:root {
  color-scheme: dark;
  --bg: #090b0e;
  --panel: #11151a;
  --panel-raised: #171c22;
  --line: #29313a;
  --text: #f2f5f7;
  --muted: #aeb9c2;
  --cyan: #55d9e8;
  --lime: #b7ed62;
  --amber: #f4c95d;
  --danger: #ff6b78;
  font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
}

* { box-sizing: border-box; }

body {
  min-width: 320px;
  margin: 0;
  color: var(--text);
  background:
    radial-gradient(circle at 12% 6%, rgb(85 217 232 / 8%), transparent 28rem),
    radial-gradient(circle at 89% 48%, rgb(244 201 93 / 6%), transparent 30rem),
    var(--bg);
}

button, input { font: inherit; }
button { cursor: pointer; }
button:disabled { cursor: not-allowed; opacity: .5; }
h1, h2, p { margin-top: 0; }

.site-header {
  height: 68px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 clamp(20px, 5vw, 72px);
  border-bottom: 1px solid var(--line);
  background: rgb(9 11 14 / 86%);
  backdrop-filter: blur(16px);
}

.brand {
  display: flex;
  align-items: center;
  gap: 11px;
  color: var(--text);
  font-weight: 800;
  letter-spacing: -.02em;
  text-decoration: none;
}

.brand-mark {
  width: 34px;
  height: 34px;
  display: grid;
  place-items: center;
  border: 1px solid #47606b;
  border-radius: 10px;
  color: var(--cyan);
  background: #10191d;
  font-size: 12px;
}

.lesson-chip {
  display: flex;
  align-items: center;
  gap: 9px;
  color: var(--muted);
  font-size: 12px;
  letter-spacing: .1em;
  text-transform: uppercase;
}

.lesson-chip span {
  padding: 6px 9px;
  border-radius: 999px;
  color: #172205;
  background: var(--lime);
  font-weight: 900;
}

main {
  width: min(1320px, calc(100% - 40px));
  margin: 0 auto;
  padding: 68px 0 78px;
}

.eyebrow {
  margin: 0 0 10px;
  color: var(--cyan);
  font-size: 12px;
  font-weight: 900;
  letter-spacing: .14em;
  text-transform: uppercase;
}

.intro {
  max-width: 980px;
  margin-bottom: 44px;
}

.intro h1 {
  max-width: 900px;
  margin-bottom: 18px;
  font-size: clamp(42px, 6vw, 76px);
  line-height: 1;
  letter-spacing: -.055em;
}

.intro-copy {
  max-width: 800px;
  margin-bottom: 22px;
  color: var(--text);
  font-size: 18px;
  line-height: 1.72;
}

.intro-copy code,
.request-trace code {
  display: inline-block;
  padding: 2px 8px;
  border: 1px solid color-mix(in srgb, var(--lime) 45%, transparent);
  border-radius: 6px;
  color: var(--lime);
  background: color-mix(in srgb, var(--lime) 11%, transparent);
  font-weight: 700;
}

.concept-tags {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
}

.concept-tags span {
  padding: 7px 11px;
  border: 1px solid var(--line);
  border-radius: 999px;
  color: #d5dde2;
  background: var(--panel);
  font-size: 12px;
  font-weight: 700;
}

.lesson-workspace,
.observation,
.architecture-explanation,
.next-lesson {
  border: 1px solid var(--line);
  border-radius: 18px;
  background: color-mix(in srgb, var(--panel) 96%, var(--cyan));
  box-shadow: 0 24px 70px rgb(0 0 0 / 20%);
}

.lesson-workspace { padding: clamp(22px, 3vw, 38px); }

.workspace-heading {
  display: flex;
  align-items: end;
  justify-content: space-between;
  gap: 32px;
  margin-bottom: 30px;
  padding-bottom: 24px;
  border-bottom: 1px solid var(--line);
}

.workspace-heading h2,
.observation-heading h2,
.architecture-explanation h2,
.next-lesson h2 {
  margin-bottom: 8px;
  font-size: clamp(25px, 3vw, 34px);
  letter-spacing: -.035em;
}

.workspace-heading > div:first-child > p:last-child,
.observation-heading > p:last-child,
.next-lesson p {
  max-width: 680px;
  margin-bottom: 0;
  color: var(--muted);
  font-size: 15px;
  line-height: 1.65;
}

.lesson-controls {
  display: flex;
  flex: 0 0 auto;
  gap: 9px;
}

.primary-action,
.secondary-action {
  min-height: 46px;
  border-radius: 11px;
  font-weight: 850;
}

.primary-action {
  border: 0;
  color: #172205;
  background: var(--lime);
}

.primary-action.start { min-width: 142px; padding: 0 20px; }
.primary-action.request-action { width: 100%; margin: 14px 0 0; }
.primary-action span { margin-right: 8px; font-size: 11px; }
.primary-action.sending { animation: buttonPulse .7s ease-in-out infinite alternate; }
.secondary-action { padding: 0 18px; border: 1px solid var(--line); color: var(--text); background: transparent; }

.lab {
  display: grid;
  grid-template-columns: minmax(340px, 1fr) minmax(130px, .3fr) minmax(340px, 1fr);
  align-items: center;
  gap: 0;
}

.node-card {
  min-width: 0;
  min-height: 480px;
  padding: 24px;
  border: 1px solid var(--line);
  border-radius: 15px;
  background: linear-gradient(145deg, rgb(23 28 34 / 98%), rgb(13 16 20 / 98%));
  box-shadow: 0 18px 55px rgb(0 0 0 / 22%);
}

.client-card,
.service-card { display: flex; flex-direction: column; }
.service-card { border-color: #4c4634; }

.endpoint-box,
.server-identity {
  display: flex;
  flex-direction: column;
  gap: 8px;
  padding: 14px;
  border: 1px solid var(--line);
  border-radius: 11px;
  background: #0a0d10;
}

.endpoint-box span,
.server-identity span {
  color: var(--muted);
  font-size: 11px;
  font-weight: 800;
  letter-spacing: .08em;
  text-transform: uppercase;
}

.endpoint-box code,
.server-identity code {
  overflow: auto;
  color: var(--cyan);
  font-size: 12px;
}

.client-response { margin-top: 18px; }
.products { height: 198px; overflow: auto; padding: 9px; border: 1px solid var(--line); border-radius: 11px; background: #0a0d10; }
.empty-copy { margin: 0; padding: 24px 12px; color: var(--muted); font-size: 13px; line-height: 1.55; text-align: center; }
.product { display: flex; align-items: center; justify-content: space-between; gap: 18px; padding: 13px 11px; border-bottom: 1px solid #20272e; }
.product:last-child { border-bottom: 0; }
.product div { min-width: 0; display: flex; flex-direction: column; gap: 4px; }
.product b { font-size: 13px; }
.product span { overflow: hidden; color: var(--muted); font-size: 12px; text-overflow: ellipsis; white-space: nowrap; }
.product strong { color: var(--lime); font: 800 12px ui-monospace, Consolas, monospace; }

.connection {
  min-width: 0;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  color: var(--muted);
}

.connection-label,
.address-label { font: 800 11px ui-monospace, Consolas, monospace; text-transform: uppercase; }
.connection-label { margin-bottom: 11px; color: var(--cyan); }
.address-label { margin-top: 10px; color: #d4dce1; }
.connection p { margin: 8px 0 0; font-size: 11px; letter-spacing: .1em; text-transform: uppercase; }
.wire { position: relative; width: 100%; height: 2px; overflow: visible; background: #34444b; }
.wire::after { position: absolute; top: -4px; right: 0; width: 8px; height: 8px; border-top: 2px solid var(--cyan); border-right: 2px solid var(--cyan); content: ""; transform: rotate(45deg); }
.packet { position: absolute; top: -4px; left: -10px; width: 10px; height: 10px; border-radius: 50%; background: var(--lime); box-shadow: 0 0 15px var(--lime); opacity: 0; }
.flowing .packet { animation: packetFlow 1.1s ease-in-out; }
.flowing .packet-two { animation-delay: .22s; }

.server-identity { margin-bottom: 18px; }
.request-log { margin-top: auto; }
.section-heading { min-height: 24px; display: flex; align-items: center; justify-content: space-between; gap: 12px; margin-bottom: 7px; color: var(--muted); font-size: 10px; font-weight: 850; letter-spacing: .08em; text-transform: uppercase; }
.request-log button { border: 0; color: var(--cyan); background: transparent; font-size: 11px; text-transform: uppercase; }
.request-log pre { height: 150px; margin: 0; overflow: auto; padding: 12px; border: 1px solid var(--line); border-radius: 10px; color: #d2d9de; background: #090b0d; font: 12px/1.6 ui-monospace, SFMono-Regular, Consolas, monospace; white-space: pre; }

.observation {
  display: grid;
  grid-template-columns: minmax(250px, .58fr) minmax(0, 1.42fr);
  gap: clamp(30px, 5vw, 72px);
  margin-top: 26px;
  padding: clamp(24px, 3.5vw, 42px);
}

.request-trace {
  display: grid;
  gap: 0;
  margin: 0;
  padding: 0;
  list-style: none;
}

.request-trace li {
  position: relative;
  display: grid;
  grid-template-columns: 34px 1fr;
  gap: 13px;
  min-height: 64px;
  color: var(--muted);
}

.request-trace li:not(:last-child)::after {
  position: absolute;
  top: 31px;
  bottom: 0;
  left: 16px;
  width: 1px;
  background: var(--line);
  content: "";
}

.request-trace li > span {
  z-index: 1;
  width: 34px;
  height: 34px;
  display: grid;
  place-items: center;
  border: 1px solid var(--line);
  border-radius: 50%;
  color: var(--muted);
  background: var(--panel-raised);
  font: 800 12px ui-monospace, Consolas, monospace;
  transition: border-color .2s ease, color .2s ease, background .2s ease, box-shadow .2s ease;
}

.request-trace li div { padding-top: 2px; }
.request-trace strong { display: block; color: #dde4e8; font-size: 14px; }
.request-trace small { display: block; margin-top: 4px; color: var(--muted); font-size: 12px; line-height: 1.5; }
.request-trace code { padding: 1px 5px; font-size: 11px; }
.request-trace li.active > span { border-color: var(--cyan); color: var(--cyan); background: rgb(85 217 232 / 10%); box-shadow: 0 0 18px rgb(85 217 232 / 24%); }
.request-trace li.complete > span { border-color: var(--lime); color: #172205; background: var(--lime); }
.request-trace li.complete:not(:last-child)::after { background: color-mix(in srgb, var(--lime) 65%, var(--line)); }
.request-trace li.complete strong { color: var(--text); }

.architecture-explanation { margin-top: 26px; padding: clamp(26px, 4vw, 50px); }
.explanation-copy { max-width: 1000px; }
.explanation-copy > p:not(.eyebrow) { margin-bottom: 16px; color: var(--text); font-size: 17px; line-height: 1.82; }
.responsibility-list { display: flex; flex-wrap: wrap; gap: 9px; margin: 5px 0 22px; padding: 0; list-style: none; }
.responsibility-list li { position: relative; border: 1px solid color-mix(in srgb, var(--amber) 30%, var(--line)); border-radius: 8px; color: #f0e2ba; background: color-mix(in srgb, var(--amber) 8%, transparent); font-size: 13px; }

.tradeoffs {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 16px;
  margin-top: 34px;
  padding-top: 30px;
  border-top: 1px solid var(--line);
}

.tradeoff-card { padding: 24px; border: 1px solid var(--line); border-radius: 13px; background: var(--panel-raised); }
.tradeoff-card.gives { border-top: 3px solid var(--lime); }
.tradeoff-card.problems { border-top: 3px solid var(--danger); }
.tradeoff-label { margin-bottom: 16px; color: var(--text); font-size: 16px; font-weight: 800; }
.tradeoff-card ul { display: grid; gap: 10px; margin: 0; padding: 0; list-style: none; }
.tradeoff-card li { position: relative; padding-left: 18px; color: #d4dde2; font-size: 14px; line-height: 1.55; }
.tradeoff-card li::before { position: absolute; top: .65em; left: 1px; width: 6px; height: 6px; border-radius: 50%; background: var(--lime); content: ""; }
.tradeoff-card.problems li::before { background: var(--danger); }

.next-lesson {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 40px;
  margin-top: 26px;
  padding: clamp(26px, 4vw, 42px);
  border-color: color-mix(in srgb, var(--cyan) 38%, var(--line));
}

.next-lesson a {
  min-height: 50px;
  display: inline-flex;
  flex: 0 0 auto;
  align-items: center;
  gap: 12px;
  padding: 0 18px;
  border: 1px solid color-mix(in srgb, var(--cyan) 45%, var(--line));
  border-radius: 11px;
  color: var(--text);
  background: rgb(85 217 232 / 9%);
  font-size: 13px;
  font-weight: 800;
  text-decoration: none;
}

.next-lesson a:hover { background: rgb(85 217 232 / 15%); }
.next-lesson a span { color: var(--cyan); font-size: 20px; }

.toast-region { position: fixed; right: 22px; bottom: 22px; z-index: 10; display: grid; gap: 8px; }
.toast { max-width: 360px; padding: 12px 15px; border: 1px solid rgb(183 237 98 / 40%); border-radius: 10px; color: var(--text); background: #182111; box-shadow: 0 14px 40px #0008; font-size: 12px; }
.toast.error { border-color: rgb(255 107 120 / 45%); background: #281318; }

html[data-theme="light"] .lesson-workspace,
html[data-theme="light"] .observation,
html[data-theme="light"] .architecture-explanation,
html[data-theme="light"] .next-lesson,
html[data-theme="light"] .tradeoff-card {
  color: var(--text);
  background: #fff;
  box-shadow: 0 18px 48px rgb(25 42 50 / 10%);
}

html[data-theme="light"] .concept-tags span,
html[data-theme="light"] .endpoint-box,
html[data-theme="light"] .server-identity,
html[data-theme="light"] .products,
html[data-theme="light"] .request-log pre,
html[data-theme="light"] .request-trace li > span {
  border-color: var(--line);
  color: var(--text);
  background: #f6f9fa;
}

html[data-theme="light"] .concept-tags span,
html[data-theme="light"] .tradeoff-card li,
html[data-theme="light"] .request-trace strong { color: var(--text); }
html[data-theme="light"] .lesson-response-summary b { color: #29434d; }
html[data-theme="light"] .responsibility-list li { color: #644a0d; }
html[data-theme="light"] .request-trace li.complete > span { color: #172205; background: var(--lime); }
html[data-theme="light"] .primary-action { color: #172205; background: #a9d85c; }
html[data-theme="light"] .secondary-action { color: var(--text); background: var(--panel-raised); }
html[data-theme="light"] .next-lesson a { color: var(--text); background: rgb(8 127 147 / 8%); }

@keyframes packetFlow { 0% { left: -10px; opacity: 0; } 12% { opacity: 1; } 88% { opacity: 1; } 100% { left: calc(100% - 4px); opacity: 0; } }
@keyframes buttonPulse { from { filter: brightness(.86); } to { filter: brightness(1.12); } }
@keyframes packetFlowVertical { 0% { top: -8px; opacity: 0; } 12% { opacity: 1; } 88% { opacity: 1; } 100% { top: calc(100% - 2px); opacity: 0; } }

@media (max-width: 960px) {
  .workspace-heading,
  .next-lesson { align-items: flex-start; flex-direction: column; }
  .lab { grid-template-columns: 1fr; gap: 18px; }
  .node-card { min-height: 0; }
  .connection { min-height: 98px; }
  .wire { width: 2px; height: 48px; }
  .wire::after { top: auto; right: -3px; bottom: 0; transform: rotate(135deg); }
  .packet { top: -8px; left: -4px; }
  .flowing .packet { animation-name: packetFlowVertical; }
  .observation { grid-template-columns: 1fr; }
}

@media (max-width: 640px) {
  .site-header { padding-inline: 16px; }
  .lesson-chip { font-size: 0; }
  .lesson-chip span { font-size: 10px; }
  main { width: min(100% - 24px, 1320px); padding-top: 42px; }
  .intro h1 { font-size: 42px; }
  .intro-copy { font-size: 16px; }
  .lesson-controls { width: 100%; }
  .lesson-controls button { flex: 1; }
  .node-card { padding: 18px; }
  .tradeoffs { grid-template-columns: 1fr; }
  .next-lesson a { width: 100%; justify-content: space-between; }
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { scroll-behavior: auto !important; animation-duration: .01ms !important; animation-iteration-count: 1 !important; transition-duration: .01ms !important; }
}