.flash-siren {
  display: inline-block;
  animation: siren 1s infinite;
}

@keyframes siren {
  0%, 100% {
    filter: brightness(0.9) drop-shadow(0 0 2px red);
  }
  50% {
  filter: brightness(1.4) drop-shadow(0 0 45px red);
  }
}