:root {
  --desk: #191715;
  --desk-2: #2c2824;
  --paper: #fbf8f1;
  --paper-2: #f3eee2;
  --ink: #2a2622;
  --ink-soft: #6b635a;
  --rule: #e3dac8;
  --accent-ink: #7a4b1e;
  --ok: #2f7d4f;
  --ok-bg: #e5f2e8;
  --bad: #b3432f;
  --bad-bg: #f8e5e0;
  --focus: #3b82f6;
  --serif: "Iowan Old Style", "Palatino Linotype", Palatino, "Book Antiqua", Georgia, serif;
  --sans: system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", sans-serif;
  --mono: ui-monospace, "Cascadia Mono", Consolas, "Courier New", monospace;
  --pw: 420px;
  --ph: 600px;
  --fs: 15px;
}

* { box-sizing: border-box; }
html, body { margin: 0; height: 100%; }
body {
  background: radial-gradient(ellipse at 50% 35%, var(--desk-2), var(--desk) 72%);
  color: #ece6dc;
  font-family: var(--sans);
  overflow: hidden;
  -webkit-font-smoothing: antialiased;
}
.sr-only { position: absolute; width: 1px; height: 1px; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; }

/* ---------- Stage and book ---------- */
.stage {
  position: fixed;
  inset: 0 0 64px 0;
  display: flex;
  align-items: center;
  justify-content: center;
  perspective: 2600px;
  padding-top: env(safe-area-inset-top);
}
.book {
  position: relative;
  width: calc(var(--pw) * 2);
  height: var(--ph);
  transform-style: preserve-3d;
  touch-action: pan-y;
  user-select: none;
  -webkit-user-select: none;
}
.book.single { width: var(--pw); }
.slot { position: absolute; top: 0; width: var(--pw); height: var(--ph); }
.slot.left { left: 0; }
.slot.right { left: var(--pw); }
.book.single .slot.left { display: none; }
.book.single .slot.right { left: 0; }
.slot:not(:empty) { box-shadow: 0 24px 50px rgba(0, 0, 0, .5), 0 4px 10px rgba(0, 0, 0, .35); }

.flipper {
  position: absolute;
  top: 0;
  width: var(--pw);
  height: var(--ph);
  transform-style: preserve-3d;
  display: none;
  z-index: 10;
  will-change: transform;
}
.flipper.active { display: block; }
.flipper.fwd { left: var(--pw); transform-origin: left center; }
.flipper.bwd { left: 0; transform-origin: right center; }
.book.single .flipper { left: 0; transform-origin: left center; }
.face { position: absolute; inset: 0; backface-visibility: hidden; -webkit-backface-visibility: hidden; }
.face.back { transform: rotateY(180deg); }
.face::after {
  content: "";
  position: absolute;
  inset: 0;
  pointer-events: none;
  background: linear-gradient(to right, rgba(0, 0, 0, var(--shade, 0)), rgba(0, 0, 0, 0) 70%);
}
.flipper.bwd .face.front::after, .flipper.fwd .face.back::after {
  background: linear-gradient(to left, rgba(0, 0, 0, var(--shade, 0)), rgba(0, 0, 0, 0) 70%);
}

/* ---------- Paper pages ---------- */
.page {
  position: absolute;
  inset: 0;
  display: flex;
  flex-direction: column;
  background-color: var(--paper);
  color: var(--ink);
  overflow: hidden;
  padding: calc(var(--pw) * .07) calc(var(--pw) * .075) calc(var(--pw) * .05);
  font-family: var(--serif);
  font-size: var(--fs);
  line-height: 1.5;
}
/* gutter shading near the spine */
.book.spread .slot.left .page, .flipper.fwd .face.back .page, .flipper.bwd .face.front .page {
  background-image: linear-gradient(to left, rgba(60, 40, 10, .13), rgba(60, 40, 10, 0) 7%);
}
.book.spread .slot.right .page, .flipper.fwd .face.front .page, .flipper.bwd .face.back .page {
  background-image: linear-gradient(to right, rgba(60, 40, 10, .13), rgba(60, 40, 10, 0) 7%);
}
.book.single .page { border-radius: 4px; }

.page-head {
  flex: none;
  display: flex;
  justify-content: space-between;
  gap: 1em;
  font-family: var(--sans);
  font-size: .6em;
  letter-spacing: .12em;
  text-transform: uppercase;
  color: var(--ink-soft);
  border-bottom: 1px solid var(--rule);
  padding-bottom: .5em;
  margin-bottom: .9em;
  white-space: nowrap;
}
.page-head span { overflow: hidden; text-overflow: ellipsis; }
.page-body { flex: 1; min-height: 0; overflow: hidden; }
.page-foot {
  flex: none;
  text-align: center;
  font-family: var(--sans);
  font-size: .65em;
  color: var(--ink-soft);
  padding-top: .6em;
  min-height: 1.8em;
}
.page.bare .page-head { visibility: hidden; }

.page-body > :first-child { margin-top: 0; }
.page-body h1 { font-size: 1.6em; line-height: 1.15; margin: .2em 0 .5em; }
.page-body h2 { font-size: 1.18em; line-height: 1.25; margin: .9em 0 .35em; color: var(--accent-ink); }
.page-body h3 { font-size: 1.02em; margin: .8em 0 .3em; }
.page-body p { margin: .45em 0; }
.page-body ul, .page-body ol { margin: .4em 0; padding-left: 1.35em; }
.page-body li { margin: .15em 0; }
.page-body li > ul, .page-body li > ol { margin: .1em 0; }
.page-body img.inline-figure { height: 1.9em; width: auto; vertical-align: middle; margin: 0 .2em; border: 1px solid var(--rule); border-radius: 3px; background: var(--paper); }
.page-body p > img.inline-figure:only-child, .page-body p > img.inline-figure:first-child:last-child { display: block; height: auto; width: min(100%, 320px); margin: .5em auto; }
.page-body table { border-collapse: collapse; width: 100%; font-family: var(--sans); font-size: .76em; margin: .6em 0; }
.page-body th, .page-body td { border-bottom: 1px solid var(--rule); padding: .35em .4em; text-align: left; vertical-align: top; }
.page-body th { font-weight: 600; color: var(--ink-soft); }
.page-body pre {
  font-family: var(--mono);
  font-size: .72em;
  line-height: 1.25;
  background: var(--paper-2);
  border: 1px solid var(--rule);
  padding: .6em .7em;
  border-radius: 4px;
  overflow: hidden;
  white-space: pre;
  margin: .6em 0;
}
.page-body code { font-family: var(--mono); font-size: .92em; }
.page-body blockquote { margin: .6em 0; padding: .35em .8em; border-left: 3px solid var(--accent-ink); background: var(--paper-2); }
.page-body blockquote p { margin: .25em 0; }
.page-body hr { border: 0; border-top: 1px solid var(--rule); margin: 1em 0; }
.page-body .shrink { font-size: .82em; }

/* chapter openers */
.chapter { margin-bottom: .6em; }
.chapter-kicker { font-family: var(--sans); font-size: .68em; font-weight: 600; letter-spacing: .16em; text-transform: uppercase; color: var(--accent-ink); }
.chapter-title { font-size: 1.75em; line-height: 1.1; margin: .2em 0 .25em; font-weight: 600; }
.chapter-meta { font-family: var(--sans); font-size: .72em; color: var(--ink-soft); }
.chapter-rule { height: 2px; width: 3em; background: var(--accent-ink); margin: .7em 0 .4em; }
.chapter-goals { font-size: .92em; }
.chapter-goals-title { font-family: var(--sans); font-size: .7em; font-weight: 600; letter-spacing: .08em; text-transform: uppercase; color: var(--ink-soft); margin-top: .6em; }

/* title page */
.title-page { height: 100%; display: flex; flex-direction: column; justify-content: space-between; text-align: center; padding: 6% 0 2%; }
.tp-brand { font-family: var(--sans); font-size: .7em; letter-spacing: .35em; text-transform: uppercase; color: var(--ink-soft); }
.tp-kicker { font-family: var(--sans); font-size: .72em; letter-spacing: .14em; text-transform: uppercase; color: var(--accent-ink); }
.tp-title { font-size: 2em; line-height: 1.1; margin: .3em 0 .4em; font-weight: 600; }
p.tp-author { font-family: var(--sans); font-size: .8em; letter-spacing: .06em; color: var(--ink-soft); margin: 0 0 .9em; }
p.tp-desc { font-size: .95em; color: var(--ink-soft); max-width: 22em; margin: 0 auto; }
.badge { display: inline-block; font-family: var(--sans); font-size: .68em; font-weight: 600; padding: .35em .8em; border-radius: 999px; }
.badge.ok { background: var(--ok-bg); color: var(--ok); }
.badge.draft { background: #f7ecd6; color: #8a5a12; }

/* contents */
.toc { list-style: none; padding: 0 !important; margin: .4em 0; }
.toc li { margin: 0; }
.toc-link {
  display: flex; align-items: baseline; gap: .4em; width: 100%;
  font: inherit; color: inherit; background: none; border: 0; padding: .35em 0; cursor: pointer; text-align: left;
}
.toc-link:hover .toc-label { color: var(--accent-ink); text-decoration: underline; }
.toc-link:focus-visible { outline: 2px solid var(--focus); outline-offset: 2px; border-radius: 3px; }
.toc-label { flex: none; max-width: 80%; }
.toc-dots { flex: 1; border-bottom: 1px dotted #b9ad97; transform: translateY(-.3em); }
.toc-page { flex: none; font-family: var(--sans); font-size: .85em; font-variant-numeric: tabular-nums; color: var(--ink-soft); }
.toc .sub .toc-label { padding-left: 1.2em; font-size: .9em; color: var(--ink-soft); }
.toc .major .toc-label { font-weight: 600; }

/* end page */
.end-page { height: 100%; display: flex; flex-direction: column; justify-content: center; text-align: center; }
.end-page h1 { font-size: 1.6em; }
.end-page ul { text-align: left; display: inline-block; margin: .6em auto; }

/* ---------- Hard covers ---------- */
.page.cover, .page.endpaper {
  background-color: var(--cover);
  color: var(--accent);
  background-image:
    radial-gradient(ellipse at 30% 18%, rgba(255, 255, 255, .13), rgba(255, 255, 255, 0) 55%),
    radial-gradient(rgba(0, 0, 0, .06) 1px, transparent 1px);
  background-size: auto, 4px 4px;
}
.page.cover { padding: calc(var(--pw) * .065); border-radius: 3px 8px 8px 3px; box-shadow: inset 6px 0 10px -4px rgba(0, 0, 0, .45), inset 0 0 0 1px rgba(0, 0, 0, .25); }
.page.cover.back-cover { border-radius: 8px 3px 3px 8px; box-shadow: inset -6px 0 10px -4px rgba(0, 0, 0, .45), inset 0 0 0 1px rgba(0, 0, 0, .25); }
.cover-frame {
  flex: 1;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: space-between;
  text-align: center;
  border: 2px solid color-mix(in srgb, var(--accent) 75%, transparent);
  outline: 1px solid color-mix(in srgb, var(--accent) 40%, transparent);
  outline-offset: -9px;
  border-radius: 4px;
  padding: 9% 9% 7%;
}
.cover-brand { font-size: 1em; letter-spacing: .38em; text-transform: uppercase; padding-left: .38em; }
.cover-center { display: flex; flex-direction: column; align-items: center; gap: 1em; width: 100%; }
.cover-title {
  font-size: 2.15em;
  line-height: 1.08;
  margin: 0;
  font-weight: 600;
  text-wrap: balance;
  text-shadow: 0 1px 0 rgba(0, 0, 0, .35), 0 -1px 0 rgba(255, 255, 255, .12);
}
.cover-author { font-family: var(--sans); font-size: .72em; letter-spacing: .24em; text-transform: uppercase; opacity: .9; margin-top: -.3em; }
.cover-art { width: 64%; color: var(--accent); filter: drop-shadow(0 1px 0 rgba(0, 0, 0, .35)); }
.cover-art svg { display: block; width: 100%; height: auto; max-height: calc(var(--ph) * .34); }
.cover-meta { font-family: var(--sans); font-size: .66em; letter-spacing: .22em; text-transform: uppercase; opacity: .85; }
.back-cover .cover-frame { justify-content: center; gap: 1.4em; }
.back-cover .cover-desc { font-size: .95em; max-width: 20em; opacity: .92; }
.page.endpaper {
  background-image:
    radial-gradient(color-mix(in srgb, var(--accent) 22%, transparent) 1.2px, transparent 1.4px),
    radial-gradient(color-mix(in srgb, var(--accent) 12%, transparent) 1.2px, transparent 1.4px);
  background-size: 18px 18px, 18px 18px;
  background-position: 0 0, 9px 9px;
  align-items: center;
  justify-content: center;
}
.endpaper-mark { font-size: 3.2em; letter-spacing: .1em; opacity: .5; font-weight: 600; }
.page.blank, .page.paper-back { background-color: var(--paper); }

/* ---------- Quizzes and tests ---------- */
.q {
  font-family: var(--sans);
  font-size: .84em;
  line-height: 1.45;
  border: 1px solid var(--rule);
  border-radius: 8px;
  padding: .7em .8em;
  margin: .55em 0;
  background: #fffdf8;
}
.q-head { display: flex; justify-content: space-between; font-size: .72em; font-weight: 600; letter-spacing: .08em; text-transform: uppercase; color: var(--ink-soft); margin-bottom: .3em; }
.q-prompt { font-family: var(--serif); font-size: 1.08em; margin-bottom: .55em; }
.q-choices { display: grid; gap: .35em; }
.q-choices.tf { grid-template-columns: 1fr 1fr; }
.choice {
  display: flex; gap: .6em; align-items: flex-start; width: 100%;
  font: inherit; color: inherit; text-align: left;
  background: #fff; border: 1px solid #d9cfbb; border-radius: 6px;
  padding: .45em .6em; min-height: 2.5em; cursor: pointer;
}
.choice:hover:not(:disabled) { border-color: var(--accent-ink); }
.choice:focus-visible, .q-btn:focus-visible, .q input:focus-visible, .q textarea:focus-visible { outline: 2px solid var(--focus); outline-offset: 1px; }
.choice[aria-checked="true"] { border-color: var(--accent-ink); background: #f6ecdc; box-shadow: inset 0 0 0 1px var(--accent-ink); }
.choice:disabled { cursor: default; }
.q.revealed .choice.is-correct { border-color: var(--ok); background: var(--ok-bg); box-shadow: inset 0 0 0 1px var(--ok); }
.q.revealed .choice.is-wrong { border-color: var(--bad); background: var(--bad-bg); box-shadow: inset 0 0 0 1px var(--bad); }
.choice-letter { flex: none; width: 1.3em; font-weight: 700; color: var(--accent-ink); }
.q-row { display: flex; gap: .5em; align-items: center; flex-wrap: wrap; margin-top: .3em; }
.q input[type="text"], .q textarea {
  font: inherit; color: var(--ink); background: #fff;
  border: 1px solid #cdbfa6; border-radius: 6px; padding: .45em .55em;
  user-select: text; -webkit-user-select: text;
}
.q input[type="text"] { width: 9em; }
.q textarea { width: 100%; resize: none; display: block; }
.q .unit { color: var(--ink-soft); }
.q-btn {
  font: 600 1em var(--sans); color: #fff; background: var(--accent-ink);
  border: 0; border-radius: 6px; padding: .45em .95em; min-height: 2.3em; cursor: pointer;
}
.q-btn:disabled { opacity: .45; cursor: default; }
.q-btn.ghost { background: transparent; color: var(--accent-ink); box-shadow: inset 0 0 0 1px currentColor; }
.q-feedback { visibility: hidden; margin-top: .55em; padding-top: .45em; border-top: 1px dashed var(--rule); }
.q.revealed .q-feedback { visibility: visible; }
.q-verdict { font-weight: 700; min-height: 1.45em; }
.q-verdict.ok { color: var(--ok); }
.q-verdict.bad { color: var(--bad); }
.q-answer { margin: .1em 0 .2em; }
.q-expl { color: #4d463f; }
.rubric-title { font-weight: 600; margin-top: .35em; }
.rubric label { display: flex; gap: .45em; align-items: flex-start; margin: .25em 0; cursor: pointer; }
.rubric input { margin-top: .25em; }
.score { font-family: var(--sans); font-size: .86em; border-radius: 8px; background: var(--paper-2); border: 1px solid var(--rule); padding: .8em; margin: .7em 0; }
.score-line { font-weight: 600; margin-bottom: .5em; }
.score-line.pass { color: var(--ok); }
.score-line.fail { color: var(--bad); }
.score-actions { display: flex; gap: .5em; flex-wrap: wrap; }
.q-intro { font-family: var(--sans); font-size: .82em; color: var(--ink-soft); }

@media (pointer: coarse) {
  .q input[type="text"], .q textarea { font-size: 16px; }
}

/* ---------- Controls ---------- */
.controls {
  position: fixed; left: 0; right: 0; bottom: 0;
  height: 64px;
  padding: 0 12px env(safe-area-inset-bottom);
  display: flex; align-items: center; justify-content: center; gap: 10px;
}
.btn {
  display: inline-flex; align-items: center; justify-content: center; gap: .2em;
  font: 500 14px var(--sans); color: #f2ece2; text-decoration: none;
  background: rgba(255, 255, 255, .08); border: 1px solid rgba(255, 255, 255, .15);
  border-radius: 999px; padding: 8px 14px; min-height: 40px; min-width: 44px; cursor: pointer;
}
.btn:hover { background: rgba(255, 255, 255, .15); }
.btn:disabled { opacity: .35; cursor: default; }
.btn:focus-visible { outline: 2px solid var(--focus); outline-offset: 2px; }
.page-label { min-width: 8.5em; text-align: center; font-size: 13px; color: #cfc6b8; font-variant-numeric: tabular-nums; }
.hint {
  position: fixed; top: 14px; left: 50%; transform: translateX(-50%);
  font-size: 13px; color: #d8d0c3; background: rgba(0, 0, 0, .38);
  padding: 7px 14px; border-radius: 999px; transition: opacity .6s; pointer-events: none; white-space: nowrap;
}
.hint.hidden { opacity: 0; }
.error { color: #f1e6d6; font-size: 16px; padding: 24px; text-align: center; }
.measure { position: absolute; left: -99999px; top: 0; width: var(--pw); height: var(--ph); visibility: hidden; pointer-events: none; }

@media (max-width: 560px) {
  .btn .label-long { display: none; }
  .page-label { min-width: 6.5em; }
}