html {
  scrollbar-gutter: stable;
}

body {
  line-height: 1.4;
  font-size: 16px;
  padding: 0 10px;
  margin: 50px auto;
  max-width: 650px;
  background: #fff;
  color: #000;
}

.lanes {
  display: flex;
  flex-direction: column;
  gap: 18px;
  margin: 20px 0 8px;
}

figure { margin: 0; }

figcaption {
  margin-top: 8px;
  text-align: center;
  font-size: 14px;
  color: #555;
}

.track {
  --ball-size: 16px;
  position: relative;
  height: 48px;
  background: #000;
}

.ball {
  position: absolute;
  top: calc(50% - var(--ball-size) / 2);
  left: 8px;
  width: var(--ball-size);
  height: var(--ball-size);
  border-radius: 50%;
  background: #fff;
  animation: bounce linear infinite alternate;
}

.ball.fast { animation-duration: 0.18s; }
.ball.slow { animation-duration: 9s; }

@keyframes bounce {
  from { left: 8px; }
  to   { left: calc(100% - var(--ball-size) - 8px); }
}