index.css

body {
background-color: black;
}

.btn {
position: absolute;
margin-top: 5%;
margin-left: 40%;
}

a {
background-color: black;
color: red;
text-decoration: none;
text-transform: uppercase;
line-height: 37px;
text-align: center;
border-radius: 40px;
width: 10%;
height: 4%;
border-style: solid;
border-width: 5px;
}

a:hover {
border-width : 0px;
background-color: black;
animation: htmlhack 8s linear infinite;
}

a:before {
content: "";
position: absolute;
top: 0%;
left: 0%;
right: 0%;
bottom: 0%;
z-index: -1;
background: linear-gradient(45deg, red, orange, yellow, lime, green, cyan, blue, violet, purple);
background-size: 400%;
opacity: 0;
transition: 0.5%;
}

a:hover:before {
filter: blur(10px);
border-radius: 40px;
opacity: 1;
animation: htmlhack 8s linear infinite;
}

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