@import url("https://fonts.googleapis.com/css2?family=Poppins:wght@400;500;600;700&family=Montserrat:wght@400;600;700&display=swap");

:root {
  --ink: #1c2329;
  --muted: #65717d;
  --line: #d9e0e5;
  --paper: #ffffff;
  --soft: #f6f8fa;
  --accent: #1f6f8b;
  --accent-dark: #164f64;
  --max-content: 820px;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  color: var(--ink);
  background: var(--paper);
  font-family: Poppins, Montserrat, Arial, sans-serif;
  font-size: 16px;
  line-height: 1.72;
}

.site-header {
  position: sticky;
  top: 0;
  z-index: 20;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 28px;
  min-height: 92px;
  padding: 18px clamp(18px, 3vw, 56px);
  border-bottom: 1px solid var(--line);
  background: rgba(255, 255, 255, .96);
  backdrop-filter: blur(10px);
}

.brand img {
  display: block;
  width: 174px;
  height: auto;
}

.site-header nav {
  display: flex;
  gap: clamp(14px, 2vw, 30px);
  align-items: center;
  flex-wrap: wrap;
  justify-content: flex-end;
}

a {
  color: var(--accent-dark);
  text-decoration-thickness: .08em;
  text-underline-offset: .18em;
}

.acronym-term {
  position: relative;
  border-bottom: 1px dotted currentColor;
  cursor: help;
  text-decoration: none;
}

.acronym-term:focus {
  outline: 2px solid color-mix(in srgb, var(--accent) 60%, white);
  outline-offset: 2px;
}

.acronym-term::after {
  position: absolute;
  left: 50%;
  bottom: calc(100% + 8px);
  z-index: 30;
  width: max-content;
  max-width: min(320px, 70vw);
  padding: 8px 10px;
  border: 1px solid var(--line);
  border-radius: 4px;
  background: #101820;
  box-shadow: 0 10px 24px rgba(16, 24, 32, .18);
  color: #fff;
  content: attr(data-definition);
  font-family: Poppins, Montserrat, Arial, sans-serif;
  font-size: 13px;
  font-style: normal;
  font-weight: 500;
  line-height: 1.35;
  opacity: 0;
  pointer-events: none;
  text-align: left;
  transform: translate(-50%, 4px);
  transition: opacity .12s ease, transform .12s ease;
  white-space: normal;
}

.acronym-term:hover::after,
.acronym-term:focus::after {
  opacity: 1;
  transform: translate(-50%, 0);
}

.endnote-ref {
  position: relative;
  border-bottom: 1px dotted currentColor;
  color: var(--accent-dark);
  font-size: .9em;
  font-weight: 600;
  text-decoration: none;
  cursor: help;
}

.endnote-ref:focus {
  outline: 2px solid color-mix(in srgb, var(--accent) 60%, white);
  outline-offset: 2px;
}

.endnote-ref::after {
  position: absolute;
  left: 50%;
  bottom: calc(100% + 8px);
  z-index: 30;
  width: max-content;
  max-width: min(420px, 78vw);
  padding: 9px 11px;
  border: 1px solid var(--line);
  border-radius: 4px;
  background: #101820;
  box-shadow: 0 10px 24px rgba(16, 24, 32, .18);
  color: #fff;
  content: attr(data-note);
  font-family: Poppins, Montserrat, Arial, sans-serif;
  font-size: 13px;
  font-style: normal;
  font-weight: 500;
  line-height: 1.4;
  opacity: 0;
  pointer-events: none;
  text-align: left;
  transform: translate(-50%, 4px);
  transition: opacity .12s ease, transform .12s ease;
  white-space: normal;
}

.endnote-ref:hover::after,
.endnote-ref:focus::after {
  opacity: 1;
  transform: translate(-50%, 0);
}

.reference-term {
  position: relative;
  border-bottom: 1px dotted currentColor;
  color: var(--accent-dark);
  font-weight: 500;
  text-decoration: none;
}

.reference-term:focus {
  outline: 2px solid color-mix(in srgb, var(--accent) 60%, white);
  outline-offset: 2px;
}

.reference-term::after {
  position: absolute;
  left: 50%;
  bottom: calc(100% + 8px);
  z-index: 30;
  width: max-content;
  max-width: min(440px, 78vw);
  padding: 9px 11px;
  border: 1px solid var(--line);
  border-radius: 4px;
  background: #101820;
  box-shadow: 0 10px 24px rgba(16, 24, 32, .18);
  color: #fff;
  content: attr(data-reference-title) ": " attr(data-reference-preview);
  font-family: Poppins, Montserrat, Arial, sans-serif;
  font-size: 13px;
  font-style: normal;
  font-weight: 500;
  line-height: 1.4;
  opacity: 0;
  pointer-events: none;
  text-align: left;
  transform: translate(-50%, 4px);
  transition: opacity .12s ease, transform .12s ease;
  white-space: normal;
}

.reference-term:hover::after,
.reference-term:focus::after {
  opacity: 1;
  transform: translate(-50%, 0);
}

.site-header a {
  color: #181818;
  font-size: 14px;
  font-weight: 500;
  text-decoration: none;
}

.book-layout {
  display: grid;
  grid-template-columns: minmax(250px, 320px) minmax(0, 1fr);
  gap: clamp(28px, 4vw, 64px);
  max-width: 1420px;
  margin: 0 auto;
  padding: 38px clamp(18px, 3vw, 56px) 60px;
}

.toc-sidebar {
  position: sticky;
  top: 118px;
  max-height: calc(100vh - 140px);
  overflow: auto;
  align-self: start;
  padding-right: 10px;
  border-right: 1px solid var(--line);
  font-size: 14px;
  line-height: 1.45;
}

.toc-kicker {
  margin: 0 0 8px;
  color: var(--muted);
  font-size: 12px;
  font-weight: 600;
  letter-spacing: .08em;
  text-transform: uppercase;
}

.book-identity {
  display: grid;
  grid-template-columns: 72px minmax(0, 1fr);
  gap: 12px;
  align-items: center;
  margin: 0 0 18px;
  padding: 0 0 18px;
  border-bottom: 1px solid var(--line);
}

.book-cover-link {
  display: grid;
  place-items: center;
  width: 72px;
  aspect-ratio: 2 / 3;
  border-radius: 4px;
  overflow: hidden;
  line-height: 0;
}

.book-cover-link:focus-visible {
  outline: 3px solid rgba(0, 137, 145, .35);
  outline-offset: 3px;
}

.book-cover-thumb {
  display: block;
  width: 72px;
  aspect-ratio: 2 / 3;
  object-fit: contain;
  object-position: center center;
  border: 1px solid transparent;
  box-shadow: 0 8px 18px rgba(28, 35, 41, .14);
}

.book-cover-thumb--teal-preview {
  filter: grayscale(1) sepia(1) saturate(4.2) hue-rotate(130deg) brightness(.78) contrast(1.18);
}

.book-identity-text {
  min-width: 0;
}

.book-title-sidebar {
  margin: 0 0 4px;
  color: #111;
  font-size: 16px;
  font-weight: 700;
  line-height: 1.2;
}

.book-author-sidebar {
  margin: 0;
  color: var(--muted);
  font-size: 13px;
  line-height: 1.35;
}

.chapter-link {
  display: block;
  margin: 0 0 16px;
  color: #111;
  font-size: 17px;
  font-weight: 600;
  text-decoration: none;
}

.toc-sidebar ol {
  margin: 0;
  padding: 0;
  list-style: none;
}

.toc-sidebar li {
  margin: 0;
}

.toc-sidebar li li a {
  padding-left: 18px;
}

.toc-sidebar details {
  margin: 0;
}

.toc-sidebar summary {
  display: flex;
  gap: 6px;
  align-items: baseline;
  padding: 6px 8px;
  border-radius: 4px;
  color: #2b3944;
  cursor: pointer;
  list-style: none;
}

.toc-sidebar summary::-webkit-details-marker {
  display: none;
}

.toc-sidebar summary::before {
  content: ">";
  color: var(--muted);
  font-size: 15px;
  line-height: 1;
  transition: transform .16s ease;
}

.toc-sidebar details[open] > summary::before {
  transform: rotate(90deg);
}

.toc-sidebar summary:hover {
  background: var(--soft);
  color: var(--accent-dark);
}

.toc-summary-number {
  color: var(--muted);
  font-variant-numeric: tabular-nums;
}

.toc-group ol {
  margin: 2px 0 8px 16px;
}

.toc-sidebar a {
  display: block;
  padding: 6px 8px;
  border-radius: 4px;
  color: #2b3944;
  text-decoration: none;
}

.toc-sidebar a span {
  color: var(--muted);
  font-variant-numeric: tabular-nums;
}

.toc-sidebar a[aria-current="page"],
.toc-sidebar a:hover {
  background: var(--soft);
  color: var(--accent-dark);
}

.keep-together-heading {
  margin-top: 30px;
  padding-top: 4px;
}

.keep-together-heading span {
  color: var(--muted);
  font-variant-numeric: tabular-nums;
}

.content {
  width: min(100%, var(--max-content));
}

.reference-page {
  width: min(100% - 36px, var(--max-content));
  margin: 0 auto;
  padding: 44px 0 64px;
}

.reference-page .back-to-reading {
  margin-bottom: 24px;
}

.reference-page .back-to-reading:last-child {
  margin-top: 34px;
  margin-bottom: 0;
}

.back-to-reading[hidden] {
  display: none !important;
}

.back-to-reading {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  min-height: 36px;
  padding: 7px 12px;
  border: 1px solid var(--line);
  border-radius: 4px;
  background: var(--soft);
  color: var(--accent-dark);
  font-size: 14px;
  font-weight: 600;
  text-decoration: none;
}

.back-to-reading:hover,
.back-to-reading:focus {
  border-color: color-mix(in srgb, var(--accent) 45%, var(--line));
  background: #eef5f7;
}

.site-masthead {
  background: linear-gradient(rgba(248, 251, 252, .90), rgba(248, 251, 252, .90)), url("cog.png") 0 0 / 25px 25px repeat;
  border-bottom: 1px solid var(--line);
}

.site-masthead-inner {
  align-items: center;
  display: flex;
  gap: 28px;
  justify-content: space-between;
  margin: 0 auto;
  max-width: 1220px;
  min-height: 112px;
  padding: 22px 32px;
}

.brand-lockup {
  align-items: flex-start;
  display: flex;
  flex-direction: column;
  gap: 4px;
}

.logo {
  align-items: center;
  display: inline-flex;
  text-decoration: none;
}

.logo img {
  display: block;
  height: auto;
  width: clamp(154px, 16vw, 218px);
}

.logo-sub {
  color: var(--accent-dark);
  display: block;
  font-size: 13px;
  font-weight: 700;
  letter-spacing: .08em;
  margin-left: 4px;
  text-transform: uppercase;
}

.site-nav {
  align-items: center;
  display: flex;
  flex-wrap: wrap;
  font-size: 15px;
  gap: 18px;
  justify-content: flex-end;
}

.site-nav a,
.site-nav button {
  color: var(--text);
  font: inherit;
}

.site-nav a {
  text-decoration: none;
}

.site-nav a:hover,
.site-nav a.active {
  color: var(--accent-dark);
}

.site-nav a.active {
  font-weight: 700;
}

.site-nav button {
  background: transparent;
  border: 0;
  cursor: pointer;
  padding: 0;
}

.course-link {
  background: var(--accent);
  border-radius: 999px;
  color: #fff !important;
  font-weight: 700;
  padding: 12px 18px;
}

.reference-library-page {
  padding-top: 34px;
}

.reference-library-hero {
  display: grid;
  gap: 14px;
  margin: 0 0 28px;
  padding: 4px 0 8px;
}

.reference-library-hero h1 {
  margin-bottom: 4px;
}

.reference-library-hero p {
  max-width: 760px;
  color: var(--muted);
  font-size: 1.05rem;
}

.reference-library-actions {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
  align-items: center;
  margin-top: 4px;
}

.reference-library-search {
  align-items: center;
  background: var(--paper);
  border: 1px solid var(--line);
  border-radius: 6px;
  color: var(--muted);
  cursor: pointer;
  display: inline-flex;
  font: inherit;
  justify-content: flex-start;
  min-height: 2.5rem;
  padding: 0.5rem 0.65rem;
  text-align: left;
  width: min(100%, 34rem);
}

.reference-library-search:hover,
.reference-library-search:focus {
  background: var(--paper);
  border-color: var(--accent);
  color: var(--accent-dark);
}

.reference-category-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: 18px;
  margin: 24px 0 38px;
}

.reference-category-card {
  display: grid;
  grid-template-columns: minmax(250px, 340px) minmax(0, 1fr);
  overflow: hidden;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: #fff;
}

.reference-category-card__media {
  display: grid;
  place-items: center;
  min-height: 224px;
  padding: 18px;
  background: linear-gradient(180deg, #eef7f8 0%, #f7fbfb 100%);
}

.reference-category-card__media img {
  width: min(280px, 100%);
  height: 202px;
  object-fit: contain;
}

.reference-category-card__body {
  align-self: center;
  padding: 24px 26px;
}

.reference-category-card h2 {
  margin: 0 0 10px;
  font-size: clamp(1.45rem, 3vw, 2rem);
}

.reference-category-card p {
  margin: 0;
  max-width: 700px;
  color: var(--muted);
  font-size: 1rem;
}

.reference-browse {
  margin-top: 18px;
}

.reference-browse__header,
.reference-browse-group__header {
  display: flex;
  justify-content: space-between;
  gap: 18px;
  align-items: end;
  margin: 0 0 14px;
}

.reference-browse__header h2,
.reference-browse-group__header h3 {
  margin: 0;
}

.reference-browse-group {
  margin: 26px 0 34px;
  padding-top: 6px;
}

.reference-letter-nav {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
  margin: 0 0 18px;
}

.reference-letter-nav a {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 30px;
  height: 30px;
  border: 1px solid var(--line);
  border-radius: 4px;
  color: var(--accent-dark);
  font-size: 0.85rem;
  font-weight: 700;
  text-decoration: none;
}

.reference-letter-section {
  display: grid;
  grid-template-columns: 42px minmax(0, 1fr);
  gap: 18px;
  padding: 16px 0;
  border-top: 1px solid var(--line);
}

.reference-letter-section h4 {
  margin: 0;
  color: var(--accent-dark);
  font-size: 1rem;
}

.reference-library-list {
  margin: 0;
  padding: 0;
  list-style: none;
}

.reference-library-list li {
  margin: 0 0 14px;
}

.reference-library-list a {
  font-weight: 600;
}

.reference-library-list p {
  margin: 3px 0 0;
  color: var(--muted);
  font-size: 14px;
  line-height: 1.45;
}

.reference-library-list--compact {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  column-gap: 22px;
  row-gap: 14px;
}

.reference-library-list--compact li {
  margin: 0;
}

@media (max-width: 860px) {
  .site-masthead-inner {
    align-items: flex-start;
    flex-direction: column;
  }

  .site-nav {
    justify-content: flex-start;
    gap: 12px;
  }

  .reference-category-card,
  .reference-library-list--compact {
    grid-template-columns: 1fr;
  }

  .reference-category-card__media {
    min-height: 190px;
  }

  .reference-browse__header,
  .reference-browse-group__header {
    display: block;
  }
}

@media (max-width: 620px) {
  .reference-letter-section {
    grid-template-columns: 1fr;
    gap: 8px;
  }
}

.breadcrumb {
  margin: 0 0 18px;
  color: var(--muted);
  font-size: 14px;
}

h1 {
  margin: 0 0 24px;
  color: #101820;
  font-size: clamp(2rem, 5vw, 3.15rem);
  line-height: 1.13;
  font-weight: 600;
}

h1 span,
h2 span {
  color: var(--accent);
  font-variant-numeric: tabular-nums;
}

h2 {
  margin: 34px 0 12px;
  padding-top: 0;
  border-top: 0;
  font-size: 1.45rem;
  line-height: 1.3;
}

.example-separator {
  margin: 30px 0;
  border: 0;
  border-top: 1px solid var(--border);
}

.example-block {
  margin: 28px 0 30px;
  padding: 18px 0 2px;
  border-top: 1px solid var(--border);
  border-bottom: 1px solid var(--border);
}

.example-block p {
  margin-bottom: 14px;
}

.example-title {
  font-weight: 700;
}

.small-heading {
  margin: 26px 0 10px;
  padding-top: 0;
  border-top: 0;
  font-size: 1.05rem;
  line-height: 1.45;
  font-weight: 700;
}

p {
  margin: 0 0 18px;
}

em {
  font-style: italic;
}

strong {
  font-weight: 700;
}

sub,
sup {
  line-height: 0;
}

.table-wrap {
  overflow-x: auto;
  margin: 14px 0 24px;
}

.book-table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.96rem;
}

.book-table th,
.book-table td {
  border: 1px solid #d2d8e3;
  padding: 8px 10px;
  text-align: center;
  vertical-align: middle;
}

.book-table th {
  background: #f4f7fb;
  font-weight: 700;
}

.sub-paragraph {
  margin-left: 1.4rem;
}

.bullet-list,
.ordered-list {
  margin: 10px 0 20px 1.35rem;
  padding-left: 1.25rem;
}

.bullet-list li,
.ordered-list li {
  margin: 0 0 12px;
  padding-left: 0.2rem;
}

.bullet-list li.list-level-1,
.ordered-list li.list-level-1 {
  margin-left: 2.25rem;
}

.bullet-list li.list-level-2,
.bullet-list li.list-level-3,
.bullet-list li.list-level-4,
.ordered-list li.list-level-2,
.ordered-list li.list-level-3,
.ordered-list li.list-level-4 {
  margin-left: 3.5rem;
}


.book-home {
  margin: 0 auto;
  max-width: 920px;
  padding: 42px clamp(20px, 4vw, 56px) 64px;
}

.book-home-hero {
  display: grid;
  grid-template-columns: 112px minmax(0, 1fr);
  gap: 20px;
  align-items: center;
  max-width: 780px;
  margin: 0 0 28px;
}

.book-cover-link--home {
  width: 112px;
}

.book-cover-thumb--home {
  width: 112px;
}

.book-home h1 {
  margin: 0 0 10px;
}

.book-home-hero p {
  margin: 0;
  max-width: 660px;
}

.book-home .section-list {
  max-width: 760px;
}

@media (max-width: 640px) {
  .book-home-hero {
    grid-template-columns: 80px minmax(0, 1fr);
    gap: 14px;
  }

  .book-cover-link--home,
  .book-cover-thumb--home {
    width: 80px;
  }
}

.section-list {
  margin: 12px 0 28px;
  padding-left: 0;
  list-style: none;
}

.section-list li {
  margin: 6px 0;
}

.faq-question-list {
  margin: 8px 0 18px;
  padding-left: 1.35rem;
  font-size: 0.95rem;
}

.faq-question-list li {
  margin: 4px 0;
}

.faq-question-list a {
  text-decoration: none;
}

.faq-question-list a:hover,
.faq-question-list a:focus {
  text-decoration: underline;
}
.section-links {
  margin: 12px 0 28px;
  padding: 0;
  list-style: none;
}

.section-links li {
  margin: 6px 0;
}

.revision-questions {
  margin: 0 0 28px;
  padding-left: 1.6rem;
}

.revision-questions li {
  margin: 0 0 12px;
  padding-left: 0.25rem;
}

.figure {
  margin: 28px 0 28px;
  scroll-margin-top: 96px;
}

.figure + .figure {
  margin-top: 10px;
}

.figure img,
p img {
  display: block;
  max-width: 100%;
  height: auto;
  margin: 0 auto;
}

figcaption {
  display: block;
  margin: 8px 0 0;
  color: var(--muted);
  font-size: 14px;
  line-height: 1.45;
}

.endnotes {
  margin: 42px 0 28px;
  padding-top: 18px;
  border-top: 1px solid var(--line);
  color: var(--ink);
}

.endnotes h2 {
  margin: 0 0 12px;
  font-size: 20px;
}

.endnotes ul {
  margin: 0;
  padding: 0;
  list-style: none;
}

.endnotes li {
  margin: 0 0 10px;
  padding-left: 0.25rem;
  font-size: 14px;
  line-height: 1.55;
}

.endnote-number {
  font-weight: 600;
}

.endnote-backref {
  margin-left: 4px;
  font-size: 13px;
}

.inline-math,
.equation-body {
  font-family: "Cambria Math", "STIX Two Math", math, serif;
}

.inline-math math {
  display: inline math;
}

.display-equation {
  display: grid;
  grid-template-columns: minmax(0, 1fr) auto;
  gap: 24px;
  align-items: center;
  margin: 22px 0 24px;
  padding-left: clamp(18px, 5vw, 52px);
  font-size: 1.2rem;
  line-height: 1.45;
}

.equation-body {
  min-width: 0;
  overflow-x: auto;
  overflow-y: hidden;
  padding-block: 0.08em;
}

.equation-body math {
  max-width: 100%;
}

.legacy-equation-image {
  display: inline-block;
  max-width: 100%;
  height: auto;
  vertical-align: middle;
}

.legacy-equation-text {
  display: inline-block;
  margin-left: 0.45em;
  vertical-align: middle;
}

.equation-number {
  align-self: center;
  color: var(--ink);
  font-family: "Cambria Math", "STIX Two Math", math, serif;
  font-variant-numeric: tabular-nums;
  white-space: nowrap;
}

.equation-fallback {
  font-family: "Cambria Math", "STIX Two Math", serif;
  color: var(--accent-dark);
}

.ad-slot {
  margin: 32px 0;
  padding: 22px;
  border: 1px dashed #b8c4cc;
  background: #fafbfc;
  color: var(--muted);
  text-align: center;
  font-size: 14px;
}

.pager {
  display: flex;
  justify-content: space-between;
  gap: 18px;
  margin-top: 46px;
  padding-top: 22px;
  border-top: 1px solid var(--line);
}

.pager a {
  max-width: 48%;
  text-decoration: none;
}

.site-footer {
  border-top: 1px solid var(--line);
  padding: 34px clamp(18px, 3vw, 56px);
  color: var(--muted);
  text-align: center;
  font-size: 14px;
}

.site-footer p {
  margin: 0 0 8px;
}

@media (max-width: 900px) {
  .site-header {
    position: static;
    align-items: flex-start;
    flex-direction: column;
  }

  .site-header nav {
    justify-content: flex-start;
  }

  .book-layout {
    display: block;
    padding-top: 24px;
  }

  .toc-sidebar {
    position: static;
    max-height: none;
    margin-bottom: 30px;
    padding: 0 0 22px;
    border-right: 0;
    border-bottom: 1px solid var(--line);
  }

  .book-identity {
    grid-template-columns: 56px minmax(0, 1fr);
  }

  .book-cover-thumb {
    width: 56px;
  }

  .content {
    width: 100%;
  }
}

@media (max-width: 620px) {
  .display-equation {
    grid-template-columns: minmax(0, 1fr);
    padding-left: 0;
  }

  .equation-number {
    justify-self: end;
  }
}