:root {
  --ink: #17324D;
  --ink-soft: #2B4660;
  --blue: #4E74F9;
  --coral: #FF6B5E;
  --mint: #34B89A;
  --mist: #EEF5FF;
  --paper: #FFFFFF;
  --line: #D6E2F2;
  --muted: #657B91;
  --shadow: 0 24px 70px rgba(23, 50, 77, 0.13);
}

* {
  box-sizing: border-box;
}

html {
  color: var(--ink);
  background: var(--mist);
  scroll-behavior: smooth;
}

body {
  min-width: 320px;
  margin: 0;
  font-family: "Avenir Next", "PingFang SC", "Hiragino Sans GB", system-ui, sans-serif;
  background:
    radial-gradient(circle at 82% 10%, rgba(255, 107, 94, 0.11), transparent 25rem),
    radial-gradient(circle at 3% 28%, rgba(78, 116, 249, 0.12), transparent 30rem),
    linear-gradient(180deg, #FBFDFF 0, var(--mist) 50rem);
}

a {
  color: inherit;
}

.skip-link {
  position: fixed;
  z-index: 20;
  top: 12px;
  left: 12px;
  padding: 10px 14px;
  color: var(--paper);
  background: var(--ink);
  border-radius: 8px;
  transform: translateY(-160%);
}

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

.page-shell {
  width: min(100% - 40px, 1120px);
  margin: 0 auto;
  padding: 24px 0 36px;
}

.masthead {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 24px;
  min-height: 62px;
  padding-bottom: 18px;
  border-bottom: 1px solid var(--line);
}

.brand {
  display: inline-flex;
  align-items: center;
  gap: 13px;
  font-size: 13px;
  font-weight: 800;
  letter-spacing: 0.08em;
  text-decoration: none;
  text-transform: uppercase;
}

.brand b {
  display: inline-grid;
  width: 22px;
  height: 22px;
  place-items: center;
  margin-left: 3px;
  color: var(--paper);
  background: var(--blue);
  border-radius: 50%;
}

.brand-mark {
  display: flex;
  align-items: center;
  gap: 3px;
  height: 24px;
}

.brand-mark i {
  width: 3px;
  height: 10px;
  background: var(--blue);
  border-radius: 10px;
}

.brand-mark i:nth-child(2) { height: 22px; background: var(--coral); }
.brand-mark i:nth-child(3) { height: 16px; }
.brand-mark i:nth-child(4) { height: 7px; background: var(--mint); }

.audio-badge {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  color: var(--muted);
  font-size: 13px;
  font-weight: 700;
}

.audio-badge i {
  width: 9px;
  height: 9px;
  background: var(--coral);
  border-radius: 50%;
  box-shadow: 0 0 0 5px rgba(255, 107, 94, 0.14);
}

.hero {
  display: grid;
  grid-template-columns: minmax(0, 1fr) 330px;
  align-items: center;
  gap: clamp(56px, 10vw, 130px);
  min-height: 600px;
  padding: 72px 5% 76px;
}

.eyebrow {
  margin: 0 0 14px;
  color: var(--blue);
  font: 800 12px/1.2 "SFMono-Regular", Menlo, monospace;
  letter-spacing: 0.14em;
  text-transform: uppercase;
}

h1,
h2 {
  font-family: "Avenir Next Rounded", "Arial Rounded MT Bold", "Trebuchet MS", sans-serif;
}

h1 {
  margin: 0;
  font-size: clamp(58px, 8vw, 104px);
  font-weight: 800;
  line-height: 0.83;
  letter-spacing: -0.065em;
}

h1 span {
  color: var(--coral);
  font-size: 0.63em;
  letter-spacing: -0.045em;
}

.hero-intro {
  margin: 28px 0 32px;
  color: var(--ink-soft);
  font-size: clamp(17px, 2vw, 20px);
}

.course-facts {
  display: flex;
  gap: 0;
  margin: 0;
}

.course-facts div {
  min-width: 92px;
  padding: 0 24px;
  border-left: 1px solid var(--line);
}

.course-facts div:first-child {
  padding-left: 0;
  border-left: 0;
}

.course-facts dt {
  font: 800 22px/1 "SFMono-Regular", Menlo, monospace;
}

.course-facts dd {
  margin: 7px 0 0;
  color: var(--muted);
  font-size: 11px;
  font-weight: 800;
  letter-spacing: 0.12em;
  text-transform: uppercase;
}

.signal-card {
  position: relative;
  min-height: 390px;
  overflow: hidden;
  padding: 30px;
  color: var(--paper);
  background: var(--ink);
  border-radius: 42% 42% 34px 34px;
  box-shadow: var(--shadow);
  transform: rotate(2.5deg);
}

.signal-card::before {
  position: absolute;
  top: 18px;
  right: 18px;
  width: 18px;
  height: 18px;
  content: "";
  background: var(--coral);
  border: 6px solid rgba(255, 255, 255, 0.16);
  border-radius: 50%;
}

.signal-number {
  color: rgba(255, 255, 255, 0.08);
  font: 900 180px/0.9 "Avenir Next", sans-serif;
  letter-spacing: -0.1em;
  transform: translateX(-17px);
}

.signal-wave {
  position: absolute;
  right: 30px;
  bottom: 78px;
  left: 30px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: 80px;
}

.signal-wave i {
  width: 7px;
  height: 24%;
  background: var(--mint);
  border-radius: 99px;
  animation: signal 1.5s ease-in-out infinite alternate;
}

.signal-wave i:nth-child(2), .signal-wave i:nth-child(8) { height: 52%; animation-delay: -0.8s; }
.signal-wave i:nth-child(3), .signal-wave i:nth-child(7) { height: 82%; animation-delay: -1.2s; }
.signal-wave i:nth-child(4), .signal-wave i:nth-child(6) { height: 42%; animation-delay: -0.4s; }
.signal-wave i:nth-child(5) { height: 100%; animation-delay: -1s; background: var(--coral); }

.signal-card p {
  position: absolute;
  right: 30px;
  bottom: 24px;
  left: 30px;
  margin: 0;
  color: rgba(255, 255, 255, 0.68);
  font: 700 11px/1.55 "SFMono-Regular", Menlo, monospace;
  letter-spacing: 0.12em;
  text-align: right;
  text-transform: uppercase;
}

.unit-section {
  padding: 74px 5% 90px;
  background: rgba(255, 255, 255, 0.56);
  border: 1px solid rgba(214, 226, 242, 0.9);
  border-radius: 36px;
  box-shadow: 0 28px 80px rgba(23, 50, 77, 0.07);
  backdrop-filter: blur(14px);
}

.section-heading {
  display: grid;
  grid-template-columns: 1fr 320px;
  align-items: end;
  gap: 40px;
  margin-bottom: 34px;
}

.section-heading h2 {
  margin: 0;
  font-size: clamp(46px, 7vw, 78px);
  line-height: 0.9;
  letter-spacing: -0.055em;
}

.section-heading > p {
  margin: 0;
  color: var(--muted);
  font-size: 15px;
  line-height: 1.65;
}

.cassette {
  position: relative;
  overflow: hidden;
  padding: 28px 34px 34px;
  color: var(--paper);
  background:
    linear-gradient(145deg, rgba(255,255,255,0.055), transparent 22%),
    var(--ink);
  border: 1px solid rgba(255, 255, 255, 0.16);
  border-radius: 30px;
  box-shadow: 0 28px 62px rgba(23, 50, 77, 0.2);
}

.cassette-label {
  display: flex;
  justify-content: space-between;
  padding: 0 18px 20px;
  color: rgba(255, 255, 255, 0.58);
  font: 700 10px/1 "SFMono-Regular", Menlo, monospace;
  letter-spacing: 0.14em;
  text-transform: uppercase;
}

.reel-deck {
  position: relative;
  display: grid;
  grid-template-columns: 96px 1fr 74px 1fr 96px;
  align-items: center;
  width: min(100% - 70px, 660px);
  margin: 0 auto 26px;
  padding: 16px 22px;
  background: rgba(255, 255, 255, 0.08);
  border: 1px solid rgba(255, 255, 255, 0.14);
  border-radius: 80px;
}

.reel {
  position: relative;
  z-index: 2;
  width: 76px;
  height: 76px;
  display: block;
  background:
    radial-gradient(circle, var(--ink) 0 14%, transparent 15%),
    conic-gradient(from 10deg, var(--paper) 0 9%, transparent 9% 24%, var(--paper) 24% 33%, transparent 33% 49%, var(--paper) 49% 58%, transparent 58% 74%, var(--paper) 74% 83%, transparent 83%);
  border: 8px solid rgba(255, 255, 255, 0.16);
  border-radius: 50%;
  animation: reel-turn 12s linear infinite;
}

.reel i {
  position: absolute;
  inset: 20px;
  border: 3px solid var(--coral);
  border-radius: 50%;
}

.tape-line {
  grid-column: 1 / -1;
  grid-row: 1;
  height: 2px;
  background: rgba(255, 255, 255, 0.26);
}

.reel-deck strong {
  grid-column: 3;
  grid-row: 1;
  color: var(--paper);
  font: 900 15px/1.05 "SFMono-Regular", Menlo, monospace;
  letter-spacing: 0.08em;
  text-align: center;
}

.reel-left { grid-column: 1; grid-row: 1; }
.reel-right { grid-column: 5; grid-row: 1; }

.part-list {
  display: grid;
  gap: 12px;
}

.part-link {
  position: relative;
  display: grid;
  grid-template-columns: 76px 1fr auto;
  align-items: center;
  gap: 24px;
  min-height: 130px;
  padding: 20px 24px;
  color: var(--ink);
  background: var(--paper);
  border: 2px solid transparent;
  border-radius: 19px;
  text-decoration: none;
  transition: transform 180ms ease, border-color 180ms ease, box-shadow 180ms ease;
}

.part-link:hover {
  transform: translateX(7px);
  box-shadow: 0 14px 30px rgba(4, 18, 33, 0.16);
}

.part-link:focus-visible {
  outline: 4px solid var(--mint);
  outline-offset: 4px;
}

.part-a:hover { border-color: var(--blue); }
.part-b:hover { border-color: var(--coral); }

.part-letter {
  width: 68px;
  height: 68px;
  display: grid;
  place-items: center;
  color: var(--paper);
  background: var(--blue);
  border-radius: 50%;
  font: 900 32px/1 "Avenir Next Rounded", "Trebuchet MS", sans-serif;
  box-shadow: inset 0 0 0 8px rgba(255, 255, 255, 0.16);
}

.part-b .part-letter {
  background: var(--coral);
}

.part-copy {
  display: grid;
  gap: 6px;
}

.part-meta {
  display: flex;
  gap: 10px;
  color: var(--blue);
  font: 800 11px/1.2 "SFMono-Regular", Menlo, monospace;
  letter-spacing: 0.12em;
  text-transform: uppercase;
}

.part-b .part-meta {
  color: #D64E43;
}

.part-meta b {
  color: var(--muted);
  font-weight: 700;
}

.part-copy > strong {
  font-size: clamp(24px, 3vw, 32px);
  line-height: 1.1;
}

.part-copy small {
  color: var(--muted);
  font-size: 14px;
}

.part-action {
  display: flex;
  align-items: center;
  gap: 16px;
  font-size: 14px;
  font-weight: 800;
  white-space: nowrap;
}

.part-action i {
  width: 42px;
  height: 42px;
  display: grid;
  place-items: center;
  color: var(--paper);
  background: var(--ink);
  border-radius: 50%;
  font: 400 22px/1 system-ui, sans-serif;
  transition: transform 180ms ease;
}

.part-link:hover .part-action i {
  transform: translateX(3px);
}

.screw {
  position: absolute;
  width: 8px;
  height: 8px;
  background: rgba(255, 255, 255, 0.28);
  border-radius: 50%;
}

.screw-one { top: 16px; left: 16px; }
.screw-two { top: 16px; right: 16px; }
.screw-three { bottom: 16px; left: 16px; }
.screw-four { right: 16px; bottom: 16px; }

footer {
  display: flex;
  justify-content: space-between;
  gap: 24px;
  padding: 30px 5% 0;
  color: var(--muted);
  font-size: 12px;
  font-weight: 700;
}

footer p {
  margin: 0;
}

footer span {
  margin-right: 7px;
  color: var(--mint);
}

@keyframes reel-turn {
  to { transform: rotate(360deg); }
}

@keyframes signal {
  from { transform: scaleY(0.55); }
  to { transform: scaleY(1); }
}

@media (max-width: 820px) {
  .hero {
    grid-template-columns: 1fr 220px;
    gap: 42px;
    min-height: 520px;
    padding-inline: 2%;
  }

  .signal-card {
    min-height: 300px;
    border-radius: 40% 40% 28px 28px;
  }

  .signal-number {
    font-size: 125px;
  }

  .unit-section {
    padding-inline: 30px;
  }

  .section-heading {
    grid-template-columns: 1fr;
    gap: 18px;
  }

  .section-heading > p {
    max-width: 520px;
  }
}

@media (max-width: 640px) {
  .page-shell {
    width: min(100% - 20px, 1120px);
    padding-top: 12px;
  }

  .masthead {
    gap: 12px;
    min-height: 54px;
  }

  .brand {
    max-width: 210px;
    font-size: 10px;
  }

  .audio-badge {
    flex-shrink: 0;
    font-size: 11px;
  }

  .hero {
    grid-template-columns: 1fr;
    gap: 34px;
    min-height: auto;
    padding: 58px 10px 62px;
  }

  h1 {
    font-size: clamp(56px, 19vw, 78px);
  }

  .course-facts div {
    min-width: 0;
    flex: 1;
    padding-inline: 16px;
  }

  .signal-card {
    width: min(100%, 330px);
    min-height: 190px;
    justify-self: end;
    border-radius: 95px 95px 26px 26px;
    transform: rotate(1.5deg);
  }

  .signal-number {
    font-size: 88px;
  }

  .signal-wave {
    right: 22px;
    bottom: 52px;
    left: 150px;
    height: 50px;
  }

  .signal-card p {
    right: 22px;
    bottom: 18px;
    left: auto;
  }

  .unit-section {
    padding: 48px 12px 12px;
    border-radius: 26px;
  }

  .section-heading {
    padding: 0 8px;
  }

  .cassette {
    padding: 24px 10px 10px;
    border-radius: 22px;
  }

  .cassette-label {
    padding-inline: 14px;
  }

  .reel-deck {
    grid-template-columns: 62px 1fr 50px 1fr 62px;
    width: calc(100% - 38px);
    padding: 10px 14px;
  }

  .reel {
    width: 54px;
    height: 54px;
    border-width: 6px;
  }

  .reel i {
    inset: 13px;
  }

  .reel-deck strong {
    font-size: 10px;
  }

  .part-link {
    grid-template-columns: 54px 1fr;
    gap: 14px;
    min-height: 148px;
    padding: 18px 14px;
  }

  .part-letter {
    width: 52px;
    height: 52px;
    font-size: 25px;
    box-shadow: inset 0 0 0 6px rgba(255, 255, 255, 0.16);
  }

  .part-copy small {
    font-size: 12px;
    line-height: 1.45;
  }

  .part-action {
    grid-column: 2;
    justify-content: space-between;
    font-size: 13px;
  }

  .part-action i {
    width: 36px;
    height: 36px;
  }

  footer {
    flex-direction: column;
    gap: 8px;
    padding-bottom: 8px;
    text-align: center;
  }
}

@media (prefers-reduced-motion: reduce) {
  html {
    scroll-behavior: auto;
  }

  *,
  *::before,
  *::after {
    scroll-behavior: auto !important;
    animation-duration: 0.001ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.001ms !important;
  }
}