style.css

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
  height: 100vh;
  background: #000;
  display: flex;
  justify-content: center;
  align-items: center;
  margin: 0;
  flex-wrap: wrap;
}

button {
  border: none;
  outline: none;
  padding: 20px 70px;
  background: linear-gradient(to left, #4699ff, #00e5ff);
  margin: 50px;
  border-radius: 30em;
  font-size: 20px;
  color: white;
  cursor: pointer;
  position: relative;
  text-transform: uppercase;
}

button:nth-child(2) {
  background: linear-gradient(to left, #37ff73, #57ffe0);
}

button:nth-child(3) {
  background: linear-gradient(to left, #fa6c9f, #ffe140);
}

button::before {
  content: "";
  width: 100%;
  height: 100%;
  background: linear-gradient(to left, #4699ff, #00e5ff);
  left: 0;
  top: 15px;
  border-radius: 30em;
  filter: blur(50px);
  z-index: -9999999;
  position: absolute;
}

button:nth-child(2)::before {
  background: linear-gradient(to left, #37ff73, #57ffe0);
}

button:nth-child(3)::before {
  background: linear-gradient(to left, #fa6c9f, #ffe140);
}

button:hover {
  filter: brightness(1.2);
  animation: animate is linear forwards;
}

@keyframes animate {
  0% {
    filter: hue-rotate(0deg);
  }
  100% {
    filter: hue-rotate(360deg);
  }
}