SOURCE

<div class="btn"></div>

.btn{
  width:100px;
  height:100px;
  border-radius:100%;  
  margin:100px;
  background: linear-gradient(45deg,#5c89ff, #15bcfd);  
  animation: btnani 1.5s infinite ease-out;
}
@keyframes btnani{
  0%{
    box-shadow:0 0 0 0 rgba(56,162,254,1);
  }
  100%{
    box-shadow:0 0 0 50px rgba(56,162,254,0);
  }
}