:root {
  color-scheme: dark;
  font-family: "PingFang SC", "Microsoft YaHei", "Noto Sans CJK SC", sans-serif;
  --background: #0d151b;
  --surface: #15212a;
  --ink: #eee8dc;
  --muted: #b9c0bb;
  --line: #46565f;
  --cyan: #8ac5cb;
  --red: #ed9487;
  --amber: #e3b874;
}

* { box-sizing: border-box; }
body { margin: 0; background: var(--background); color: var(--ink); }
button, input { font: inherit; }
a, button, input { -webkit-tap-highlight-color: transparent; }
a:focus-visible, button:focus-visible, input:focus-visible { outline: 3px solid var(--amber); outline-offset: 4px; }
button:disabled, input:disabled { opacity: .5; cursor: default; }
.page { max-width: 1240px; margin: 0 auto; padding: 24px 32px 20px; }
.page-header { display: flex; justify-content: space-between; gap: 24px; align-items: flex-start; padding-bottom: 16px; border-bottom: 1px solid var(--line); }
h1 { font-size: 28px; line-height: 1.4; margin: 0 0 8px; font-weight: 650; }
.intro { margin: 0; color: var(--muted); font-size: 16px; line-height: 1.6; }
.back-link { color: var(--muted); white-space: nowrap; padding: 9px 0; text-underline-offset: 5px; font-size: 15px; }
.back-link:hover { color: var(--amber); }
.lesson { display: grid; grid-template-columns: minmax(0, 1.65fr) minmax(320px, 1fr); gap: 32px; padding: 18px 0; }
h2 { font-size: 18px; line-height: 1.5; margin: 0; font-weight: 600; }
.section-heading { display: flex; justify-content: space-between; align-items: center; gap: 12px; margin-bottom: 12px; }
.teaching-label { font-size: 13px; color: var(--amber); white-space: nowrap; }
.diagram { margin: 0; }
.diagram svg { display: block; width: 100%; aspect-ratio: 7 / 4; max-height: 280px; background: var(--surface); border: 1px solid var(--line); border-radius: 4px; }
.diagram text { font: 26px "PingFang SC", "Microsoft YaHei", sans-serif; fill: var(--muted); }
.diagram .baseline-label { font-size: 24px; fill: #acb5b9; }
.diagram .friendly-label { fill: var(--cyan); }
.diagram .target-label { fill: var(--red); }
.diagram .meeting-label { fill: var(--amber); font-weight: 600; }
.grid-line { fill: none; stroke: #23323b; stroke-width: 1; }
.triangle-fill { fill: #223238; fill-opacity: .55; }
.baseline { fill: none; stroke: #708088; stroke-width: 2; }
.target-direction { fill: none; stroke: var(--red); stroke-width: 2; stroke-dasharray: 6 8; opacity: .75; }
.target-route { fill: none; stroke: var(--red); stroke-width: 4; stroke-dasharray: 10 7; }
.friendly-route { fill: none; stroke: var(--cyan); stroke-width: 4; }
.target-arrow { fill: none; stroke: var(--red); stroke-width: 1.5; }
.friendly-arrow { fill: none; stroke: var(--cyan); stroke-width: 1.5; }
.equal-tick { stroke: var(--ink); stroke-width: 3; }
.equal-angle { fill: none; stroke: var(--amber); stroke-width: 3; }
.friendly-start { fill: var(--background); stroke: var(--cyan); stroke-width: 3; }
.target-start { fill: var(--background); stroke: var(--red); stroke-width: 3; }
.meeting-ring { fill: none; stroke: var(--amber); stroke-width: 2; }
.meeting-point { fill: var(--amber); }
.moving-halo { fill: var(--background); stroke: var(--ink); stroke-width: 1; }
.friendly-moving-dot { fill: var(--background); stroke: var(--cyan); stroke-width: 3; }
.target-moving-dot { fill: var(--red); }
#target-moving .moving-halo { fill: none; }
.route-legend { display: flex; flex-wrap: wrap; gap: 8px 24px; padding: 12px 0 14px; color: var(--muted); font-size: 14px; line-height: 1.6; }
.route-legend span { display: inline-flex; align-items: center; gap: 8px; }
.swatch { width: 22px; border-top: 3px solid var(--cyan); }
.swatch.target { border-color: var(--red); border-top-style: dashed; }
.playback { display: flex; gap: 16px; justify-content: space-between; align-items: center; min-height: 44px; }
.playback-buttons { display: flex; gap: 10px; flex-shrink: 0; }
button { display: inline-flex; align-items: center; justify-content: center; gap: 7px; height: 44px; min-width: 100px; border: 1px solid var(--cyan); border-radius: 4px; background: var(--cyan); color: #102027; padding: 0 13px; font-size: 15px; font-weight: 600; cursor: pointer; }
button:hover { filter: brightness(1.12); }
button svg { width: 20px; height: 20px; fill: none; stroke: currentColor; stroke-width: 2; stroke-linecap: round; stroke-linejoin: round; }
#play-icon { fill: currentColor; stroke-width: 1; }
.reset-button { min-width: 88px; background: transparent; color: var(--ink); border-color: var(--line); }
#play-status { margin: 0; font-size: 14px; line-height: 1.5; text-align: right; color: var(--muted); }
.progress-row { display: flex; align-items: center; gap: 12px; margin-top: 12px; font-size: 13px; color: var(--muted); min-height: 28px; }
.progress-row label { flex-shrink: 0; }
input[type="range"] { width: 100%; min-width: 0; height: 24px; margin: 0; accent-color: var(--cyan); cursor: pointer; }
output { width: 4ch; flex-shrink: 0; text-align: right; font-variant-numeric: tabular-nums; }
.explanation { border-left: 1px solid var(--line); padding-left: 28px; }
.steps { padding: 0; margin: 14px 0; list-style: none; display: grid; gap: 18px; }
.steps li { display: flex; gap: 14px; align-items: flex-start; }
.step-number { font-size: 16px; line-height: 27px; color: var(--amber); font-variant-numeric: tabular-nums; }
h3 { font-size: 17px; line-height: 1.6; margin: 0 0 6px; font-weight: 600; }
.steps p, .function-note { margin: 0; font-size: 15px; line-height: 1.7; color: var(--muted); }
.function-note { border-top: 1px solid var(--line); padding-top: 14px; }
.function-note strong { color: var(--ink); font-weight: 500; }
.noscript { color: var(--amber); font-size: 14px; line-height: 1.6; }

@media (max-width: 900px) {
  .page { padding: 24px; }
  .lesson { grid-template-columns: minmax(0, 1.35fr) minmax(270px, 1fr); gap: 24px; }
  .section-heading { align-items: flex-start; flex-direction: column; gap: 4px; }
  .explanation { padding-left: 20px; }
  .steps { gap: 16px; }
  .playback { flex-wrap: wrap; gap: 8px; }
  #play-status { text-align: left; }
}

@media (max-width: 700px) {
  .page { padding: 20px 16px 24px; }
  .page-header { flex-direction: column; gap: 12px; padding-bottom: 16px; }
  h1 { font-size: 23px; }
  .intro { font-size: 15px; }
  .back-link { padding: 2px 0; min-height: 32px; }
  .lesson { display: block; padding: 18px 0; }
  .section-heading { flex-direction: row; flex-wrap: wrap; align-items: center; gap: 6px 12px; }
  h2 { font-size: 17px; }
  .diagram .baseline-label { font-size: 27px; }
  .diagram svg { max-height: none; }
  .diagram text { font-size: 28px; }
  .route-legend { gap: 6px 16px; font-size: 13px; }
  .playback { flex-wrap: nowrap; }
  #play-status { font-size: 13px; text-align: right; }
  .explanation { border-left: 0; border-top: 1px solid var(--line); padding: 20px 0 0; margin-top: 20px; }
  .steps { gap: 18px; }
  .steps p { font-size: 15px; }
}

@media (max-width: 360px) {
  .page { padding-left: 12px; padding-right: 12px; }
  .playback { flex-wrap: wrap; }
  #play-status { text-align: left; }
}