.glow {
  animation: glow;
  animation-delay: 1s;
  animation-duration: 2s;
  animation-direction: alternate;
  animation-timing-function: ease-in;
  animation-iteration-count: infinite;
  filter: drop-shadow(0px 0px 0px green);
}

@keyframes glow {
  from {filter: drop-shadow(0px 0px 0px #00ff00);}
  to {filter: drop-shadow(0px 0px 5px #00ff00);}
}