<button class="btn">Hover me</button>
@import url("https://fonts.googleapis.com/css2?family=Poppins:wght@200;300;400;500;600;700&display=swap");

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-family: "Poppins", sans-serif;
}

body {
  width: 100%;
  height: 100vh;
  justify-content: center;
  align-items: center;
  display: flex;
  background-color: #1c1524;
}

.btn {
  width: 220px;
  font-size: 20px;
  height: 50px;
  border: none;
  outline: none;
  color: #fff;
  background: #111;
  cursor: pointer;
  position: relative;
  z-index: 0;
  border-radius: 10px;
}

.btn:before {
  content: "";
  background: linear-gradient(
    124deg,
    #ff2400,
    #e81d1d,
    #e8b71d,
    #e3e81d,
    #1de840,
    #1ddde8,
    #2b1de8,
    #dd00f3,
    #dd00f3
  );
  position: absolute;
  top: -2px;
  left: -2px;
  background-size: 400%;
  z-index: -1;
  filter: blur(5px);
  width: calc(100% + 4px);
  height: calc(100% + 4px);
  animation: glowing 20s linear infinite;
  opacity: 0;
  transition: opacity 0.3s ease-in-out;
  border-radius: 10px;
}

.btn:hover:before {
  opacity: 1;
}

.btn:after {
  z-index: -1;
  content: "";
  position: absolute;
  width: 100%;
  height: 100%;
  background: #111;
  left: 0;
  top: 0;
  border-radius: 10px;
}

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

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.