:root {
  --bg: #efece4;
  --paper: #f7f5ef;
  --ink: #1c1915;
  --muted: #5e584f;
  --line: #1c1915;
  --soft: #d9d3c7;
  --accent: #134e3a;
  --accent-ink: #f3f7f4;
  --ink-hover: #3c362f;
  --paper-hover: #e4ddd2;
  --bar-hover: rgba(247, 245, 239, 0.16);
  --bad: #8c2f24;
  --ok: #134e3a;
  --code: #f0ebe1;
  --space: 8px;
  color-scheme: light;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  background: var(--bg);
  color: var(--ink);
  font: 17px/1.55 "Liberation Sans", "Nimbus Sans", "DejaVu Sans", sans-serif;
}

body, button, input, textarea { font-family: inherit; }

a { color: inherit; }

:focus-visible {
  outline: 2px solid var(--ink);
  outline-offset: 2px;
}

.cm-editor:focus,
.cm-editor:focus-visible,
.cm-editor :focus,
.cm-editor :focus-visible,
#notes textarea:focus,
#notes textarea:focus-visible,
.snippet-source:focus,
.snippet-source:focus-visible {
  outline: none;
  box-shadow: none;
}

.top {
  display: flex;
  align-items: center;
  gap: 24px;
  padding: 0 24px;
  height: 56px;
  background: var(--ink);
  color: var(--paper);
  position: sticky;
  top: 0;
  z-index: 30;
}

.top-panel {
  display: flex;
  align-items: center;
  gap: 24px;
  flex: 1;
  min-width: 0;
}

.top-side {
  display: flex;
  align-items: center;
  gap: 16px;
  margin-left: auto;
}

.top-side form { margin: 0; }

.burger,
.home-link,
.top-topics { display: none; }

.brand {
  color: var(--paper);
  text-decoration: none;
  font-weight: 700;
  letter-spacing: 0.04em;
}

.top-nav { display: flex; gap: 16px; flex: 1; }

.top-nav a {
  color: var(--paper);
  text-decoration: none;
  border-radius: 2px;
  padding: 6px 8px;
  transition: background-color 220ms ease, color 220ms ease;
}

.top-nav a:hover { background: var(--bar-hover); }

.who { font-size: 14px; }

.top form { margin: 0; }

.top button {
  background: transparent;
  color: var(--paper);
  border-color: var(--paper);
  padding: 6px 10px;
}

.top button:hover {
  background: var(--bar-hover);
  color: var(--paper);
  border-color: var(--paper);
}

.meter {
  margin: 0;
  display: flex;
  gap: 12px;
  font-variant-numeric: tabular-nums;
  font-size: 14px;
}

main {
  width: min(1120px, calc(100% - 32px));
  margin: 32px auto 64px;
}

h1, h2, h3 { line-height: 1.2; font-weight: 650; letter-spacing: -0.02em; }

h1 { font-size: 40px; margin: 0 0 12px; }

h2 { font-size: 26px; margin: 0; }

h3 { font-size: 20px; margin: 0; }

.kicker, .source, .count, .mins, .hint {
  color: var(--muted);
  font-size: 14px;
}

.kicker { margin: 0 0 8px; letter-spacing: 0.04em; text-transform: uppercase; }

.lead { font-size: 18px; max-width: 68ch; margin: 0; }

.intro { margin-bottom: 32px; }

.tracks, .lesson-list, .notes, .rail ol {
  list-style: none;
  margin: 0;
  padding: 0;
}

.tracks {
  display: grid;
  grid-template-columns: 1fr;
  gap: 16px;
}

.track, .lesson-list a, .notes a {
  display: block;
  text-decoration: none;
  background: var(--paper);
  border: 1px solid var(--line);
  padding: 20px;
  transition: background-color 160ms ease, transform 160ms ease;
}

.track:hover, .lesson-list a:hover, .notes a:hover {
  background: #fff;
}

.track header, .block-head, .row, .pager {
  display: flex;
  justify-content: space-between;
  gap: 16px;
  align-items: baseline;
}

.track p { margin: 12px 0; max-width: 70ch; }

.squares { display: flex; flex-wrap: wrap; gap: 4px; padding: 0; margin: 8px 0 0; list-style: none; }

.sq {
  width: 14px;
  height: 14px;
  border: 1px solid var(--line);
  background: transparent;
  transition: background-color 160ms ease;
}

.sq.is-done, .lesson-list a.is-done .idx { background: var(--accent); color: var(--accent-ink); }

.sources { margin-top: 40px; border-top: 1px solid var(--line); padding-top: 20px; }

.sources ul { padding-left: 18px; max-width: 70ch; }

.sources li { margin: 6px 0; }

.page-head { margin-bottom: 24px; }

.crumb { margin: 0 0 12px; font-size: 14px; }

.lesson-list { display: grid; gap: 8px; }

.lesson-list a {
  display: grid;
  grid-template-columns: 48px 1fr auto;
  gap: 12px;
  align-items: center;
}

.lesson-list .idx {
  font-variant-numeric: tabular-nums;
  border: 1px solid var(--line);
  width: 40px;
  height: 40px;
  display: grid;
  place-items: center;
}

.lesson-list em {
  display: block;
  color: var(--muted);
  font-style: normal;
  font-size: 15px;
}

.lesson-layout {
  display: grid;
  grid-template-columns: 240px minmax(0, 1fr);
  gap: 28px;
  align-items: start;
}

.rail {
  position: sticky;
  top: 16px;
  max-height: calc(100vh - 32px);
  overflow: auto;
}

.rail a {
  display: block;
  text-decoration: none;
  padding: 8px 10px;
  border: 1px solid transparent;
  font-size: 14px;
  line-height: 1.35;
  transition: border-color 160ms ease, background-color 160ms ease;
}

.rail a.is-current { border-color: var(--line); background: var(--paper); }

.rail a.is-done { box-shadow: inset 3px 0 0 var(--accent); }

.prose { max-width: 68ch; font-size: 18px; line-height: 1.7; }

.prose h2 { margin: 32px 0 10px; font-size: 22px; }

.prose h3 { margin: 24px 0 8px; font-size: 18px; }

.prose p, .prose ul, .prose ol { margin: 8px 0 12px; }

.prose ul, .prose ol { padding-left: 20px; }

.prose li { margin: 4px 0; }

.prose code, .editor-label, textarea, .output, .notes pre {
  font-family: "Liberation Mono", "DejaVu Sans Mono", ui-monospace, monospace;
}

.prose code {
  background: var(--code);
  padding: 0 4px;
  font-size: 0.92em;
}

.prose pre {
  background: #fbfaf6;
  border: 1px solid var(--soft);
  padding: 14px 16px;
  overflow-x: auto;
  line-height: 1.55;
  margin: 0;
}

.snippet {
  margin: 18px 0 22px;
  max-width: 100%;
  border: 1px solid var(--line);
  background: #fbfaf6;
}

.snippet-bar {
  display: flex;
  align-items: center;
  gap: 12px;
  margin: 0;
  padding: 6px 10px;
  background: var(--ink);
  color: var(--paper);
  font-family: "Liberation Mono", "DejaVu Sans Mono", ui-monospace, monospace;
  font-size: 12px;
}

.snippet-kicker {
  letter-spacing: 0.04em;
  text-transform: uppercase;
  color: #d9d3c7;
}

.snippet-file {
  flex: 1;
  min-width: 0;
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
}

.snippet-bar .snippet-copy {
  margin-left: auto;
  background: transparent;
  color: var(--paper);
  border-color: #8a8378;
  padding: 4px 8px;
  font-size: 12px;
  font-family: "Liberation Sans", "Nimbus Sans", "DejaVu Sans", sans-serif;
}

.snippet-frame {
  display: grid;
  grid-template-columns: auto minmax(0, 1fr);
  overflow: auto;
  max-height: min(420px, 70vh);
}

.snippet-gutter,
.snippet-code {
  margin: 0;
  padding: 12px 0;
  border: 0;
  background: transparent;
  font-family: "Liberation Mono", "DejaVu Sans Mono", ui-monospace, monospace;
  font-size: 13.5px;
  line-height: 1.6;
  tab-size: 4;
}

.snippet-gutter {
  padding-left: 12px;
  padding-right: 10px;
  text-align: right;
  color: #6d675e;
  background: #f3efe6;
  border-right: 1px solid var(--soft);
  user-select: none;
  position: sticky;
  left: 0;
}

.snippet-code {
  padding-left: 12px;
  padding-right: 16px;
  min-width: max-content;
}

.snippet-code code {
  font-family: inherit;
}

.prose .snippet-code,
.prose .snippet-gutter {
  margin: 0;
  border: 0;
  background: transparent;
}

.prose .snippet-gutter { background: #f3efe6; }

.snippet-source {
  position: fixed;
  left: 0;
  top: 0;
  width: 1px;
  height: 1px;
  opacity: 0;
}

.tok-kw { color: #1c1915; font-weight: 700; }
.tok-str { color: #8c2f24; }
.tok-cmt { color: #6d675e; }
.tok-num { color: #1d4e89; }

.bridge, .callout {
  margin: 0 0 20px;
  padding: 14px 16px 6px;
  background: var(--paper);
  border: 1px solid var(--soft);
  max-width: 68ch;
}

.callout { margin: 20px 0; border-left: 4px solid var(--accent); }

.callout.mistake { border-left-color: var(--bad); }

.callout-kicker {
  margin: 0 0 8px;
  font-size: 12px;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  font-weight: 700;
}

.callout p, .bridge p { margin: 0 0 10px; }

.continue { margin: 18px 0 0; }

.continue a {
  display: inline-block;
  background: var(--ink);
  color: var(--paper);
  text-decoration: none;
  padding: 10px 14px;
  transition: transform 160ms ease, background-color 160ms ease;
}

.continue a:hover {
  background: var(--ink-hover);
  border-color: var(--ink-hover);
}

.track h2 { display: flex; gap: 12px; align-items: center; }

.track h2 .idx {
  font-variant-numeric: tabular-nums;
  font-size: 14px;
  border: 1px solid var(--line);
  width: 36px;
  height: 36px;
  display: grid;
  place-items: center;
}

.prose pre code { background: none; padding: 0; }

.prose table {
  width: 100%;
  border-collapse: collapse;
  margin: 12px 0 16px;
  font-size: 15px;
}

.prose th, .prose td {
  border: 1px solid var(--line);
  padding: 6px 8px;
  text-align: left;
  vertical-align: top;
}

.prose th { background: var(--code); }

.prose blockquote {
  margin: 12px 0;
  padding: 8px 14px;
  border-left: 3px solid var(--accent);
  background: var(--paper);
}

.prose aside,
.prose .callout {
  margin: 20px 0;
  padding: 14px 16px 6px;
  border: 1px solid var(--soft);
  border-left: 4px solid var(--accent);
  background: var(--paper);
}

.prose aside.mistake,
.prose .callout.mistake { border-left-color: var(--bad); }

.prose svg { width: 100%; height: auto; margin: 12px 0; background: var(--paper); border: 1px solid var(--line); }

.block { margin-top: 36px; padding-top: 8px; }

.block-head { margin-bottom: 12px; }

fieldset {
  border: 1px solid var(--line);
  background: var(--paper);
  margin: 0 0 12px;
  padding: 12px 14px 8px;
}

legend { padding: 0 6px; font-weight: 650; }

.choice {
  display: flex;
  gap: 10px;
  align-items: flex-start;
  padding: 6px;
  margin: 4px 0;
  cursor: pointer;
  transition: background-color 160ms ease;
}

.choice:hover { background: var(--paper-hover); }

.choice.right { background: #e5f0ea; }

.choice.bad { background: #f6e4e1; }

.explain { color: var(--muted); font-size: 15px; }

button, .text-btn, .continue a {
  appearance: none;
  background: var(--ink);
  color: var(--paper);
  border: 1px solid var(--ink);
  padding: 10px 14px;
  text-decoration: none;
  cursor: pointer;
  font-size: 15px;
  transition: background-color 220ms ease, color 220ms ease, border-color 220ms ease, box-shadow 220ms ease;
}

button[type="button"], .text-btn {
  background: transparent;
  color: var(--ink);
}

button:hover,
.text-btn:hover,
.continue a:hover,
.top-nav a:hover,
.top button:hover,
.snippet-bar .snippet-copy:hover,
.ide-bar button:hover,
.burger:hover {
  background: var(--paper);
  color: var(--ink);
  border-color: var(--paper);
}

button:disabled { opacity: 0.6; cursor: wait; transform: none; }

.login {
  display: grid;
  gap: 12px;
  max-width: 360px;
}

.login label {
  display: grid;
  gap: 6px;
  font-size: 14px;
}

.login input {
  border: 1px solid var(--line);
  background: var(--paper);
  color: var(--ink);
  padding: 10px 12px;
  font: inherit;
}

.ide { margin-top: 8px; max-width: 100%; }

.ide-bar {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 12px;
  padding: 6px 10px;
  border: 1px solid var(--line);
  border-bottom: 0;
  background: var(--ink);
  color: var(--paper);
  font-family: "Liberation Mono", "DejaVu Sans Mono", ui-monospace, monospace;
  font-size: 12px;
  letter-spacing: 0.04em;
}

.ide-bar button {
  background: transparent;
  color: var(--paper);
  border-color: #8a8378;
  padding: 4px 8px;
  font-size: 12px;
  margin-left: auto;
}

.cm-host .cm-content ::selection,
.cm-host .cm-content *::selection {
  background: #7eb6ef;
}

.cm-host .cm-editor {
  border: 1px solid var(--line);
  height: min(28rem, 62dvh);
  min-height: 220px;
  max-height: 80dvh;
  resize: vertical;
  overflow: hidden;
}

.cm-host .cm-editor.cm-focused {
  outline: none;
  box-shadow: none;
}

.ide-scroll {
  display: grid;
  grid-template-columns: 3.2em minmax(0, 1fr);
  max-height: 480px;
  overflow: auto;
  border: 1px solid var(--line);
  background: #fbfaf6;
}

.ide-gutter, .ide-paint, .ide textarea {
  margin: 0;
  font-family: "Liberation Mono", "DejaVu Sans Mono", ui-monospace, monospace;
  font-size: 14px;
  line-height: 1.55;
  tab-size: 4;
}

.ide-gutter {
  padding: 12px 8px;
  text-align: right;
  color: var(--muted);
  background: var(--paper);
  border-right: 1px solid var(--soft);
  user-select: none;
}

.ide-code { position: relative; min-width: 0; }

.ide-paint {
  padding: 12px 12px 24px 8px;
  white-space: pre;
  pointer-events: none;
  min-height: 280px;
}

.ide textarea {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  min-height: 0;
  padding: 12px 12px 24px 8px;
  border: 0;
  resize: none;
  overflow: hidden;
  color: var(--ink);
  background: transparent;
  white-space: pre;
}

.ide.is-on textarea {
  color: transparent;
  caret-color: var(--ink);
}

.ide textarea::selection { background: #d5e4dc; }

#notes textarea {
  width: 100%;
  min-height: 140px;
  resize: vertical;
  border: 1px solid var(--line);
  background: var(--paper);
  color: var(--ink);
  padding: 12px;
  font-family: inherit;
  font-size: 16px;
  line-height: 1.5;
  outline: none;
  box-shadow: none;
}

.row { margin: 10px 0; align-items: center; }

.verdict { margin-top: 12px; }

.verdict:not(:has(.output:not(:empty))) { display: none; }

.verdict-kicker {
  margin: 0;
  padding: 6px 12px;
  background: var(--ink);
  color: var(--paper);
  font-size: 12px;
  letter-spacing: 0.06em;
  text-transform: uppercase;
}

.verdict.is-ok .verdict-kicker { background: #243028; }

.verdict.is-bad .verdict-kicker { background: #3a2420; }

.verdict.is-pending .verdict-kicker { background: #3c362f; }

.output {
  min-height: 0;
  white-space: pre-wrap;
  background: #fbfaf6;
  border: 1px solid var(--line);
  border-top: 0;
  padding: 0;
  margin: 0;
  font-size: 13.5px;
  line-height: 1.55;
}

.output:not(:empty) { padding: 12px 14px; }

.output.is-ok { border-color: #243028; }

.output.is-bad { border-color: var(--bad); }

.badge {
  margin: 0;
  font-size: 13px;
  letter-spacing: 0.04em;
  text-transform: uppercase;
}

.badge.is-ok { color: var(--ok); }

.badge.is-bad { color: var(--bad); }

.pager {
  margin-top: 28px;
  padding-top: 16px;
  border-top: 1px solid var(--line);
  align-items: center;
}

.notes { display: grid; gap: 16px; }

.notes pre {
  white-space: pre-wrap;
  background: var(--paper);
  border: 1px solid var(--line);
  border-top: 0;
  margin: 0;
  padding: 12px 14px;
}

@media (max-width: 1100px) {
  .lesson-layout { grid-template-columns: 200px minmax(0, 1fr); gap: 18px; }
  .prose, .lead, .bridge, .callout { max-width: none; }
}

@media (max-width: 860px) {
  h1 { font-size: 32px; }
  .lesson-layout { grid-template-columns: 1fr; }
  .rail { display: none; }
  .lesson-list a { grid-template-columns: 40px 1fr; }
  .mins { display: none; }
  .top { height: 56px; flex-wrap: nowrap; padding: 0 12px; gap: 12px; }
  .top .burger {
    display: inline-flex;
    align-items: center;
    gap: 8px;
    margin-left: auto;
    background: transparent;
    color: var(--paper);
    border-color: #8a8378;
    padding: 6px 10px;
    min-height: 40px;
  }
  .burger-box {
    display: inline-flex;
    flex-direction: column;
    justify-content: center;
    gap: 4px;
    width: 16px;
  }
  .burger-box span {
    display: block;
    height: 2px;
    background: currentColor;
  }
  .top.is-open .burger-box span:nth-child(1) { transform: translateY(6px) rotate(45deg); }
  .top.is-open .burger-box span:nth-child(2) { opacity: 0; }
  .top.is-open .burger-box span:nth-child(3) { transform: translateY(-6px) rotate(-45deg); }
  .burger-box span { transition: transform 220ms ease, opacity 220ms ease; }
  .top-panel {
    display: none;
    position: absolute;
    top: 56px;
    left: 0;
    right: 0;
    flex-direction: column;
    align-items: stretch;
    gap: 0;
    max-height: calc(100dvh - 56px);
    overflow: auto;
    background: var(--ink);
    border-top: 1px solid #3c362f;
    padding: 8px 0 16px;
  }
  .top.is-open .top-panel { display: flex; }
  .top-nav, .top-side {
    flex-direction: column;
    align-items: stretch;
    gap: 0;
    margin: 0;
  }
  .top-nav a, .top-topics a, .top-side .who {
    display: block;
    padding: 12px 16px;
  }
  .home-link { display: block; }
  .top-topics { display: block; border-top: 1px solid #3c362f; margin-top: 8px; padding-top: 8px; }
  .top-topics ol { list-style: none; margin: 0; padding: 0; }
  .top-topics-title { font-weight: 700; }
  .top-topics a.is-current { background: var(--paper); color: var(--ink); }
  .top-topics a.is-done { box-shadow: inset 3px 0 0 var(--paper); }
  .top-side { border-top: 1px solid #3c362f; margin-top: 8px; padding-top: 8px; }
  .top-side form { padding: 4px 16px 8px; }
  .top-side button { width: 100%; }
  .meter { padding: 8px 16px 0; justify-content: space-between; }
  .pager, .track header, .block-head, .row { flex-wrap: wrap; }
  .pager { flex-direction: column; align-items: stretch; }
  .prose table { display: block; overflow-x: auto; }
  main { width: min(1120px, calc(100% - 20px)); margin: 20px auto 48px; }
}

@media (max-width: 640px) {
  body { font-size: 16px; }
  h1 { font-size: 28px; }
  h2 { font-size: 22px; }
  .lead, .prose { font-size: 16px; }
  .lesson-list a { padding: 14px; }
  .track, .notes a { padding: 14px; }
  button, .text-btn, .continue a { min-height: 44px; }
  .snippet-bar button, .ide-bar button { min-height: 0; }
  .row button, .pager button, .pager .text-btn { width: 100%; text-align: center; }
  .ide-bar, .snippet-bar { flex-wrap: wrap; }
  .snippet-code, .snippet-gutter { font-size: 12.5px; }
  .cm-host .cm-editor { height: min(24rem, 58dvh); min-height: 200px; }
  .who { font-size: 13px; }
  fieldset { padding: 10px; }
  .login { max-width: none; }
}

@media (max-width: 420px) {
  .top { padding: 0 10px; }
  h1 { font-size: 24px; }
  .kicker { font-size: 12px; }
  .burger { padding: 6px 8px; }
}

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  *, *::before, *::after {
    animation: none !important;
    transition: none !important;
  }
}