:root {
  color-scheme: light dark;
  --page: #fff;
  --ink: #171717;
  --muted: #4b5563;
  --panel: #f4f6f8;
  --panel-strong: #e6edf4;
  --border: #52606d;
  --link: #0645ad;
  --visited: #5a287d;
  --accent: #7a2e0b;
  --focus: #ffbf47;
  --measure: 74ch;
  --reader-font: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  --reader-size: 1.125rem;
  --reader-leading: 1.65;
  --letter-space: normal;
  --word-space: normal;
}

* { box-sizing: border-box; }

html {
  background: var(--page);
  color: var(--ink);
  font-size: 100%;
  scroll-behavior: smooth;
}

body {
  min-inline-size: 0;
  margin: 0;
  overflow-wrap: anywhere;
  background: var(--page);
  color: var(--ink);
  font-family: var(--reader-font);
  font-size: var(--reader-size);
  line-height: var(--reader-leading);
  letter-spacing: var(--letter-space);
  word-spacing: var(--word-space);
}

a {
  color: var(--link);
  text-decoration-thickness: .12em;
  text-underline-offset: .16em;
}

a:visited { color: var(--visited); }
a:hover { text-decoration-thickness: .2em; }

:focus-visible {
  outline: .22rem solid var(--focus);
  outline-offset: .2rem;
  border-radius: .12rem;
}

.skip-link {
  position: fixed;
  z-index: 20;
  inset-block-start: .5rem;
  inset-inline-start: .5rem;
  padding: .65rem .9rem;
  transform: translateY(-180%);
  background: var(--ink);
  color: var(--page);
  font-weight: 800;
}

.skip-link:focus { transform: translateY(0); }

.site-header,
main,
.site-footer {
  inline-size: min(100% - 2rem, var(--measure));
  margin-inline: auto;
}

.site-header {
  padding-block: 2.5rem 1.25rem;
  border-block-end: .18rem solid var(--border);
}

.site-footer {
  padding-block: 1.5rem 2.5rem;
  border-block-start: .18rem solid var(--border);
  color: var(--muted);
  font-size: .9em;
}

main { padding-block: 1.5rem 3rem; }
article { margin-block: 2rem; }
p, li, dd { max-inline-size: 70ch; }

.eyebrow {
  margin: 0;
  color: var(--accent);
  font-size: .82em;
  font-weight: 800;
  letter-spacing: .06em;
  text-transform: uppercase;
}

h1, h2, h3, h4, h5 {
  line-height: 1.2;
  text-wrap: balance;
}

h1 { margin-block: .2rem .5rem; font-size: clamp(2rem, 7vw, 3.5rem); }
h2 { margin-block-start: 2.4rem; font-size: clamp(1.55rem, 5vw, 2.15rem); }
h3 { margin-block-start: 2rem; font-size: clamp(1.25rem, 4vw, 1.6rem); }

.subtitle {
  max-inline-size: 58ch;
  margin-block: .4rem 1rem;
  color: var(--muted);
  font-size: 1.08em;
}

.view-nav ul,
.action-list {
  display: flex;
  flex-wrap: wrap;
  gap: .6rem 1.15rem;
  margin: 0;
  padding: 0;
  list-style: none;
}

.view-nav a,
.primary-action {
  display: inline-block;
  min-block-size: 2.75rem;
  padding: .45rem .75rem;
  border: .12rem solid var(--border);
  border-radius: .35rem;
  background: var(--panel);
  color: var(--ink);
  font-weight: 750;
}

.view-nav a[aria-current="page"] {
  border-color: var(--accent);
  background: var(--accent);
  color: var(--page);
  text-decoration: none;
}

.reader-intro,
.preferences,
.chapter-toc,
.book-card,
.equation-card,
.structured-block,
.proof-figure {
  margin-block: 1.5rem;
  padding: clamp(1rem, 3vw, 1.5rem);
  border: .12rem solid var(--border);
  border-radius: .45rem;
  background: var(--panel);
}

.reader-intro h2,
.preferences h2,
.chapter-toc h2,
.book-card h2,
.equation-card h2,
.structured-block > h3 { margin-block-start: 0; }

.chapter-toc ol {
  columns: 2 18rem;
  column-gap: 2.5rem;
  margin-block-end: 0;
  padding-inline-start: 1.5rem;
}

.chapter-toc li { break-inside: avoid; margin-block: .35rem; }
.chapter-toc .toc-level-4 { margin-inline-start: 1rem; }

.book-card { border-inline-start: .45rem solid var(--accent); }
.structured-block.definition { border-inline-start: .45rem solid var(--accent); }
.structured-block.theorem,
.structured-block.proposition,
.structured-block.corollary { border-inline-start: .45rem double var(--accent); }
.structured-block.exercise,
.structured-block.problem { border-style: dashed; }

.source-block { margin-block: 1.2rem; }
.source-cite { margin-block: .35rem 0; color: var(--muted); font-size: .78em; }
.reader-correction {
  display: inline;
  color: var(--muted);
  font-size: .9em;
  font-style: italic;
}

.formula-help {
  margin-block: .75rem;
  padding: .55rem .75rem;
  border: .08rem dashed var(--border);
  border-radius: .3rem;
}

.formula-help summary { cursor: pointer; font-weight: 750; }
.formula-help dt { margin-block-start: .55rem; font-weight: 750; }
.formula-help dd { margin-inline-start: 0; }

.formula-unit {
  display: inline-block;
  min-inline-size: 0;
  max-inline-size: 100%;
  vertical-align: middle;
}
.math-scroll {
  display: block;
  max-inline-size: 100%;
  overflow-x: auto;
  overflow-y: hidden;
}
.math-formula {
  display: inline-block;
  margin-inline: .08em;
  font-size: 1.05em;
}
.spoken-formula {
  margin-inline: .08em;
  padding-inline: .12em;
  border-block-end: .12em dotted var(--accent);
  font-weight: 650;
}

.proof-figure { min-inline-size: 0; margin-inline: 0; break-inside: avoid; }
.proof-figure figcaption { margin-block-start: 0; font-size: 1.1em; }
.proof-steps { display: grid; gap: 0; margin: 1rem 0; padding: 0; list-style: none; }
.proof-step {
  display: grid;
  min-inline-size: 0;
  grid-template-columns: minmax(9rem, 13rem) minmax(0, 1fr);
  gap: .75rem;
  align-items: baseline;
  padding: .55rem .25rem;
  border-block-end: .08rem solid var(--border);
}
.step-content { min-inline-size: 0; max-inline-size: 100%; }
.formula-help { min-inline-size: 0; max-inline-size: 100%; overflow-wrap: anywhere; }

.proof-step.conclusion,
.proof-step:last-child { border-block-start: .22rem solid var(--ink); font-weight: 800; }
.step-label { color: var(--muted); font-size: .88em; font-weight: 750; }
.long-description { margin-block-end: 0; padding-block-start: .8rem; border-block-start: .08rem dashed var(--border); }

.table-wrap { max-inline-size: 100%; overflow-x: auto; }
table { inline-size: 100%; border-collapse: collapse; }
caption { margin-block-end: .5rem; text-align: start; font-weight: 800; }
th, td { padding: .55rem; border: .08rem solid var(--border); text-align: start; vertical-align: top; }
th { background: var(--panel-strong); }

.preferences-form[hidden] { display: none; }
.preference-grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(min(100%, 12rem), 1fr)); gap: 1rem; }
.preference-grid label { display: grid; gap: .3rem; font-weight: 700; }

select, button {
  min-block-size: 2.75rem;
  max-inline-size: 100%;
  padding: .45rem .65rem;
  border: .12rem solid var(--border);
  border-radius: .3rem;
  background: var(--page);
  color: var(--ink);
  font: inherit;
}

button { cursor: pointer; font-weight: 800; }
.preference-actions { margin-block: 1rem 0; }
.status { min-block-size: 1.5em; color: var(--muted); }
kbd, code, .source-code { font-family: ui-monospace, "Cascadia Mono", "Segoe UI Mono", monospace; }
kbd { padding: .08em .3em; border: .08rem solid var(--border); border-radius: .2rem; background: var(--panel-strong); }

.source-lines { margin: 0; padding-inline-start: 4.5rem; }
.source-lines li { max-inline-size: none; padding-inline-start: .6rem; }
.source-lines li:target { background: #fff3a6; color: #171717; outline: .16rem solid #7a2e0b; }
.source-code { display: block; inline-size: 100%; white-space: pre-wrap; overflow-wrap: anywhere; tab-size: 2; }

.sr-only {
  position: absolute;
  inline-size: 1px;
  block-size: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}

[data-font="serif"] { --reader-font: Georgia, "Times New Roman", serif; }
[data-font="mono"] { --reader-font: "Cascadia Mono", Consolas, ui-monospace, monospace; }
[data-size="large"] { --reader-size: 1.32rem; }
[data-size="extra-large"] { --reader-size: 1.55rem; }
[data-spacing="generous"] { --reader-leading: 1.85; --letter-space: .035em; --word-space: .12em; }
[data-spacing="extra"] { --reader-leading: 2; --letter-space: .06em; --word-space: .2em; }

[data-theme="dark"] {
  --page: #111827; --ink: #f9fafb; --muted: #d1d5db; --panel: #1f2937;
  --panel-strong: #374151; --border: #d1d5db; --link: #93c5fd;
  --visited: #e9b8ff; --accent: #f0abfc; --focus: #fde047;
}

[data-theme="sepia"] {
  --page: #f6f0df; --ink: #2b2518; --muted: #554c37; --panel: #eee4ca;
  --panel-strong: #e2d4b4; --border: #655a40; --link: #004c99;
  --visited: #6b2c70; --accent: #7a2e0b; --focus: #ffbf47;
}

[data-theme="contrast"] {
  --page: #000; --ink: #fff; --muted: #fff; --panel: #000; --panel-strong: #000;
  --border: #fff; --link: #0ff; --visited: #f9f; --accent: #ff0; --focus: #0f0;
}

@media (prefers-color-scheme: dark) {
  body:not([data-theme]), body[data-theme="system"] {
    --page: #111827; --ink: #f9fafb; --muted: #d1d5db; --panel: #1f2937;
    --panel-strong: #374151; --border: #d1d5db; --link: #93c5fd;
    --visited: #e9b8ff; --accent: #f0abfc; --focus: #fde047;
  }
}

@media (forced-colors: active) { :focus-visible { outline-color: Highlight; } }
@media (prefers-reduced-motion: reduce) { html { scroll-behavior: auto; } }

@media (max-width: 34rem) {
  .site-header, main, .site-footer { inline-size: min(100% - 1rem, var(--measure)); }
  .view-nav ul, .action-list { display: grid; grid-template-columns: 1fr; }
  .view-nav a, .primary-action { inline-size: 100%; }
  .proof-step { grid-template-columns: 1fr; gap: .15rem; }
  .source-lines { padding-inline-start: 3.25rem; }
}

@media print {
  :root { color-scheme: light; --page: #fff; --ink: #000; --muted: #333; --panel: #fff; --border: #000; --link: #000; --visited: #000; --accent: #000; }
  body { font-size: 11pt; line-height: 1.45; }
  .site-header, main, .site-footer { inline-size: 100%; max-inline-size: none; }
  .skip-link, .view-nav, .preferences, .status { display: none !important; }
  a { color: inherit; text-decoration: underline; }
}