<div class="circle"></div>

body {
  margin: 40px;
}
.circle {
  background: green;
  border-radius: 100%;
  width: 100px;
  height: 100px;
  -webkit-animation: pulsing 2s infinite;
  animation: pulsing 2s infinite;
}

@-webkit-keyframes pulsing {
  0% {
    -webkit-transform: scale(0.5, 0.5);
    transform: scale(0.5, 0.5)
  }
  50% {
    -webkit-transform: scale(1.0, 1.0);
    transform: scale(1.0, 1.0);
  }
  100% {
    -webkit-transform: scale(0.5, 0.5);
    transform: scale(0.5, 0.5);
  }
}

@keyframes pulsing {
  0% {
    -webkit-transform: scale(0.5, 0.5);
    transform: scale(0.5, 0.5)
  }
  50% {
    -webkit-transform: scale(1.0, 1.0);
    transform: scale(1.0, 1.0);
  }
  100% {
    -webkit-transform: scale(0.5, 0.5);
    transform: scale(0.5, 0.5);
  }
}