:root {
  color-scheme: light;
  --ground: #f2f5f7;
  --surface: #ffffff;
  --surface-2: #e9eef2;
  --ink: #16212b;
  --ink-2: #4f6070;
  --line: #d5dde4;
  --accent: #0d6f6e;
  --accent-ink: #ffffff;
  --accent-soft: #d8eeed;
  --slow: #a9531b;
  --slow-soft: #f6e5d8;
  --done: #4f6070;
  --display: "Bricolage Grotesque", "Avenir Next", "Helvetica Neue", Arial, sans-serif;
  --body: "Source Sans 3", "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  --mono: "JetBrains Mono", "SF Mono", Menlo, Consolas, monospace;
  --measure: 62ch;
  --radius: 8px;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    color-scheme: dark;
    --ground: #0f161b;
    --surface: #161f26;
    --surface-2: #1d2830;
    --ink: #e4ebf0;
    --ink-2: #99aab7;
    --line: #2b3841;
    --accent: #43c0bb;
    --accent-ink: #08201f;
    --accent-soft: #163f3e;
    --slow: #e69a5e;
    --slow-soft: #3a2616;
    --done: #7d8f9c;
  }
}
:root[data-theme="dark"] {
  color-scheme: dark;
  --ground: #0f161b;
  --surface: #161f26;
  --surface-2: #1d2830;
  --ink: #e4ebf0;
  --ink-2: #99aab7;
  --line: #2b3841;
  --accent: #43c0bb;
  --accent-ink: #08201f;
  --accent-soft: #163f3e;
  --slow: #e69a5e;
  --slow-soft: #3a2616;
  --done: #7d8f9c;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  * { transition: none !important; }
}
body {
  margin: 0;
  background: var(--ground);
  color: var(--ink);
  font-family: var(--body);
  font-size: 17px;
  line-height: 1.55;
  -webkit-font-smoothing: antialiased;
}
a { color: var(--accent); text-decoration-thickness: 1px; text-underline-offset: 3px; }
a:hover { text-decoration-thickness: 2px; }
:focus-visible { outline: 2px solid var(--accent); outline-offset: 3px; border-radius: 3px; }
h1, h2, h3 { font-family: var(--display); font-weight: 700; line-height: 1.15; text-wrap: balance; margin: 0; }
p { margin: 0; }
ul, ol { margin: 0; padding: 0; }

.skip {
  position: absolute; left: 16px; top: -60px; padding: 8px 12px;
  background: var(--accent); color: var(--accent-ink); border-radius: var(--radius); z-index: 20;
}
.skip:focus { top: 12px; }

.eyebrow {
  font-family: var(--mono); font-size: 12px; letter-spacing: 0.08em; text-transform: uppercase;
  color: var(--ink-2);
}

/* Top bar */
.topbar {
  position: sticky; top: env(safe-area-inset-top, 0px); z-index: 10;
  background: color-mix(in srgb, var(--surface) 88%, transparent);
  backdrop-filter: blur(10px);
  border-bottom: 1px solid var(--line);
}
.topbar-inner {
  max-width: 1180px; margin: 0 auto; padding: 12px 24px;
  display: flex; align-items: center; justify-content: space-between; gap: 24px; flex-wrap: wrap;
}
.brand { font-family: var(--display); font-weight: 700; font-size: 18px; color: var(--ink); text-decoration: none; }
.progress { display: flex; align-items: center; gap: 12px; min-width: 200px; flex: 1 1 200px; max-width: 420px; }
.progress-track { flex: 1; height: 8px; border-radius: 999px; background: var(--surface-2); overflow: hidden; }
.progress-fill { height: 100%; width: 0; background: var(--accent); transition: width 240ms ease; }
.progress-text { font-family: var(--mono); font-size: 12px; color: var(--ink-2); white-space: nowrap; font-variant-numeric: tabular-nums; }

main { max-width: 1180px; margin: 0 auto; padding: 0 24px 64px; }

/* Hero */
.hero {
  display: grid; grid-template-columns: minmax(0, 1.2fr) minmax(280px, 0.8fr);
  gap: 40px; align-items: center; padding-block: 56px 40px;
  border-bottom: 1px solid var(--line);
}
.hero h1 { font-size: clamp(34px, 4.4vw, 54px); margin-top: 12px; letter-spacing: -0.01em; }
.lede { max-width: var(--measure); margin-top: 20px; font-size: 18px; color: var(--ink-2); }
.hero-actions { display: flex; gap: 12px; flex-wrap: wrap; margin-top: 28px; }
.button {
  display: inline-block; padding: 10px 18px; border-radius: var(--radius);
  background: var(--accent); color: var(--accent-ink); text-decoration: none; font-weight: 600;
  border: 1px solid var(--accent);
}
.button.ghost { background: transparent; color: var(--accent); }
.button:hover { filter: brightness(1.05); }

.loop { margin: 0; }
.loop svg { width: 100%; height: auto; display: block; max-width: 100%; }
.loop-edges path { stroke: var(--ink-2); stroke-width: 2; }
.loop-arrow { fill: var(--ink-2); }
.loop-nodes rect { fill: var(--surface); stroke: var(--line); stroke-width: 1.5; }
.loop-nodes text { fill: var(--ink); font-family: var(--display); font-weight: 700; font-size: 15px; text-anchor: middle; }
.loop-nodes .loop-ghost { stroke-dasharray: 4 4; }
.loop-nodes .loop-ghost-text { fill: var(--ink-2); font-weight: 500; font-size: 13px; }
.loop-nodes .loop-key { fill: var(--slow-soft); stroke: var(--slow); }
.loop-nodes .loop-key-text { fill: var(--slow); }
.loop figcaption { margin-top: 10px; font-size: 14px; color: var(--ink-2); text-align: center; }

/* Layout: rail + content */
main { display: grid; grid-template-columns: 1fr; }
.hero { grid-column: 1 / -1; }
@media (min-width: 900px) {
  main { grid-template-columns: 230px minmax(0, 1fr); column-gap: 48px; }
}
.rail { padding-block: 32px; }
.rail ol { list-style: none; display: flex; gap: 6px; flex-wrap: wrap; }
.rail a {
  display: flex; align-items: baseline; gap: 8px; padding: 6px 10px; border-radius: 6px;
  color: var(--ink-2); text-decoration: none; font-size: 14px; line-height: 1.3;
}
.rail a:hover { background: var(--surface-2); color: var(--ink); }
.rail a.is-current { background: var(--accent-soft); color: var(--ink); }
.rail-num { font-family: var(--mono); font-size: 12px; color: var(--accent); min-width: 12px; }
.rail-title { flex: 1; }
.rail-count { font-family: var(--mono); font-size: 11px; color: var(--ink-2); font-variant-numeric: tabular-nums; }
@media (min-width: 900px) {
  .rail { position: sticky; top: 72px; align-self: start; max-height: calc(100vh - 90px); overflow-y: auto; }
  .rail ol { flex-direction: column; gap: 2px; }
}
@media (max-width: 899px) {
  .rail-title { display: none; }
  .rail a { padding: 6px 12px; border: 1px solid var(--line); }
  .rail li:first-child .rail-title, .rail li:last-child .rail-title { display: inline; }
}

.content { min-width: 0; padding-block: 32px; display: grid; gap: 56px; }
section h2 { font-size: clamp(26px, 3vw, 34px); }
.section-lede { max-width: var(--measure); margin-top: 12px; color: var(--ink-2); }

/* Foundations */
.two-columns { display: grid; grid-template-columns: repeat(auto-fit, minmax(260px, 1fr)); gap: 20px; margin-top: 24px; }
.column { padding: 20px 22px; border-radius: var(--radius); background: var(--surface); border: 1px solid var(--line); }
.column h3 { font-size: 15px; letter-spacing: 0.02em; margin-bottom: 12px; }
.column.slow h3 { color: var(--slow); }
.column.fast h3 { color: var(--ink-2); }
.column ul { list-style: none; display: grid; gap: 6px; }
.column li { padding-left: 14px; position: relative; }
.column li::before { content: ""; position: absolute; left: 0; top: 0.65em; width: 6px; height: 6px; border-radius: 50%; background: currentColor; }
.column.slow li::before { background: var(--slow); }
.column.fast li { color: var(--ink-2); }

/* Projects */
.project { padding-top: 8px; }
.project-head .eyebrow { color: var(--accent); }
.project h2 { margin-top: 8px; }
.goal { max-width: var(--measure); margin-top: 12px; font-size: 18px; }
.project-meta { display: grid; grid-template-columns: repeat(auto-fit, minmax(260px, 1fr)); gap: 20px; margin-top: 24px; }
.meta-block h3 { font-size: 13px; letter-spacing: 0.06em; text-transform: uppercase; color: var(--ink-2); font-family: var(--mono); font-weight: 600; }
.learn { list-style: none; display: flex; flex-wrap: wrap; gap: 8px; margin-top: 10px; }
.learn li {
  padding: 4px 10px; border-radius: 999px; font-size: 14px;
  background: var(--slow-soft); color: var(--slow); border: 1px solid transparent;
}
.meta-block.demo p { margin-top: 10px; max-width: 48ch; }

.courses-block { margin-top: 28px; }
.courses-head { display: flex; align-items: baseline; justify-content: space-between; gap: 12px; flex-wrap: wrap; }
.courses-block h3 { font-size: 20px; }
.foundations .courses-block h3 { font-size: 18px; margin-bottom: 4px; }
.section-count { font-family: var(--mono); font-size: 12px; color: var(--ink-2); font-variant-numeric: tabular-nums; }

.courses { list-style: none; margin-top: 12px; border-top: 1px solid var(--line); }
.course {
  display: grid; grid-template-columns: 34px minmax(0, 1fr) auto; gap: 14px; align-items: start;
  padding: 14px 0; border-bottom: 1px solid var(--line);
}
.course.is-done .course-head a { color: var(--done); }
.course.is-done .note { color: var(--done); }
.tick-cell { position: relative; padding-top: 2px; }
.tick-cell input { position: absolute; opacity: 0; width: 22px; height: 22px; margin: 0; }
.tick {
  display: block; width: 22px; height: 22px; border-radius: 6px;
  border: 1.5px solid var(--ink-2); background: var(--surface); cursor: pointer; position: relative;
}
.tick::after {
  content: ""; position: absolute; left: 7px; top: 3px; width: 5px; height: 10px;
  border: solid var(--accent-ink); border-width: 0 2px 2px 0; transform: rotate(45deg); opacity: 0;
}
.tick-cell input:checked + .tick { background: var(--accent); border-color: var(--accent); }
.tick-cell input:checked + .tick::after { opacity: 1; }
.tick-cell input:focus-visible + .tick { outline: 2px solid var(--accent); outline-offset: 3px; }
.course-head { display: flex; flex-wrap: wrap; align-items: baseline; gap: 4px 10px; }
.course-head a { font-weight: 600; color: var(--ink); }
.course-head a:hover { color: var(--accent); }
.by { color: var(--ink-2); font-size: 15px; }
.note { margin-top: 4px; color: var(--ink-2); max-width: var(--measure); }
.kind {
  font-family: var(--mono); font-size: 11px; letter-spacing: 0.04em; text-transform: uppercase;
  padding: 3px 8px; border-radius: 4px; background: var(--surface-2); color: var(--ink-2); white-space: nowrap; margin-top: 3px;
}
.kind-university { background: var(--accent-soft); color: var(--accent); }
.kind-paper { background: var(--slow-soft); color: var(--slow); }
@media (max-width: 560px) {
  .course { grid-template-columns: 34px minmax(0, 1fr); }
  .kind { grid-column: 2; justify-self: start; margin-top: 0; }
}

/* Ladder */
.ladder-steps { list-style: none; counter-reset: step; display: grid; gap: 0; margin-top: 24px; border-left: 2px solid var(--line); }
.ladder-steps li { position: relative; padding: 10px 0 10px 24px; display: grid; gap: 2px; }
.ladder-steps li::before {
  content: ""; position: absolute; left: -7px; top: 18px; width: 12px; height: 12px; border-radius: 50%;
  background: var(--ground); border: 2px solid var(--accent);
}
.ladder-steps li:last-child::before { background: var(--accent); }
.ladder-from { font-family: var(--mono); font-size: 12px; color: var(--ink-2); letter-spacing: 0.04em; text-transform: uppercase; }
.ladder-steps strong { font-family: var(--display); font-size: 20px; font-weight: 700; }

/* Method */
.method-list { list-style: none; display: grid; gap: 14px; margin-top: 20px; max-width: var(--measure); }
.method-list li { padding-left: 18px; position: relative; }
.method-list li::before { content: ""; position: absolute; left: 0; top: 0.7em; width: 8px; height: 2px; background: var(--accent); }

.footer { border-top: 1px solid var(--line); }
.footer p { max-width: 1180px; margin: 0 auto; padding: 24px; font-size: 14px; color: var(--ink-2); }

@media (max-width: 899px) {
  .hero { grid-template-columns: 1fr; padding-block: 40px 28px; }
  .loop { max-width: 420px; margin: 0 auto; }
  .topbar-inner, main { padding-left: 16px; padding-right: 16px; }
  .footer p { padding: 20px 16px; }
}