:root {
  color-scheme: light;
  --ink: #263b39; --muted: #586964; --accent: #176b61; --accent-dark: #10564f;
  --line: #d5ded8; --soft: #edf5f1; --paper: #fffefb; --success: #246044;
  --cell: clamp(25px, 6vw, 36px); --row-height: 44px;
}
* { box-sizing: border-box; }
body { margin: 0; background: #f3f3ed; color: var(--ink); font: 18px/1.55 system-ui, -apple-system, "Segoe UI", sans-serif; }
button, input, textarea { font: inherit; }
button, a, input, summary { touch-action: manipulation; }
button { min-height: 48px; padding: 12px 18px; border: 1px solid transparent; border-radius: 11px; background: var(--accent); color: #fff; font-weight: 650; cursor: pointer; }
button:hover:not(:disabled) { background: var(--accent-dark); }
button:disabled { cursor: default; opacity: .65; }
a { color: var(--accent); text-underline-offset: 3px; }
:focus-visible { outline: 3px solid #ab691c; outline-offset: 4px; }
[hidden] { display: none !important; }
.skip-link { position: absolute; z-index: 10; left: 12px; top: -100px; padding: 12px; background: white; }
.skip-link:focus { top: 8px; }
.page-header { display: flex; align-items: center; justify-content: space-between; gap: 12px; max-width: 1168px; margin: auto; padding: 20px 24px 16px; }
.brand { font-size: 20px; font-weight: 800; text-decoration: none; }
.quiet { border-color: var(--line); background: var(--paper); color: var(--accent); font-size: 16px; font-weight: 600; }
.quiet:hover:not(:disabled) { background: var(--soft); border-color: #9cb9ab; }
.page { max-width: 1168px; margin: auto; padding: 0 24px 36px; }
.intro { margin: 8px 0 22px; }
.eyebrow { margin: 0 0 5px; color: var(--muted); font-size: 15px; }
h1 { margin: 0 0 8px; font-size: clamp(28px, 5vw, 38px); line-height: 1.2; letter-spacing: -.025em; }
.intro p:last-child { margin: 0; color: var(--muted); }
.workspace { padding: 28px; border: 1px solid var(--line); border-radius: 22px; background: var(--paper); box-shadow: 0 6px 24px #263b3905; }
.exercise-header { display: flex; align-items: center; justify-content: space-between; flex-wrap: wrap; gap: 10px 18px; }
.exercise-header h2 { margin: 0; font-size: 29px; font-variant-numeric: tabular-nums; }
#step-counter { color: var(--muted); font-size: 14px; }
#topic-progress { margin: 12px 0 26px; color: var(--muted); font-size: 14px; }
.progress-track { height: 5px; margin: 8px 0 24px; overflow: hidden; border-radius: 8px; background: #e9ede7; }
.progress-fill { display: block; height: 100%; border-radius: inherit; background: var(--accent); }
.lesson-layout { display: grid; grid-template-columns: minmax(0, 1.05fr) minmax(0, 1fr); gap: 28px; align-items: start; }
.visual-pane { min-width: 0; padding-right: 28px; border-right: 1px solid var(--line); }
.lesson-complete .visual-pane { grid-column: 1 / -1; padding-right: 0; border-right: 0; }
.notebook-scroll { overflow-x: auto; margin: 0 0 10px; padding: 16px 10px 22px; border: 1px solid #e2e9e1; border-radius: 12px; background-color: #fffefa; background-image: linear-gradient(#d9e4df66 1px, transparent 1px), linear-gradient(90deg, #d9e4df66 1px, transparent 1px); background-size: var(--row-height) var(--row-height); scrollbar-width: thin; scrollbar-color: #b0c6bb transparent; }
.notebook { position: relative; display: flex; align-items: flex-start; width: max-content; min-width: 190px; max-width: none; margin: 0 auto; padding-top: 24px; font-family: "DejaVu Sans Mono", Consolas, monospace; font-size: clamp(24px, 5vw, 30px); font-variant-numeric: tabular-nums; line-height: 1.15; }
.working { padding-right: 14px; }
.number-row { display: grid; grid-template-columns: var(--columns); min-height: var(--row-height); align-items: center; }
.number-row > span, .number-row > button { grid-row: 1; display: flex; align-items: center; justify-content: center; min-width: 0; min-height: var(--row-height); padding: 0; text-align: center; }
.number-row > button { width: var(--cell); border: 1px solid transparent; border-radius: 6px; background: transparent; color: var(--ink); font: inherit; }
.number-row > button:hover:not(:disabled) { border-color: #84ad9f; background: #edf5f1; }
.number-row > button:focus-visible { position: relative; z-index: 2; outline-offset: 1px; }
.number-row > button[aria-pressed="true"], .prefix-digit[aria-pressed="true"] { border-color: #7fafa0; background: #dbeee6; color: #114e45; }
.current { border-radius: 4px; background: #e1eee7; outline: 2px solid #4b8976; outline-offset: -2px; }
.used { color: #60706a; }
.pending-digit { border: 2px dashed #8ca79b; border-radius: 4px; background: #edf5f1; }
.dividend-comma, .quotient-comma { color: var(--accent); font-weight: 700; }
.subtraction { border-bottom: 2px solid var(--ink); }
.minus { color: var(--ink); }
.right-side { min-width: calc(var(--cell) * 2); padding-left: 10px; border-left: 2px solid var(--ink); }
.divisor { min-height: var(--row-height); padding: 5px 6px 4px; border-bottom: 2px solid var(--ink); letter-spacing: .08em; }
.quotient { min-height: var(--row-height); padding: 8px 6px 0; white-space: nowrap; }
.slot { display: inline-block; min-width: .65em; color: #8d9a92; text-align: center; }
.slot.next-slot { border-radius: 4px; background: #e1eee7; color: var(--accent); outline: 2px solid #82ae9c; outline-offset: 1px; }
.notebook-marks { position: absolute; z-index: 1; inset: 0; width: 100%; height: 100%; overflow: visible; color: var(--accent); pointer-events: none; }
.notebook-marks .arch, .notebook-marks .first-arch, .notebook-marks .bring-arrow { fill: none; stroke: currentColor; stroke-width: 2.4; stroke-linecap: round; stroke-linejoin: round; }
.notebook-marks marker path { fill: currentColor; }
.notebook-note { min-height: 24px; margin: 10px 0 0; color: var(--muted); font-size: 15px; }
.carry-note { color: var(--accent); font-weight: 650; }
.question { min-width: 0; padding-top: 2px; }
.step-name { margin: 0 0 7px; color: var(--accent); font-size: 15px; font-weight: 700; }
.question h2 { max-width: 620px; margin: 0; font-size: clamp(21px, 2.4vw, 26px); line-height: 1.4; letter-spacing: -.012em; }
#answer-note { margin: 10px 0 18px; color: var(--muted); font-size: 16px; }
label { display: block; margin-bottom: 6px; font-size: 16px; }
.answer-line { display: flex; align-items: stretch; flex-wrap: wrap; gap: 10px; }
input { flex: 0 1 145px; width: 145px; min-width: 0; padding: 10px 14px; border: 2px solid #80978b; border-radius: 10px; background: #fff; color: var(--ink); font-size: 25px; font-variant-numeric: tabular-nums; line-height: 1.3; }
input[readonly] { border-color: #7da589; background: #f0f6ef; color: var(--success); }
.answer-line button { flex: 1; min-width: 150px; max-width: 340px; }
#feedback { min-height: 28px; margin: 12px 0 0; font-size: 17px; }
#feedback.good { color: var(--success); }
#feedback.error { color: #963a25; }
#answer-options { display: flex; gap: 10px; margin-bottom: 12px; }
#answer-options button { min-width: 80px; border: 2px solid var(--line); background: white; color: var(--ink); }
#answer-options button[aria-pressed="true"] { border-color: var(--accent); background: var(--soft); }
#answer-options button:hover:not(:disabled) { background: var(--soft); }
.help-actions { display: grid; gap: 2px; justify-items: start; }
.help-actions > #help-area { width: 100%; }
#decimal-shift { margin-bottom: 14px; }
#normalization-record { margin-bottom: 12px; color: var(--muted); font-size: 15px; }
#normalization-record > summary { padding: 8px 0; }
#normalization-record[open] > summary { margin-bottom: 5px; }
.help-toggle { padding-left: 0; border-color: transparent; background: transparent; color: var(--accent); font-size: 16px; font-weight: 500; text-decoration: underline; text-underline-offset: 4px; }
.help-toggle:hover:not(:disabled) { background: transparent; color: var(--accent-dark); }
#help-content { margin-top: 4px; padding: 16px; border: 1px solid #d6e5da; border-radius: 12px; background: var(--soft); font-size: 16px; }
#hint-text { margin: 0 0 12px; }
#multiples { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 6px 8px; margin: 8px 0 16px; font-size: 16px; font-variant-numeric: tabular-nums; }
.table-note { grid-column: 1 / -1; margin: 0 0 5px; font-size: 14px; }
#multiples span, #multiples button { min-width: 0; min-height: 40px; padding: 6px 8px; border: 1px solid #d6e2d9; border-radius: 7px; background: white; color: var(--ink); font-size: inherit; font-weight: 500; text-align: center; }
#multiples button:hover:not(:disabled) { border-color: #84ad9f; background: #e1eee7; }
#revealed { margin: 10px 0 0; font-weight: 650; }
#revealed:empty { display: none; }
#normalization { margin: 0 0 12px; padding: 16px; border: 1px solid #cedfd3; border-radius: 12px; background: var(--soft); font-size: 18px; }
#normalization p { margin: 5px 0; }
.normalization-pair { display: grid; gap: 18px; }
.normalization-row { min-width: 0; }
.normalization-label { display: block; margin-bottom: 4px; color: var(--muted); font-size: 14px; }
.normalization-equation { display: grid; grid-template-columns: minmax(0, 1fr) 66px minmax(0, 1fr); align-items: center; gap: 8px; text-align: center; }
.original-number, .changed-number { font-size: clamp(23px, 3vw, 29px); font-weight: 600; font-variant-numeric: tabular-nums; overflow-wrap: anywhere; }
.changed-number { padding: 6px 4px; border: 1px dashed #b8c9be; border-radius: 7px; }
.changed-number.earned, .earned { color: var(--accent); }
.changed-number.earned { border-style: solid; border-color: #a7c5b4; background: #fffefa; }
.pending { color: #718478; }
.transform-arrow { display: flex; flex-direction: column; align-items: center; justify-content: center; gap: 1px; min-width: 0; color: var(--accent); }
.transform-arrow small { font-size: 15px; line-height: 1.25; white-space: nowrap; }
.transform-arrow svg { display: block; width: 60px; height: 16px; overflow: visible; }
.transform-arrow path { fill: none; stroke: currentColor; stroke-width: 2; stroke-linecap: round; stroke-linejoin: round; }
.decimal-cue, #decimal-note:not(:empty) { margin-top: 14px; padding: 12px 14px; border-left: 3px solid #80a999; border-radius: 0 8px 8px 0; background: var(--soft); font-size: 15px; }
.decimal-cue p { margin: 0; }
.decimal-equality { display: block; margin: 7px 0; color: var(--ink); font-size: 22px; font-weight: 650; font-variant-numeric: tabular-nums; }
#completion { margin-top: 22px; padding-top: 20px; border-top: 1px solid var(--line); }
#completion h2 { margin: 0; font-size: 26px; }
#completion-text { max-width: 650px; margin: 10px 0; }
#completion .primary { display: block; width: 100%; max-width: 520px; margin: 18px 0 10px; }
#completion .quiet { width: 100%; max-width: 520px; margin: 0 0 8px; }
#next-topic { display: block; }
.small { color: var(--muted); font-size: 15px; }
#results { margin-top: 18px; padding: 14px 18px; border: 1px solid var(--line); border-radius: 12px; background: var(--paper); font-size: 16px; }
summary { min-height: 30px; font-weight: 650; cursor: pointer; }
textarea { display: block; width: 100%; margin-top: 12px; padding: 12px; border: 1px solid var(--line); border-radius: 8px; color: var(--ink); font-size: 16px; }
footer { margin-top: 22px; color: var(--muted); font-size: 14px; }
footer p { margin-bottom: 7px; }
dialog { width: calc(100% - 28px); max-width: 700px; max-height: 85dvh; padding: 24px; border: 1px solid var(--line); border-radius: 18px; background: var(--paper); color: var(--ink); }
dialog::backdrop { background: #1b302b80; }
.dialog-header { display: flex; align-items: center; justify-content: space-between; flex-wrap: wrap; gap: 16px; }
.dialog-header h2 { margin: 0; font-size: 26px; }
dialog > p { color: var(--muted); font-size: 16px; }
#topics-list { display: grid; gap: 18px; }
.topic-group { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 9px; }
.topic-group h3 { grid-column: 1 / -1; margin: 2px 0 3px; color: var(--muted); font-size: 15px; font-weight: 650; }
#topics-list button { min-width: 0; padding: 13px 15px; border: 1px solid var(--line); background: #fff; color: var(--ink); font-size: 17px; text-align: left; }
#topics-list button:hover:not(:disabled) { border-color: #a1b8aa; background: #f4f8f2; }
#topics-list button[aria-current="true"] { border: 2px solid var(--accent); padding: 12px 14px; background: var(--soft); }
#topics-list small { display: block; margin-top: 3px; color: var(--muted); font-size: 14px; font-weight: 400; }
#topics-list .topic-progress { margin-top: 8px; color: var(--accent); font-size: 13px; }
.microproblem { padding: 14px 10px; font-size: 32px; }
.storage-message, #storage-notice { margin: 12px 0; padding: 12px 15px; border: 1px solid #b99555; border-radius: 10px; background: #fff3d9; font-size: 16px; }

@media (max-width: 960px) and (min-width: 761px) {
  .workspace { padding: 22px; }
  .lesson-layout { gap: 20px; }
  .visual-pane { padding-right: 20px; }
  .answer-line input { flex-basis: 112px; width: 112px; }
  .answer-line button { min-width: 125px; padding-right: 12px; padding-left: 12px; }
  .normalization-equation { grid-template-columns: minmax(0, 1fr) 46px minmax(0, 1fr); gap: 4px; }
  .transform-arrow svg { width: 44px; }
}
@media (max-width: 760px) {
  .page, .page-header { max-width: 650px; }
  .lesson-layout { grid-template-columns: minmax(0, 1fr); gap: 20px; }
  .visual-pane { padding-right: 0; border-right: 0; }
  .question { padding-top: 20px; border-top: 1px solid var(--line); }
  .notebook-scroll { padding-top: 10px; padding-bottom: 16px; }
  #topic-progress { margin-bottom: 20px; }
}
@media (max-width: 480px) {
  :root { --row-height: 42px; }
  .page { padding: 0 12px 28px; }
  .page-header { padding: 12px; }
  .brand { font-size: 18px; }
  .page-header .quiet { padding: 10px 12px; font-size: 14px; }
  .workspace { padding: 18px 12px; border-radius: 16px; }
  .intro { margin-top: 5px; margin-bottom: 18px; }
  .intro p:last-child { font-size: 16px; }
  .exercise-header h2 { font-size: 26px; }
  .answer-line { gap: 8px; }
  .answer-line input { flex-basis: 112px; width: 112px; }
  .answer-line button { min-width: 125px; padding: 12px 10px; font-size: 17px; }
  .notebook-scroll { padding-right: 3px; padding-left: 3px; }
  .working { padding-right: 10px; }
  .right-side { padding-left: 7px; }
  .dialog-header { gap: 10px; }
  dialog { padding: 16px; }
  .topic-group { grid-template-columns: minmax(0, 1fr); }
  #multiples { gap: 5px 7px; font-size: 15px; }
  #normalization { padding: 14px 10px; }
  .normalization-equation { grid-template-columns: minmax(0, 1fr) 56px minmax(0, 1fr); }
  .transform-arrow svg { width: 52px; }
}
@media (prefers-reduced-motion: no-preference) {
  button, .current { transition: background-color .15s, border-color .15s; }
  .notebook-marks .bring-arrow { animation: arrow-appear .6s ease-out both; }
  .notebook-marks .bring-arrow[pathLength="1"] { stroke-dasharray: 1; animation: arrow-trace .7s ease-out both; }
  @keyframes arrow-appear { from { opacity: 0; } to { opacity: 1; } }
  @keyframes arrow-trace { from { stroke-dashoffset: 1; } to { stroke-dashoffset: 0; } }
}
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { animation: none !important; transition: none !important; scroll-behavior: auto !important; }
}