:root {
      --bg:     #fff;
      --text:   #333333;
      --muted:  #686868;
      --line:   #e5e5e5;
      --dark:   #f7f8fa;
      --teal:   #1a7a5a;
      --green:  #2d6a4f;
      --amber:  #8a5a10;
      --red:    #a04020;
      --purple: #6b46a8;
    }

    *, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

    body {
      font-family: 'Inter', sans-serif;
      background: var(--bg);
      color: var(--text);
      font-size: 14px;
      line-height: 1.65;
      display: flex;
      min-height: 100vh;
    }

    /* ── NAV ── */
    nav {
      width: 164px;
      flex-shrink: 0;
      padding: 32px 20px;
      display: flex;
      flex-direction: column;
      gap: 3px;
      position: sticky;
      top: 0;
      height: 100vh;
      overflow-y: auto;
    }
    .nav-logo {
      font-family: 'Source Serif 4', serif;
      font-size: 14px;
      font-weight: 600;
      color: var(--text);
      text-decoration: none;
      margin-bottom: 18px;
    }
    nav a {
      color: var(--muted);
      text-decoration: none;
      font-size: 12px;
      padding: 3px 0;
      transition: color .15s;
      white-space: nowrap;
      overflow: hidden;
      text-overflow: ellipsis;
    }
    nav a:hover { color: var(--text); }
    nav a.active { color: var(--teal); font-weight: 600; }
    .nav-div  { width: 100%; height: 1px; background: var(--line); margin: 10px 0; }
    .nav-lbl  {
      font-size: 9.5px;
      letter-spacing: .09em;
      text-transform: uppercase;
      color: var(--muted);
      opacity: .55;
      margin-top: 6px;
      margin-bottom: 2px;
    }

    /* ── VERTICAL LINE ── */
    .vline { width: 1px; background: var(--line); flex-shrink: 0; }

    /* ── MAIN ── */
    main {
      flex: 1;
      padding: 40px 52px 80px;
      max-width: 860px;
    }

    /* ── MODULE HEADER ── */
    .mod-header { margin-bottom: 28px; }
    .mod-tagline {
      display: flex;
      align-items: center;
      gap: 8px;
      margin-bottom: 8px;
    }
    .track-badge {
      font-size: 10px;
      font-weight: 700;
      letter-spacing: .06em;
      text-transform: uppercase;
      color: #fff;
      background: var(--teal);
      padding: 2px 8px;
      border-radius: 3px;
    }
    .mod-meta { font-size: 11px; color: var(--muted); }

    h1 {
      font-family: 'Source Serif 4', serif;
      font-size: 26px;
      font-weight: 400;
      line-height: 1.2;
      margin-bottom: 10px;
    }
    .mod-intro {
      font-size: 14px;
      color: var(--muted);
      max-width: 580px;
      line-height: 1.65;
    }

    /* ── OBJECTIVES ── */
    .objectives {
      background: var(--dark);
      border-left: 3px solid var(--teal);
      border-radius: 0 6px 6px 0;
      padding: 16px 20px;
      margin-bottom: 40px;
    }
    .obj-title {
      font-size: 10.5px;
      text-transform: uppercase;
      letter-spacing: .09em;
      color: var(--teal);
      font-weight: 600;
      margin-bottom: 10px;
    }
    .objectives ul { list-style: none; display: flex; flex-direction: column; gap: 5px; }
    .objectives li {
      font-size: 13px;
      padding-left: 16px;
      position: relative;
    }
    .objectives li::before {
      content: '→';
      position: absolute;
      left: 0;
      color: var(--teal);
      font-size: 11px;
      top: 2px;
    }

    /* ── HEADINGS ── */
    h2 {
      font-family: 'Source Serif 4', serif;
      font-size: 17px;
      font-weight: 500;
      margin-bottom: 14px;
      padding-bottom: 9px;
      border-bottom: 1px solid var(--line);
    }

    .section { margin-bottom: 44px; }
    p { margin-bottom: 12px; font-size: 14px; line-height: 1.7; }
    p:last-child { margin-bottom: 0; }
    strong { font-weight: 600; }

    /* ── CONCEPT CARDS (3-col) ── */
    .concept-grid {
      display: grid;
      grid-template-columns: repeat(3, 1fr);
      gap: 8px;
      margin: 16px 0;
    }
    .concept-card {
      border: 1px solid var(--line);
      border-radius: 6px;
      padding: 12px 14px;
    }
    .cc-title { font-size: 12.5px; font-weight: 600; margin-bottom: 4px; }
    .cc-body  { font-size: 12px; color: var(--muted); line-height: 1.5; }

    /* ── FORMULA BOX ── */
    .formula-box {
      background: var(--dark);
      border: 1px solid var(--line);
      border-radius: 6px;
      padding: 16px 20px;
      margin: 16px 0;
    }
    .f-label {
      font-size: 10px;
      font-family: 'Inter', sans-serif;
      text-transform: uppercase;
      letter-spacing: .08em;
      color: var(--muted);
      margin-bottom: 7px;
    }
    .f-main {
      font-family: 'Source Serif 4', serif;
      font-size: 16px;
      line-height: 1.6;
      color: var(--text);
    }
    .f-main + .f-main { margin-top: 6px; }
    .f-vars {
      font-size: 12px;
      color: var(--muted);
      margin-top: 10px;
      line-height: 1.65;
      font-family: 'Inter', sans-serif;
    }

    /* ── CALLOUT BOXES ── */
    .insight {
      border-left: 3px solid var(--amber);
      background: #fef9f0;
      border-radius: 0 6px 6px 0;
      padding: 12px 16px;
      margin: 16px 0;
    }
    .insight-lbl {
      font-size: 10px; font-weight: 700; text-transform: uppercase;
      letter-spacing: .07em; color: var(--amber); margin-bottom: 5px;
    }
    .insight p { font-size: 13px; color: var(--text); }

    .val-box {
      border-left: 3px solid var(--purple);
      background: #f7f4fd;
      border-radius: 0 6px 6px 0;
      padding: 12px 16px;
      margin: 16px 0;
    }
    .val-lbl {
      font-size: 10px; font-weight: 700; text-transform: uppercase;
      letter-spacing: .07em; color: var(--purple); margin-bottom: 5px;
    }
    .val-box p { font-size: 13px; color: var(--text); }

    /* ── TABLE ── */
    .data-table {
      width: 100%;
      border-collapse: collapse;
      font-size: 13px;
      margin: 12px 0;
    }
    .data-table th {
      text-align: left;
      padding: 7px 10px;
      border-bottom: 1px solid var(--line);
      font-size: 10.5px;
      font-weight: 600;
      text-transform: uppercase;
      letter-spacing: .06em;
      color: var(--muted);
    }
    .data-table td {
      padding: 8px 10px;
      border-bottom: 1px solid var(--dark);
      vertical-align: top;
    }
    .data-table tr:last-child td { border-bottom: none; }
    .data-table .num  { font-variant-numeric: tabular-nums; font-weight: 500; }
    .data-table .hi   { color: var(--teal); font-weight: 600; }

    /* ── SIMULATION BOX ── */
    .sim-box {
      border: 1px solid var(--line);
      border-radius: 8px;
      overflow: hidden;
      margin: 20px 0;
    }
    .sim-head {
      background: var(--dark);
      border-bottom: 1px solid var(--line);
      padding: 9px 16px;
      font-size: 10.5px;
      font-weight: 600;
      text-transform: uppercase;
      letter-spacing: .07em;
      color: var(--muted);
      display: flex;
      align-items: center;
      gap: 6px;
    }
    .sim-head::before { content: '◉'; color: var(--teal); font-size: 9px; }
    .sim-inner {
      display: grid;
      grid-template-columns: 1fr 1fr;
    }
    .sim-controls {
      padding: 18px 20px;
      border-right: 1px solid var(--line);
      display: flex;
      flex-direction: column;
      gap: 14px;
    }
    .sim-output {
      padding: 18px 20px;
      display: flex;
      flex-direction: column;
      gap: 0;
      justify-content: center;
    }

    /* ── SLIDERS ── */
    .sr { display: flex; flex-direction: column; gap: 4px; }
    .sr-lbl {
      display: flex;
      justify-content: space-between;
      font-size: 12px;
      color: var(--muted);
    }
    .sr-val { color: var(--text); font-weight: 600; font-variant-numeric: tabular-nums; }
    input[type="range"] {
      -webkit-appearance: none;
      width: 100%; height: 3px;
      background: var(--line);
      border-radius: 2px;
      outline: none;
      cursor: pointer;
    }
    input[type="range"]::-webkit-slider-thumb {
      -webkit-appearance: none;
      width: 13px; height: 13px;
      border-radius: 50%;
      background: var(--teal);
      cursor: pointer;
    }
    input[type="range"]::-moz-range-thumb {
      width: 13px; height: 13px;
      border-radius: 50%;
      background: var(--teal);
      border: none;
      cursor: pointer;
    }

    /* ── OUTPUT ROWS ── */
    .out-row {
      padding: 10px 0;
      border-bottom: 1px solid var(--line);
    }
    .out-row:last-child { border-bottom: none; padding-bottom: 0; }
    .out-val {
      font-family: 'Source Serif 4', serif;
      font-size: 19px;
      font-weight: 500;
      font-variant-numeric: tabular-nums;
    }
    .out-lbl { font-size: 11px; color: var(--muted); }
    .out-pos  { color: var(--green); }
    .out-neg  { color: var(--red); }
    .out-hi   { color: var(--teal); }

    /* ── CANVAS ── */
    .canvas-wrap { position: relative; width: 100%; margin: 16px 0; }
    canvas { display: block; width: 100%; border: 1px solid var(--line); border-radius: 6px; }

    /* ── TABS ── */
    .tab-row { display: flex; gap: 4px; margin-bottom: 12px; }
    .tab-btn {
      font-size: 12px; padding: 4px 12px;
      border: 1px solid var(--line);
      border-radius: 4px;
      background: var(--bg);
      color: var(--muted);
      cursor: pointer;
      font-family: 'Inter', sans-serif;
      transition: all .15s;
    }
    .tab-btn.active {
      background: var(--teal); color: #fff; border-color: var(--teal);
    }

    /* ── MODULE NAV ── */
    .mod-nav {
      display: flex;
      justify-content: flex-end;
      align-items: center;
      margin-top: 56px;
      padding-top: 20px;
      border-top: 1px solid var(--line);
    }
    .mod-nav a {
      color: var(--teal);
      text-decoration: none;
      font-size: 13px;
      font-weight: 500;
    }
    .mod-nav a:hover { opacity: .75; }

    .footer {
      margin-top: 20px;
      font-size: 11px;
      color: var(--muted);
    }
  
    /* 2026 learning-quality pass */
    .skip-link{position:absolute;left:-9999px;top:8px;z-index:999;padding:8px 12px;background:var(--text);color:var(--bg);border-radius:4px}
    .skip-link:focus{left:8px}
    .lesson-frame{border-left:3px solid var(--teal);background:var(--dark);border-radius:0 6px 6px 0;padding:14px 18px;margin:0 0 34px}
    .lesson-kicker{font-size:10.5px;font-weight:700;letter-spacing:.08em;text-transform:uppercase;color:var(--teal);margin-bottom:5px}
    .lesson-frame p{font-size:13px;margin:0}
    .lesson-review,.lesson-sources{margin:40px 0}
    .lesson-review details{border-bottom:1px solid var(--line);padding:10px 0}
    .lesson-review summary{cursor:pointer;font-weight:600;font-size:13px}
    .lesson-review details p{color:var(--muted);font-size:13px;margin:8px 0 0}
    .lesson-sources ul{padding-left:18px;display:grid;gap:6px}
    .lesson-sources a{color:var(--teal);text-underline-offset:2px}
    .source-date{font-size:11px;color:var(--muted);margin-top:10px}
    button,input,select,a{touch-action:manipulation}
    button:focus-visible,input:focus-visible,select:focus-visible,a:focus-visible,summary:focus-visible{outline:3px solid color-mix(in srgb,var(--teal) 45%,transparent);outline-offset:3px}
    canvas{max-width:100%;height:auto}
    @media(max-width:760px){
      body{display:block}
      nav{width:100%;height:auto;position:static;padding:22px 20px;overflow:visible}
      nav a{white-space:normal;overflow:visible;text-overflow:clip;padding:5px 0}
      .vline{display:none}
      main{width:100%;max-width:none;padding:28px 20px 64px}
      .concept-grid,.sim-inner,.sim-2col,.sim-controls-row{grid-template-columns:1fr!important;display:grid!important}
      .sim-controls,.sim-ctrl{border-right:0!important;border-bottom:1px solid var(--line)}
      .data-table{display:block;overflow-x:auto;-webkit-overflow-scrolling:touch}
      .tab-row{flex-wrap:wrap}
    }
    @media(prefers-reduced-motion:reduce){*,*::before,*::after{scroll-behavior:auto!important;transition:none!important;animation:none!important}}