:root {
  --bg: #ffffff;
  --fg: #171717;
  --card: #fafafa;
  --card-fg: #171717;
  --border: #e8e8e8;
  --muted: #ededed;
  --muted-fg: #595959;
  --blue: #2563eb;
  --blue-hover: #1d4ed8;
  --font-body: 'Source Serif 4', 'Source Serif Pro', Georgia, serif;
  --font-heading: 'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
  --font-mono: 'Source Code Pro', 'SFMono-Regular', Consolas, monospace;
  --era-foundations: #1a5276;
  --era-internet: #117864;
  --era-cloud: #6c3483;
  --era-ai: #b9770e;
}

*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

html, body {
  height: 100%;
  font-family: var(--font-body);
  color: var(--fg);
  background: var(--bg);
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

a { text-decoration: none; }
button { font-family: inherit; }

.app {
  height: 100vh;
  display: flex;
  flex-direction: column;
  overflow: hidden;
}

/* ─── Header ─── */
.header {
  flex-shrink: 0;
  border-bottom: 1px solid var(--border);
  padding: 1.25rem 1.5rem;
}
@media (min-width: 768px) {
  .header { padding: 1.5rem 2.5rem; }
}
.header-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  flex-wrap: wrap;
}
.header h1 {
  font-family: var(--font-body);
  font-size: 1.25rem;
  font-weight: 700;
  letter-spacing: -0.025em;
  color: var(--fg);
  line-height: 1.4;
}
@media (min-width: 768px) {
  .header h1 { font-size: 1.5rem; }
}
.header .subtitle {
  font-family: var(--font-body);
  font-size: 0.875rem;
  color: var(--muted-fg);
  margin-top: 0.25rem;
}
@media (min-width: 768px) {
  .header .subtitle { font-size: 1rem; }
}
.header-link {
  display: inline-flex;
  align-items: center;
  gap: 0.375rem;
  font-family: var(--font-body);
  font-size: 0.875rem;
  color: var(--muted-fg);
  transition: color 0.15s;
}
.header-link:hover { color: var(--fg); }
@media (min-width: 768px) {
  .header-link { font-size: 1rem; }
}
.header-link svg { width: 14px; height: 14px; }

/* ─── Era Navigation ─── */
.era-nav {
  flex-shrink: 0;
  border-bottom: 1px solid var(--border);
  position: relative;
}
.era-nav-scroll {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  overflow-x: auto;
  padding: 0.75rem 1rem;
  scrollbar-width: none;
}
.era-nav-scroll::-webkit-scrollbar { display: none; }
@media (min-width: 768px) {
  .era-nav-scroll { padding: 0.75rem 2.5rem; }
}
.era-btn {
  flex-shrink: 0;
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  padding: 0.625rem 1rem;
  border-radius: 0.5rem;
  font-family: var(--font-heading);
  font-size: 1rem;
  font-weight: 600;
  cursor: pointer;
  transition: background-color 0.2s, color 0.2s, border-color 0.2s;
  border: 1.5px solid transparent;
  background: none;
}
.era-btn.active {
  color: white;
  border-color: transparent;
}
.era-btn:not(.active) {
  color: var(--fg);
}
.era-btn:not(.active):hover {
  color: var(--fg);
}
.era-btn .year-range {
  font-size: 0.875rem;
  font-weight: 400;
}
.era-btn.active .year-range {
  color: rgba(255,255,255,0.7);
}
.era-btn:not(.active) .year-range {
  color: var(--muted-fg);
}
.nav-divider {
  width: 1px;
  height: 1.5rem;
  background: var(--border);
  margin: 0 0.25rem;
  flex-shrink: 0;
}
.discussion-nav-btn {
  flex-shrink: 0;
  padding: 0.625rem 1rem;
  border-radius: 0.5rem;
  font-family: var(--font-heading);
  font-size: 1rem;
  font-weight: 600;
  color: var(--muted-fg);
  border: 1px solid var(--border);
  background: none;
  cursor: pointer;
  transition: background-color 0.15s, color 0.15s;
}
.discussion-nav-btn:hover {
  background: var(--card);
  color: var(--fg);
}

/* ─── Nav scroll arrows ─── */
.nav-fade-left, .nav-fade-right {
  position: absolute;
  top: 0;
  bottom: 0;
  width: 3rem;
  z-index: 10;
  pointer-events: none;
}
.nav-fade-left {
  left: 0;
  background: linear-gradient(to right, var(--bg), transparent);
}
.nav-fade-right {
  right: 0;
  background: linear-gradient(to left, var(--bg), transparent);
}
.nav-arrow {
  position: absolute;
  top: 50%;
  transform: translateY(-50%);
  z-index: 20;
  width: 1.75rem;
  height: 1.75rem;
  border-radius: 50%;
  background: var(--bg);
  border: 1px solid var(--border);
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--muted-fg);
  cursor: pointer;
  transition: color 0.15s;
}
.nav-arrow:hover { color: var(--fg); }
.nav-arrow-left { left: 0.25rem; }
.nav-arrow-right { right: 0.25rem; }

/* ─── Main timeline area ─── */
.timeline-main {
  flex: 1;
  min-height: 0;
  position: relative;
}

.scroll-arrow {
  display: none;
  position: absolute;
  top: 50%;
  transform: translateY(-50%);
  z-index: 20;
  width: 2.5rem;
  height: 2.5rem;
  border-radius: 50%;
  background: rgba(255,255,255,0.9);
  backdrop-filter: blur(4px);
  border: 1px solid var(--border);
  align-items: center;
  justify-content: center;
  color: var(--muted-fg);
  cursor: pointer;
  transition: color 0.15s, border-color 0.15s;
}
.scroll-arrow:hover {
  color: var(--fg);
  border-color: rgba(0,0,0,0.2);
}
@media (min-width: 768px) {
  .scroll-arrow { display: flex; }
}
.scroll-arrow-left { left: 0.5rem; }
.scroll-arrow-right { right: 0.5rem; }

.timeline-scroll {
  height: 100%;
  overflow-x: auto;
  overflow-y: hidden;
  scroll-behavior: smooth;
  scrollbar-width: thin;
  -webkit-overflow-scrolling: touch;
}

.timeline-track {
  display: inline-flex;
  height: 100%;
  align-items: stretch;
  gap: 0;
  padding: 1rem 1rem;
}
@media (min-width: 768px) {
  .timeline-track { padding: 2rem 3rem; }
}

/* ─── Era Section ─── */
.era-section {
  display: flex;
  flex-direction: column;
  height: 100%;
  flex-shrink: 0;
}
.era-header {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  margin-bottom: 1.25rem;
  padding: 0 0.75rem;
}
.era-accent-bar {
  width: 6px;
  height: 3rem;
  border-radius: 9999px;
  flex-shrink: 0;
}
.era-header h2 {
  font-family: var(--font-heading);
  font-size: 1.125rem;
  font-weight: 700;
  color: var(--fg);
  letter-spacing: -0.025em;
  line-height: 1.2;
}
@media (min-width: 768px) {
  .era-header h2 { font-size: 1.25rem; }
}
.era-header .era-years {
  font-family: var(--font-heading);
  font-size: 1rem;
  color: var(--muted-fg);
  margin-top: 0.125rem;
}

.milestone-row {
  flex: 1;
  display: flex;
  align-items: stretch;
  gap: 0.75rem;
  padding: 0 0.25rem;
}

.milestone-connector {
  display: flex;
  align-items: center;
}
.milestone-connector-line {
  width: 1.5rem;
  height: 1px;
  opacity: 0.3;
}

.era-connector {
  display: flex;
  align-items: center;
  padding: 0 0.5rem;
}
.era-connector-line {
  width: 3rem;
  height: 1px;
  background: var(--border);
}

/* ─── Milestone Card ─── */
.milestone-card {
  display: flex;
  flex-direction: column;
  width: 85vw;
  flex-shrink: 0;
  border-radius: 0.75rem;
  border: 1px solid var(--border);
  background: var(--card);
  padding: 1rem;
  cursor: pointer;
  transition: border-color 0.2s, box-shadow 0.2s, transform 0.2s;
  text-align: left;
  overflow: hidden;
  font-family: var(--font-body);
}
@media (min-width: 640px) {
  .milestone-card { width: 14rem; padding: 1.25rem; }
}
@media (min-width: 768px) {
  .milestone-card { width: 16rem; }
}
.milestone-card:hover {
  border-color: rgba(0,0,0,0.2);
  box-shadow: 0 1px 3px rgba(0,0,0,0.08);
  transform: translateY(-2px);
}
.milestone-card:active {
  transform: scale(0.98);
}
.milestone-card .year-badge {
  display: inline-block;
  font-family: var(--font-heading);
  font-size: 0.75rem;
  font-weight: 700;
  padding: 0.25rem 0.625rem;
  border-radius: 0.375rem;
  color: white;
  margin-bottom: 0.75rem;
}
.milestone-card h3 {
  font-family: var(--font-heading);
  font-size: 1rem;
  font-weight: 600;
  line-height: 1.4;
  color: var(--fg);
  margin-bottom: 0.5rem;
  display: -webkit-box;
  -webkit-line-clamp: 2;
  -webkit-box-orient: vertical;
  overflow: hidden;
}
.milestone-card .snippet {
  font-size: 0.875rem;
  color: var(--muted-fg);
  line-height: 1.6;
  flex: 1;
  display: -webkit-box;
  -webkit-line-clamp: 4;
  -webkit-box-orient: vertical;
  overflow: hidden;
}
.milestone-card .read-more {
  margin-top: 0.75rem;
  padding-top: 0.75rem;
  border-top: 1px solid var(--border);
  font-family: var(--font-heading);
  font-size: 0.875rem;
  font-weight: 500;
  color: var(--muted-fg);
  transition: color 0.15s;
}
.milestone-card:hover .read-more { color: var(--fg); }

/* ─── Discussion end card ─── */
.discussion-end-card {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  width: 13rem;
  height: 100%;
  border-radius: 0.75rem;
  border: 1px dashed var(--border);
  padding: 1.5rem;
  cursor: pointer;
  transition: border-color 0.15s, background-color 0.15s;
  background: none;
  margin-left: 1rem;
  margin-right: 2rem;
}
.discussion-end-card:hover {
  border-color: rgba(0,0,0,0.3);
  background: var(--card);
}
.discussion-end-card span {
  font-family: var(--font-heading);
  font-size: 1rem;
  font-weight: 500;
  color: var(--muted-fg);
  transition: color 0.15s;
}
.discussion-end-card:hover span { color: var(--fg); }

/* ─── Footer ─── */
.footer {
  flex-shrink: 0;
  border-top: 1px solid var(--border);
  padding: 0.75rem 1.5rem;
}
@media (min-width: 768px) {
  .footer { padding: 0.75rem 2.5rem; }
}
.footer-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 0.5rem;
  flex-wrap: wrap;
}
.footer p {
  font-family: var(--font-heading);
  font-size: 0.875rem;
  color: var(--muted-fg);
}
.footer a {
  color: var(--muted-fg);
  transition: color 0.15s;
}
.footer a:hover { color: var(--fg); }
.footer .nav-hint-desktop { display: none; }
.footer .nav-hint-mobile { display: block; }
@media (min-width: 640px) {
  .footer .nav-hint-desktop { display: block; }
  .footer .nav-hint-mobile { display: none; }
}

/* ─── Modal ─── */
.modal-overlay {
  display: none;
  position: fixed;
  inset: 0;
  z-index: 50;
  align-items: center;
  justify-content: center;
  padding: 1rem;
}
.modal-overlay.active { display: flex; }

.modal-backdrop {
  position: absolute;
  inset: 0;
  background: rgba(255,255,255,0.8);
  backdrop-filter: blur(4px);
  -webkit-backdrop-filter: blur(4px);
}

.modal-dialog {
  position: relative;
  width: 100%;
  max-width: 42rem;
  max-height: 90vh;
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 0.75rem;
  box-shadow: 0 4px 24px rgba(0,0,0,0.1);
  overflow: hidden;
  display: flex;
  flex-direction: column;
  animation: modalIn 0.25s ease;
}
@keyframes modalIn {
  from { opacity: 0; transform: translateY(20px) scale(0.97); }
  to { opacity: 1; transform: translateY(0) scale(1); }
}

.modal-header {
  flex-shrink: 0;
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  padding: 1.25rem;
  padding-bottom: 0.75rem;
  border-bottom: 1px solid var(--border);
}
@media (min-width: 768px) {
  .modal-header { padding: 1.5rem; padding-bottom: 1rem; }
}
.modal-header-left { flex: 1; padding-right: 1rem; }
.modal-meta {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  flex-wrap: wrap;
  margin-bottom: 0.5rem;
}
.modal-year-badge {
  font-family: var(--font-heading);
  font-size: 0.875rem;
  font-weight: 700;
  padding: 0.25rem 0.625rem;
  border-radius: 0.375rem;
  color: white;
}
.modal-era-name {
  font-family: var(--font-heading);
  font-size: 0.875rem;
  color: var(--muted-fg);
}
.modal-title {
  font-family: var(--font-body);
  font-size: 1.125rem;
  font-weight: 600;
  color: var(--fg);
  line-height: 1.4;
}
@media (min-width: 768px) {
  .modal-title { font-size: 1.25rem; }
}
.modal-close {
  flex-shrink: 0;
  width: 2rem;
  height: 2rem;
  border-radius: 0.375rem;
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--muted-fg);
  background: none;
  border: none;
  cursor: pointer;
  font-size: 1.25rem;
  transition: color 0.15s, background-color 0.15s;
}
.modal-close:hover {
  color: var(--fg);
  background: var(--bg);
}

.modal-body {
  flex: 1;
  overflow-y: auto;
  padding: 1.25rem;
}
@media (min-width: 768px) {
  .modal-body { padding: 1.5rem; }
}

/* ─── Content rendering ─── */
.content p {
  font-family: var(--font-body);
  font-size: 1rem;
  line-height: 1.8;
  margin-bottom: 1.25rem;
}
.content strong {
  font-weight: 600;
  color: var(--fg);
}
.content pre {
  background: var(--muted);
  border: 1px solid var(--border);
  border-radius: 0.5rem;
  padding: 1rem;
  overflow-x: auto;
  font-family: var(--font-mono);
  font-size: 0.875rem;
  line-height: 1.5;
  margin: 1rem 0;
}
.content code {
  font-family: var(--font-mono);
  font-size: 0.875rem;
}
.content code.inline-code {
  background: var(--muted);
  padding: 0.125rem 0.375rem;
  border-radius: 0.25rem;
  font-size: 0.875em;
}
.content blockquote {
  border-left: 2px solid var(--border);
  padding-left: 1rem;
  margin: 1rem 0;
  font-style: italic;
  color: var(--muted-fg);
}
.content blockquote p {
  font-family: var(--font-body);
  font-size: 1rem;
  line-height: 1.8;
  margin-bottom: 0.25rem;
}
.content blockquote .attribution {
  font-style: normal;
  font-size: 0.875rem;
  margin-top: 0.5rem;
}
.content .ref-header {
  margin-top: 1.5rem;
  padding-top: 1rem;
  border-top: 1px solid var(--border);
}
.content .ref-header h3 {
  font-family: var(--font-heading);
  font-size: 0.875rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  margin-bottom: 0.75rem;
  color: var(--fg);
}
.content .ref-entry {
  font-family: var(--font-body);
  font-size: 0.875rem;
  line-height: 1.6;
  margin-bottom: 0.5rem;
  color: var(--muted-fg);
  padding-left: 1rem;
  text-indent: -1rem;
}
.citation-tag {
  display: inline;
  font-family: var(--font-mono);
  font-size: 0.75rem;
  color: var(--muted-fg);
  background: var(--muted);
  padding: 0.125rem 0.25rem;
  border-radius: 0.25rem;
  margin: 0 0.125rem;
}
.ref-link {
  color: var(--blue);
}
.ref-link:hover {
  color: var(--blue-hover);
}
.content .rule-line {
  padding-left: 1rem;
  border-left: 2px solid var(--border);
  margin-bottom: 0.5rem;
}
.content .example-block {
  background: var(--muted);
  border: 1px solid var(--border);
  border-radius: 0.5rem;
  padding: 1rem;
  margin: 1rem 0;
  font-family: var(--font-mono);
  font-size: 0.875rem;
  line-height: 1.5;
}
.content .katex-display {
  margin: 1rem 0;
  overflow-x: auto;
  padding: 0.5rem 0;
}
.content a {
  text-decoration: none;
}

/* ─── Discussion topics accordion ─── */
.discussion-section .topic-group {
  border: 1px solid var(--border);
  border-radius: 0.5rem;
  margin-bottom: 0.75rem;
  overflow: hidden;
}
.discussion-section .topic-header {
  width: 100%;
  text-align: left;
  padding: 0.875rem 1.25rem;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 0.75rem;
  background: none;
  border: none;
  cursor: pointer;
  transition: background-color 0.15s;
  font-family: var(--font-heading);
  font-size: 1rem;
  font-weight: 500;
  color: var(--fg);
  line-height: 1.4;
}
.discussion-section .topic-header:hover {
  background: rgba(0,0,0,0.02);
}
.discussion-section .topic-toggle {
  font-size: 0.875rem;
  color: var(--muted-fg);
  flex-shrink: 0;
}
.discussion-section .topic-body {
  display: none;
  padding: 0 1.25rem 1.25rem;
}
.discussion-section .topic-body.expanded {
  display: block;
}