:root {
  --ink: #172238;
  --muted: #59657a;
  --paper: #f7f9ff;
  --white: #ffffff;
  --line: #dce2f1;
  --blue: #3857e9;
  --blue-dark: #253ab3;
  --blue-soft: #e9edff;
  --cyan: #58d8ec;
  --lime: #b7ee4a;
  --yellow: #ffd95a;
  --coral: #ff7a73;
  --shadow: 0 12px 30px rgba(38, 55, 110, .11);
  --radius: 20px;
  font-family: "Avenir Next", Avenir, "Segoe UI", system-ui, sans-serif;
  color: var(--ink);
  background: var(--paper);
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body { margin: 0; min-width: 320px; font-size: 1rem; line-height: 1.6; }
a { color: inherit; }
[hidden] { display: none !important; }
.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; }
.shell { width: min(1120px, calc(100% - 40px)); margin-inline: auto; }
.skip-link { position: fixed; top: 10px; left: 10px; z-index: 100; padding: 10px 16px; background: var(--ink); color: white; border-radius: 10px; transform: translateY(-150%); }
.skip-link:focus { transform: translateY(0); }

.site-header { position: relative; z-index: 10; background: var(--white); border-bottom: 1px solid var(--line); }
.header-inner { min-height: 74px; display: flex; align-items: center; justify-content: space-between; gap: 24px; }
.brand { display: inline-flex; align-items: center; gap: 10px; color: var(--ink); text-decoration: none; font-size: 1.08rem; font-weight: 750; letter-spacing: -.02em; }
.brand strong { color: var(--blue); }
.brand-mark { display: grid; place-items: center; width: 38px; height: 38px; border-radius: 12px; background: var(--blue); color: white; font: 800 1.25rem/1 Georgia, serif; transform: rotate(-3deg); }
.main-nav { display: flex; align-items: center; gap: 8px; }
.main-nav a { min-height: 44px; display: inline-flex; align-items: center; padding: 0 15px; border-radius: 12px; color: #39465d; text-decoration: none; font-weight: 650; }
.main-nav a:hover { background: var(--blue-soft); color: var(--blue-dark); }
.main-nav .nav-contribute { background: var(--ink); color: white; }
.main-nav .nav-contribute:hover { background: var(--blue); color: white; }

.home-intro { position: relative; overflow: hidden; padding: 62px 0 72px; background: var(--blue-soft); }
.home-intro::before { content: ""; position: absolute; inset: 0; opacity: .55; background-image: linear-gradient(rgba(56,87,233,.08) 1px, transparent 1px), linear-gradient(90deg, rgba(56,87,233,.08) 1px, transparent 1px); background-size: 28px 28px; mask-image: linear-gradient(90deg, #000, transparent 66%); }
.home-intro-grid { position: relative; display: grid; grid-template-columns: 1fr .88fr; align-items: center; gap: 74px; }
.eyebrow { margin: 0 0 10px; color: var(--blue-dark); font-size: .8rem; font-weight: 850; letter-spacing: .12em; }
.intro-copy h1 { max-width: 720px; margin: 0; font-size: clamp(3rem, 6.5vw, 5.7rem); line-height: .98; letter-spacing: -.065em; }
.intro-copy h1 span { position: relative; color: var(--blue); }
.intro-copy h1 span::after { content: ""; position: absolute; left: 0; right: 2%; bottom: -8px; height: 9px; border-radius: 50%; border-top: 5px solid var(--lime); transform: rotate(-1deg); }
.intro-lede { max-width: 590px; margin: 28px 0; color: #46546b; font-size: 1.17rem; line-height: 1.65; }
.primary-button, .secondary-button { min-height: 50px; display: inline-flex; align-items: center; justify-content: center; gap: 12px; padding: 0 20px; border-radius: 14px; font-weight: 800; text-decoration: none; }
.primary-button { background: var(--blue); color: white; box-shadow: 0 8px 0 var(--blue-dark); }
.primary-button:hover { transform: translateY(2px); box-shadow: 0 6px 0 var(--blue-dark); }

.math-board { position: relative; min-height: 335px; padding: 46px 34px 34px; border: 4px solid var(--ink); border-radius: 28px; background: var(--white); box-shadow: 14px 16px 0 var(--cyan); transform: rotate(2deg); }
.board-label { position: absolute; top: 18px; left: 24px; color: var(--muted); font-size: .72rem; font-weight: 850; letter-spacing: .12em; }
.equation { min-height: 185px; display: flex; align-items: center; justify-content: center; gap: clamp(10px, 2.5vw, 22px); font: 750 clamp(2rem, 4.5vw, 3.6rem)/1 Georgia, serif; }
.fraction { display: inline-grid; grid-template-rows: 1fr 1fr; text-align: center; }
.fraction span:first-child { padding: 0 9px 4px; border-bottom: 4px solid currentColor; }
.fraction span:last-child { padding-top: 5px; }
.fraction.answer { color: var(--blue); }
.board-note { width: fit-content; margin-inline: auto; padding: 9px 14px; border-radius: 10px; background: #fff2b8; color: #624c00; font-weight: 750; transform: rotate(-2deg); }
.shape { position: absolute; }
.shape-circle { width: 28px; height: 28px; right: 22px; top: 20px; border: 6px solid var(--coral); border-radius: 50%; }
.shape-triangle { width: 0; height: 0; left: 24px; bottom: 26px; border-left: 14px solid transparent; border-right: 14px solid transparent; border-bottom: 24px solid var(--lime); transform: rotate(-12deg); }
.shape-spark { right: 25px; bottom: 19px; color: var(--blue); font-size: 1.8rem; }

.topics-section { padding: 78px 0 92px; }
.section-heading-row { display: flex; align-items: end; justify-content: space-between; gap: 40px; margin-bottom: 32px; }
.section-heading-row h2, .help-strip h2 { margin: 0; font-size: clamp(2rem, 4vw, 3rem); line-height: 1.1; letter-spacing: -.045em; }
.section-heading-row > p { max-width: 390px; margin: 0 0 4px; color: var(--muted); }
.topic-tools { display: flex; align-items: center; justify-content: space-between; gap: 18px; margin-bottom: 8px; padding: 12px; border: 1px solid var(--line); border-radius: 18px; background: var(--white); }
.topic-search { min-width: min(340px, 100%); display: flex; align-items: center; gap: 8px; min-height: 48px; padding: 0 14px; border: 2px solid transparent; border-radius: 12px; background: #f2f5fb; color: var(--muted); }
.topic-search:focus-within { border-color: var(--blue); background: white; }
.topic-search > span[aria-hidden] { font-size: 1.45rem; line-height: 1; transform: rotate(-15deg); }
.topic-search input { width: 100%; border: 0; outline: 0; background: transparent; color: var(--ink); font: inherit; }
.topic-search input::placeholder { color: #717b8d; }
.grade-filters { display: flex; gap: 6px; overflow-x: auto; scrollbar-width: none; }
.grade-filters::-webkit-scrollbar { display: none; }
.filter-button { min-height: 44px; flex: none; padding: 0 13px; border: 0; border-radius: 11px; background: transparent; color: #4b5870; font: 700 .88rem/1 inherit; cursor: pointer; }
.filter-button:hover { background: var(--blue-soft); }
.filter-button.is-active { background: var(--ink); color: white; }
.results-count { min-height: 24px; margin: 8px 4px 14px; color: var(--muted); font-size: .86rem; font-weight: 650; }
.topic-grid { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 20px; }
.topic-card { min-height: 330px; display: flex; flex-direction: column; justify-content: space-between; padding: 24px; border: 2px solid var(--line); border-radius: var(--radius); background: var(--white); box-shadow: 0 4px 0 var(--line); text-decoration: none; transition: .18s ease; }
.topic-card:hover { border-color: var(--blue); box-shadow: var(--shadow); transform: translateY(-4px); }
.topic-card-top { display: flex; justify-content: space-between; align-items: start; gap: 12px; }
.topic-icon { display: grid; place-items: center; width: 54px; height: 54px; border-radius: 16px; background: var(--blue-soft); color: var(--blue); font: 800 1.6rem/1 Georgia, serif; }
.topic-card:nth-child(3n+2) .topic-icon { background: #e2f9fb; color: #087d90; }
.topic-card:nth-child(3n+3) .topic-icon { background: #effbd7; color: #567e08; }
.grade-pill { padding: 5px 9px; border-radius: 999px; background: #f1f3f8; color: #556177; font-size: .78rem; font-weight: 750; }
.topic-subject { margin: 24px 0 5px; color: var(--blue); font-size: .78rem; font-weight: 850; letter-spacing: .1em; text-transform: uppercase; }
.topic-card h3 { margin: 0; font-size: 1.35rem; line-height: 1.25; letter-spacing: -.025em; }
.topic-card h3 + p { margin: 12px 0 0; color: var(--muted); line-height: 1.55; }
.card-link { margin-top: 26px; color: var(--blue); font-weight: 800; }
.no-results { padding: 50px 20px; border: 2px dashed var(--line); border-radius: var(--radius); text-align: center; background: white; }
.no-results > span { display: inline-grid; place-items: center; width: 54px; height: 54px; border-radius: 50%; background: var(--blue-soft); color: var(--blue); font-size: 1.5rem; font-weight: 800; }
.no-results h3 { margin: 14px 0 4px; font-size: 1.25rem; }
.no-results p { margin: 0; color: var(--muted); }

/* Lesson pages */
.lesson-hero { position: relative; overflow: hidden; padding: 34px 0 42px; background: var(--blue-soft); border-bottom: 1px solid #d1d8f3; }
.lesson-hero::after { content: ""; position: absolute; width: 240px; height: 240px; right: -70px; top: -120px; border: 36px solid rgba(88, 216, 236, .5); border-radius: 50%; }
.lesson-hero-inner { position: relative; z-index: 1; }
.breadcrumb { display: flex; gap: 9px; align-items: center; margin-bottom: 34px; color: var(--muted); font-size: .92rem; }
.breadcrumb a { color: var(--blue); font-weight: 750; text-decoration: none; }
.breadcrumb a:hover { text-decoration: underline; }
.lesson-title-row { display: grid; grid-template-columns: 1fr auto; align-items: center; gap: 34px; }
.lesson-title-row h1 { max-width: 820px; margin: 0; font-size: clamp(2.35rem, 5vw, 4.5rem); line-height: 1.02; letter-spacing: -.055em; }
.lesson-summary { max-width: 700px; margin: 16px 0 0; color: #4b5871; font-size: 1.12rem; }
.lesson-symbol { display: grid; place-items: center; width: 126px; height: 126px; border: 4px solid var(--ink); border-radius: 30px; background: var(--yellow); box-shadow: 9px 10px 0 var(--cyan); font: 800 3.4rem/1 Georgia, serif; transform: rotate(3deg); }
.lesson-meta { display: flex; flex-wrap: wrap; gap: 0; margin: 36px 0 0; }
.lesson-meta div { min-width: 160px; padding: 0 28px; border-left: 1px solid #c7cfea; }
.lesson-meta div:first-child { padding-left: 0; border-left: 0; }
.lesson-meta dt { color: #667087; font-size: .75rem; font-weight: 800; letter-spacing: .1em; text-transform: uppercase; }
.lesson-meta dd { margin: 2px 0 0; font-weight: 800; }
.lesson-layout { display: grid; grid-template-columns: 220px minmax(0, 720px); justify-content: center; gap: 70px; padding-top: 58px; padding-bottom: 90px; }
.lesson-main { grid-column: 2; grid-row: 1; min-width: 0; }
.lesson-nav { position: sticky; top: 24px; grid-column: 1; grid-row: 1; align-self: start; padding: 18px; border: 1px solid var(--line); border-radius: 16px; background: white; }
.lesson-nav > p { margin: 0 0 9px; font-size: .78rem; font-weight: 850; letter-spacing: .1em; text-transform: uppercase; }
.lesson-nav ol { margin: 0; padding: 0; list-style: none; }
.lesson-nav li + li { margin-top: 3px; }
.lesson-nav li a { display: block; padding: 7px 8px; border-radius: 8px; color: var(--muted); font-size: .88rem; line-height: 1.35; text-decoration: none; }
.lesson-nav li a:hover { background: var(--blue-soft); color: var(--blue-dark); }
.back-link { display: block; margin-top: 14px; padding: 12px 8px 0; border-top: 1px solid var(--line); color: var(--blue); font-size: .88rem; font-weight: 800; text-decoration: none; }
.interactive-lab { margin-bottom: 58px; padding: 28px; scroll-margin-top: 20px; border: 3px solid var(--ink); border-radius: 24px; background: var(--white); box-shadow: 9px 10px 0 var(--cyan); font-family: "Avenir Next", Avenir, "Segoe UI", sans-serif; }
.lab-heading { display: flex; align-items: start; justify-content: space-between; gap: 20px; }
.lab-heading .eyebrow { margin-bottom: 4px; color: var(--blue); }
.lab-heading h2 { margin: 0; font-size: clamp(1.65rem, 3vw, 2.2rem); line-height: 1.1; letter-spacing: -.04em; }
.lab-badge { flex: none; padding: 7px 10px; border-radius: 999px; background: var(--blue-soft); color: var(--blue-dark); font-size: .78rem; font-weight: 800; }
.lab-intro { margin: 10px 0 24px; color: var(--muted); }
.demo-loading, .demo-note { color: var(--muted); }
.lab-controls { display: grid; gap: 14px; margin-bottom: 18px; }
.lab-controls.two-up { grid-template-columns: repeat(2, minmax(0, 1fr)); }
.lab-controls.three-up { grid-template-columns: repeat(3, minmax(0, 1fr)); }
.lab-controls fieldset { min-width: 0; margin: 0; padding: 16px; border: 1px solid var(--line); border-radius: 14px; }
.lab-controls legend { padding: 0 6px; color: var(--ink); font-size: .85rem; font-weight: 800; }
.lab-control { display: block; color: #46546b; font-size: .88rem; font-weight: 750; }
.lab-control + .lab-control { margin-top: 14px; }
.control-row { display: flex; align-items: center; gap: 10px; margin-top: 6px; }
.lab-control input[type="range"] { width: 100%; min-width: 0; height: 28px; accent-color: var(--blue); cursor: pointer; }
.lab-control output { min-width: 36px; padding: 4px 7px; border-radius: 8px; background: var(--ink); color: white; font-size: .84rem; font-weight: 800; text-align: center; }
.number-input { width: 100%; min-height: 48px; margin-top: 7px; padding: 8px 12px; border: 2px solid var(--line); border-radius: 11px; background: white; color: var(--ink); font: inherit; font-weight: 750; }
.number-input:focus { border-color: var(--blue); outline: 0; }
.single-control { max-width: 280px; margin-bottom: 18px; }
.demo-stage { min-height: 190px; display: grid; place-items: center; padding: 22px; overflow: hidden; border-radius: 18px; background: #f4f6fb; }
.demo-result { width: 100%; display: flex; align-items: center; justify-content: space-between; gap: 16px; margin: 20px 0 0; padding-top: 16px; border-top: 1px solid var(--line); color: var(--muted); }
.demo-result strong { color: var(--blue-dark); font-size: 1.18rem; text-align: right; }
.fraction-stage { display: block; }
.fraction-bars { width: 100%; display: grid; grid-template-columns: 1fr auto 1fr; align-items: center; gap: 14px; }
.fraction-bar strong { display: block; margin-top: 7px; text-align: center; }
.fraction-pieces { height: 74px; display: grid; grid-template-columns: repeat(var(--parts), 1fr); overflow: hidden; border: 3px solid var(--ink); border-radius: 12px; background: white; }
.fraction-pieces span + span { border-left: 2px solid var(--ink); }
.fraction-bar.tone-blue .is-filled { background: var(--blue); }
.fraction-bar.tone-cyan .is-filled { background: var(--cyan); }
.math-operator { color: var(--ink); font: 800 2rem/1 Georgia, serif; }
.place-grid { width: 100%; display: grid; grid-template-columns: repeat(3, 1fr); gap: 10px; }
.place-grid > div { padding: 16px 8px; border: 2px solid var(--ink); border-radius: 14px; background: white; text-align: center; }
.place-grid > div:nth-child(1) { box-shadow: inset 0 -8px 0 var(--cyan); }
.place-grid > div:nth-child(2) { box-shadow: inset 0 -8px 0 var(--lime); }
.place-grid > div:nth-child(3) { box-shadow: inset 0 -8px 0 var(--yellow); }
.place-grid span { display: block; color: var(--blue); font-size: 2.2rem; font-weight: 850; line-height: 1; }
.place-grid strong, .place-grid small { display: block; }
.place-grid strong { margin-top: 8px; font-size: .84rem; }
.place-grid small { color: var(--muted); }
.dot-array { display: grid; grid-template-columns: repeat(var(--columns), 19px); gap: 9px; place-content: center; }
.dot-array span { width: 19px; height: 19px; border: 2px solid var(--ink); border-radius: 50%; background: var(--yellow); box-shadow: 2px 2px 0 var(--coral); }
.number-line { width: 100%; padding: 45px 14px 30px; }
.line-track { position: relative; height: 5px; border-radius: 999px; background: var(--ink); }
.line-track > span { position: absolute; top: -7px; transform: translateX(-50%); }
.line-track > span i { display: block; width: 2px; height: 18px; margin-inline: auto; background: var(--ink); }
.line-track > span b { display: block; margin-top: 5px; color: var(--muted); font-size: .73rem; font-style: normal; }
.line-journey { position: absolute; top: -4px; height: 13px; border-radius: 999px; background: var(--cyan); }
.line-marker { position: absolute; top: -13px; width: 25px; height: 25px; border: 3px solid white; border-radius: 50%; transform: translateX(-50%); box-shadow: 0 0 0 2px var(--ink); }
.line-marker b { position: absolute; top: -29px; left: 50%; transform: translateX(-50%); color: var(--ink); font-size: .74rem; font-style: normal; white-space: nowrap; }
.marker-start { background: var(--yellow); }
.marker-result { background: var(--blue); }
.equation-challenge { text-align: center; }
.balance-figure { position: relative; width: 150px; height: 58px; margin: 0 auto 10px; }
.balance-figure span { position: absolute; left: 15px; right: 15px; top: 18px; height: 5px; border-radius: 4px; background: var(--ink); }
.balance-figure i { position: absolute; left: 50%; top: 18px; width: 5px; height: 35px; background: var(--ink); transform: translateX(-50%); }
.balance-figure b { position: absolute; left: 50%; bottom: 0; width: 48px; height: 8px; border-radius: 5px; background: var(--ink); transform: translateX(-50%); }
.challenge-equation { margin: 0 0 18px; color: var(--blue); font: 800 2.3rem/1.2 Georgia, serif; }
.answer-form { max-width: 360px; margin-inline: auto; text-align: left; }
.answer-form > label, .reflection-form > label { display: block; margin-bottom: 7px; color: #46546b; font-weight: 750; }
.answer-form > div { display: grid; grid-template-columns: 1fr auto; gap: 8px; }
.answer-form .number-input { margin: 0; }
.lab-button { min-height: 48px; padding: 0 18px; border: 0; border-radius: 11px; background: var(--blue); color: white; font: inherit; font-size: .92rem; font-weight: 800; cursor: pointer; }
.lab-button:hover { background: var(--blue-dark); }
.text-button { min-height: 44px; padding: 4px 8px; border: 0; background: transparent; color: var(--blue); font: inherit; font-size: .88rem; font-weight: 750; text-decoration: underline; cursor: pointer; }
.answer-feedback { min-height: 25px; margin: 12px 0 5px; color: var(--muted); font-size: .9rem; }
.answer-feedback.is-correct { color: #357000; font-weight: 800; }
.answer-feedback.is-trying { color: #9a4300; }
.triangle-stage, .graph-stage { display: block; text-align: center; }
.triangle-figure { width: min(100%, 390px); height: 230px; }
.triangle-figure text { fill: var(--ink); font: 800 18px "Avenir Next", sans-serif; }
.triangle-figure .hypotenuse-label { fill: var(--blue); }
.graph-figure { width: min(100%, 430px); max-height: 300px; border: 1px solid var(--line); border-radius: 12px; background: white; }
.reflection-form textarea { width: 100%; margin-bottom: 10px; padding: 12px; resize: vertical; border: 2px solid var(--line); border-radius: 11px; color: var(--ink); font: inherit; }
.reflection-form textarea:focus { border-color: var(--blue); outline: 0; }
.lesson-content { font-family: Georgia, "Times New Roman", serif; font-size: 1.1rem; line-height: 1.85; }
.lesson-content h2 { margin: 56px 0 16px; scroll-margin-top: 20px; color: var(--ink); font-family: "Avenir Next", Avenir, "Segoe UI", sans-serif; font-size: 1.75rem; line-height: 1.25; letter-spacing: -.035em; }
.lesson-content h2:first-child { margin-top: 0; }
.lesson-content p { margin: 0 0 22px; }
.lesson-content strong { color: #111b2c; }
.lesson-content ul, .lesson-content ol { margin: 0 0 26px; padding-left: 1.35em; }
.lesson-content li { padding-left: .35em; margin: 9px 0; }
.lesson-content table { width: 100%; margin: 26px 0; border-spacing: 0; border: 1px solid var(--line); border-radius: 14px; overflow: hidden; background: white; font-family: "Avenir Next", Avenir, "Segoe UI", sans-serif; text-align: center; }
.lesson-content th { padding: 12px; background: var(--blue-soft); color: var(--blue-dark); font-size: .88rem; }
.lesson-content td { padding: 14px 12px; border-top: 1px solid var(--line); font-weight: 750; }
.lesson-content blockquote { margin: 28px 0; padding: 20px 22px; border-left: 6px solid var(--lime); border-radius: 0 14px 14px 0; background: #f1f9e3; color: #354216; }
.lesson-content .MathJax { font-size: 1.08em !important; }
.lesson-content mjx-container[display="true"] { margin: 30px 0 !important; padding: 24px 12px; overflow-x: auto; border-radius: 16px; background: var(--white); box-shadow: inset 0 0 0 1px var(--line); }
.community-box { margin-top: 66px; padding: 28px; border: 2px solid var(--ink); border-radius: 22px; background: #f0fbd9; box-shadow: 7px 8px 0 var(--lime); }
.community-heading { display: flex; align-items: center; gap: 16px; margin-bottom: 20px; }
.community-heading .eyebrow { margin-bottom: 4px; color: #587b11; }
.community-heading h2 { margin: 0; font-size: 1.65rem; line-height: 1.2; letter-spacing: -.03em; }
.community-icon { display: grid; place-items: center; width: 52px; height: 52px; border-radius: 16px; background: white; font-size: 1.5rem; }
#tips { margin-bottom: 18px; color: #4c5a37; }
#tips ul { margin: 0; padding-left: 1.25em; }
#tips li + li { margin-top: 12px; }
#tips cite { color: #66744e; font-size: .92rem; }
.community-box .secondary-button { border: 1px solid #8aae3b; background: white; color: #3f5e08; }
.community-box .secondary-button:hover { background: var(--yellow); }
.lesson-end { display: flex; align-items: center; justify-content: space-between; gap: 18px; margin-top: 52px; padding-top: 22px; border-top: 1px solid var(--line); color: var(--muted); }
.lesson-end a { color: var(--blue); font-weight: 800; text-decoration: none; }

.help-strip { padding: 46px 0; background: var(--ink); color: white; }
.help-strip-inner { display: grid; grid-template-columns: auto 1fr auto; align-items: center; gap: 24px; }
.help-icon { display: grid; place-items: center; width: 74px; height: 74px; border-radius: 22px; background: var(--yellow); font-size: 2rem; transform: rotate(-4deg); }
.help-strip .eyebrow { color: var(--cyan); }
.help-strip h2 { font-size: clamp(1.6rem, 3vw, 2.35rem); }
.help-strip p:last-child { margin: 8px 0 0; color: #bac5d8; }
.secondary-button { background: var(--lime); color: #1e3100; }
.secondary-button:hover { background: var(--yellow); }
.coming-soon { padding: 10px 14px; border: 1px solid #536079; border-radius: 12px; color: #d4dbea; font-weight: 700; text-align: center; }

.site-footer { padding: 30px 0; background: #10192a; color: #c6cfdf; }
.footer-inner { display: flex; align-items: center; justify-content: space-between; gap: 30px; }
.brand-footer { color: white; }
.brand-footer .brand-mark { width: 32px; height: 32px; border-radius: 10px; font-size: 1rem; }
.footer-inner p { margin: 0; }

:focus-visible { outline: 3px solid var(--coral); outline-offset: 3px; }
@media (prefers-reduced-motion: no-preference) { .topic-card, .primary-button { transition: transform .18s ease, box-shadow .18s ease, border-color .18s ease; } }

@media (max-width: 820px) {
  .home-intro { padding-top: 48px; }
  .home-intro-grid { grid-template-columns: 1fr; gap: 54px; }
  .intro-copy { text-align: center; }
  .intro-lede { margin-inline: auto; }
  .math-board { width: min(520px, 96%); margin-inline: auto; }
  .topic-grid { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .topic-tools { align-items: stretch; flex-direction: column; }
  .topic-search { width: 100%; }
  .help-strip-inner { grid-template-columns: auto 1fr; }
  .help-strip-inner > :last-child { grid-column: 2; justify-self: start; }
  .lesson-layout { grid-template-columns: minmax(0, 720px); gap: 0; }
  .lesson-main { grid-column: 1; }
  .lesson-nav { display: none; }
}

@media (max-width: 560px) {
  .shell { width: min(100% - 28px, 1120px); }
  .header-inner { min-height: 66px; }
  .main-nav > a:first-child { display: none; }
  .main-nav a { padding-inline: 11px; font-size: .9rem; }
  .home-intro { padding: 42px 0 58px; }
  .intro-copy h1 { font-size: clamp(2.65rem, 14vw, 4rem); }
  .intro-lede { font-size: 1.05rem; }
  .math-board { min-height: 270px; padding-inline: 16px; box-shadow: 9px 10px 0 var(--cyan); }
  .equation { min-height: 155px; gap: 9px; font-size: clamp(1.9rem, 10vw, 2.8rem); }
  .section-heading-row { display: block; }
  .section-heading-row > p { margin-top: 14px; }
  .topic-tools { margin-inline: -4px; padding: 8px; }
  .grade-filters { width: 100%; }
  .topics-section { padding: 60px 0 70px; }
  .topic-grid { grid-template-columns: 1fr; }
  .topic-card { min-height: 292px; }
  .help-strip-inner { grid-template-columns: 1fr; text-align: center; }
  .help-icon, .help-strip-inner > :last-child { grid-column: auto; justify-self: center; }
  .footer-inner { flex-direction: column; align-items: flex-start; gap: 12px; }
  .lesson-hero { padding-top: 26px; }
  .breadcrumb { margin-bottom: 26px; }
  .lesson-title-row { grid-template-columns: 1fr; }
  .lesson-symbol { width: 88px; height: 88px; grid-row: 1; font-size: 2.5rem; border-radius: 24px; }
  .lesson-title-row > div:first-child { grid-row: 2; }
  .lesson-meta { display: grid; grid-template-columns: 1fr 1fr; row-gap: 18px; }
  .lesson-meta div, .lesson-meta div:first-child { min-width: 0; padding: 0 14px; border-left: 1px solid #c7cfea; }
  .lesson-meta div:nth-child(odd) { padding-left: 0; border-left: 0; }
  .lesson-layout { padding-top: 38px; padding-bottom: 70px; }
  .interactive-lab { margin-inline: -4px; margin-bottom: 46px; padding: 20px 16px; border-radius: 20px; box-shadow: 6px 7px 0 var(--cyan); }
  .lab-heading { display: block; }
  .lab-badge { display: inline-block; margin-top: 12px; }
  .lab-controls.two-up, .lab-controls.three-up { grid-template-columns: 1fr; }
  .demo-stage { min-height: 170px; padding: 16px 10px; }
  .fraction-pieces { height: 58px; }
  .demo-result { align-items: flex-start; flex-direction: column; }
  .demo-result strong { font-size: 1rem; text-align: left; }
  .place-grid { gap: 5px; }
  .place-grid > div { padding-inline: 3px; }
  .place-grid span { font-size: 1.8rem; }
  .place-grid strong { font-size: .7rem; }
  .dot-array { grid-template-columns: repeat(var(--columns), 15px); gap: 6px; }
  .dot-array span { width: 15px; height: 15px; }
  .number-line { padding-inline: 7px; }
  .challenge-equation { font-size: 2rem; }
  .lesson-content { font-size: 1.04rem; }
  .lesson-content h2 { margin-top: 44px; font-size: 1.55rem; }
  .community-box { padding: 22px 18px; }
  .lesson-end { align-items: flex-start; flex-direction: column; }
}