog{width:calc(100% - 1.2rem)}.explorer-controls{gap:.8rem}.explorer-controls label:first-child{flex-basis:100%;min-width:0}.explorer-controls label:not(:first-child){flex:1}.explorer-add{font-size:1.6rem;gap:.6rem .1rem}.explorer-add .explorer-row-label{font-size:.75rem}.explorer-result{padding:.75rem}.explorer-equation{font-size:1.55rem}.explorer-original-row{gap:.5rem}}
@media(max-width:390px){.brand-title{font-size:1rem}.brand-mark{width:42px;height:42px}.board{padding:.85rem}.math-stack{font-size:1.55rem;border-spacing:.1rem .5rem}.math-row .eq{padding:0 .22em}.addition{font-size:1.65rem}.scale-group .math-stack{font-size:1.45rem}.checks .math-stack{font-size:1.5rem}.prep-grid .math-stack{font-size:1.5rem}.paper-check .math-stack{font-size:1.4rem}.paper-check details{padding:.75rem}.start-grid h1{font-size:2.3rem}.explain{font-size:1.08rem}.choice-math{font-size:1.65rem}.explorer-add{grid-template-columns:30px minmax(0,.85fr) minmax(0,1.2fr) 14px minmax(0,.85fr);font-size:1.35rem;column-gap:2px}.explorer-add .explorer-row-label{font-size:.73rem;max-width:30px;white-space:normal;padding:0}.explorer-add .explorer-col-label{font-size:.73rem}.explorer-add>span{padding-inline:0}.explorer-equation{font-size:1.4rem;gap:.3rem}.explorer-original-row{gap:.3rem}}
@container(max-width:760px){.start-grid,.lesson-grid,.finish-grid,.prep-grid{grid-template-columns:1fr}.prep-grid>section+section{border-left:0;border-top:1px solid var(--line);padding:1.2rem 0 0}.section-heading{display:block}.step-tag{display:inline-block;margin-bottom:.6rem}.context-row{display:block}.context-row button{max-width:none;margin-top:.6rem}}
@media print{.site-header,nav,.lesson-controls,footer,dialog,.paper-actions,.paper-tabs,.context-row button,#replay{display:none!important}main{padding:0;max-width:none}.lesson-grid{display:block}.board{border:0}.explain{margin-top:1rem}body{background:white}.scene-enter{animation:none}}