html, body{
  height: 25%;
  display: flex;
  justify-content: center;
  align-items: center;
  background-color: black;
  overflow: hidden;
  transform: scale(1.5);
}
/*button*/
.btn{
  padding: 30px 80px;
  border: none;
  outline: none;
  color: white;
  cursor: pointer;
  position: relative;
  z-index: 0;
  border-radius: 20px;
}
.btn::after {
  content: "";
  z-index: -1;
  position: absolute;
  width: 100%;
  height: 100%;
  background-color: darkgray;
  left: 0;
  top: 0;
  border-radius: 20px;
}

.btn::before{
  content: "";
  background: linear-gradient(
    45deg, 
    red, orange, yellow, lime, 
    turquoise, blue, magenta, red
  );
  position: absolute;
  top: -2px;
  left: -2px;
  background-size: 600%;
  z-index: -1;
  width: calc(100% + 4px);
  height: calc(100% + 4px);
  filter: blur(8px);
  animation: glowing 20s linear infinite;
  transition: opacity .3s ease-in-out;
  border-radius: 20px;
  opacity: 0;
}

@keyframes glowing {
  0% {background-position: 0 0;}
  50% {background-position: 400% 0;}
  100% {background-position: 0 0;}
}

.btn:hover::before {
  opacity: 1;
}
.btn:active:after {
  background: transparent;
}
.btn:active {
  color: black;
  font-weight: bold;
}