Rainbow button

Button.html

<button class="btn">Test</button>

<!-- Partially based on https://web.dev/speedy-css-tip-animated-gradient-text/ -->

Button.module.css

.btn {
  cursor: pointer;
  display: inline-block;
  margin-bottom: 1rem;
  font-size: 1.1rem;
  height: 2.8rem;
  letter-spacing: 0.1rem;
  line-height: 2.8rem;
  padding: 0px 1.8rem;
  text-align: center;
  text-decoration: none;
  white-space: nowrap;
  position: relative;
  z-index: 0;
  color: rgb(255, 255, 255);
  border-color: transparent;
}

.btn:active {
  color: rgb(0, 0, 0);
}

.btn::before {
  content: "";
  position: absolute;
  top: -2px;
  left: -2px;
  z-index: -1;
  filter: blur(5px);
  width: calc(100% + 4px);
  height: calc(100% + 4px);
  opacity: 0;
  background: linear-gradient(45deg,
          rgb(255, 0, 0),
          rgb(255, 115, 0),
          rgb(255, 251, 0),
          rgb(72, 255, 0),
          rgb(0, 255, 213),
          rgb(0, 43, 255),
          rgb(122, 0, 255),
          rgb(255, 0, 200),
          rgb(255, 0, 0)
      ) 0% 0% / 400%;
  animation: 2s linear 0s infinite normal none running move-background;
  transition: opacity 0.3s ease-in-out 0s;
}

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

.btn::after {
  z-index: -1;
  content: "";
  position: absolute;
  width: 100%;
  height: 100%;
  background: rgb(17, 17, 17);
  left: 0px;
  top: 0px;
}

.btn:active::after {
  background: transparent;
}

@keyframes move-background { 
  to {
    background-position: 400% 0;
  }
}