.heartbeat {
  /* Make the pulses feel a bit physical */
  animation: heartbeat 1.6s ease-in-out infinite;
  transform-origin: center center;
}

/* Heartbeat: two quick flashes and then silence */
@keyframes heartbeat {
  0%,
  100% {
    opacity: 0.4;
    transform: scale(1);
  }

  /* First beat */
  5% {
    opacity: 1;
    transform: scale(1.10);
  }
  10% {
    opacity: 0.2;
    transform: scale(1.0);
  }

  /* Second beat, slightly stronger */
  15% {
    opacity: 1;
    transform: scale(1.18);
  }
  20% {
    opacity: 0.2;
    transform: scale(1.0);
  }

  /* 20%–100%: “diastole” (dark, no signal) */
}