<a href="#" id="btn-gra" class="btn-gra">hover me</a>
body{
  background-color: #000;
  margin: 0;
  padding: 0;
}
#btn-gra{
  position: absolute;
  top: 50%;
  left: 50%;
  height: 60px;
  width: 200px;
  transform: transfer(-50%, -50%);
  font-family: sans-serif;
  text-align: center;
  text-decoration: none;
  line-height: 60px;
  color: #fff;
  text-transform: uppercase;
  font-size: 24px;
  box-sizing: border-box;
  background: linear-gradient(90deg, #03a9f4, #f441a5, #ffeb3b, #03a9f4);
  background-size: 400%;
  border-radius: 30px;
  z-index: 1;
}

#btn-gra:hover{
  color: #fff;
  text-decoration: none;
  animation: animate 8s linear infinite;
}

@keyframes animate
{
  0%
  {
    background-position: 0%;
  }
  100%
  {
    background-position: 400%;
  }
}

#btn-gra:before{
  content: '';
  position: absolute;
  top: -5px;
  left: -5px;
  bottom: -5px;
  right: -5px;
  z-index: -1;
  background: linear-gradient(90deg, #03a9f4, #f441a5, #ffeb3b, #03a9f4);
  background-size: 400%;
  border-radius: 40px;
  filter: blur(20px);
  opacity: 0;
  transition: 0.5s;
}

#btn-gra:hover:before{
  filter: blur(20px);
  opacity: 1;
  animation: animate 8s linear infinite;
}

External CSS

  1. https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.7/css/bootstrap.min.css
  2. https://cdnjs.cloudflare.com/ajax/libs/animate.css/3.5.2/animate.min.css

External JavaScript

This Pen doesn't use any external JavaScript resources.