<button class="btn btn-gradient">
  Hover Me
</button>

@import url(https://fonts.googleapis.com/css?family=Lato);

body {
  display: flex;
  height: 100vh;
  justify-content: center;
  align-items: center;
  text-align: center;
  background: #222;
}

.btn {
  position: relative;
  padding: 1rem 3rem;
  font-family: Lato, sans-serif;
  font-size: 1.5rem;
  line-height: 1.5;
  color: black;
  text-decoration: none;
  background-color: white;
  border: transparent;
  outline: transparent;
  cursor: pointer;
  user-select: none;
  white-space: nowrap;
  animation: glow 8s linear infinite;

  &-gradient {
    color: white;
    background: linear-gradient(90deg, #00dbde, #fc00ff, #00dbde);
    background-size: 300%;
    border-radius: 2em;

    &::before {
      position: absolute;
      content: "";
      top: -5px;
      left: -5px;
      bottom: -5px;
      right: -5px;
      z-index: -1;
      background: inherit;
      background-size: inherit;
      border-radius: 4em;
      opacity: 0;
      transition: 0.5s;
    }

    &:hover::before {
      opacity: 1;
      filter: blur(20px);
      animation: glow 8s linear infinite;
    }
  }
}

@keyframes glow {
  to {
    background-position: 300%;
  }
}