<button class="btn">CLICK ME</button>
html, body {
  width: 100%;
  height: 100%;
  overflow: hidden;
}
body {
  display: grid;
  place-content: center;
}
.btn {
  padding: 6px;
  font-size: 1.4em;
  position: relative;
  border: none;
  background: transparent;
  border-radius: 4px;
  box-sizing: border-box;
  width: 200px;
  height: 100px;
  overflow: hidden;
}
.btn:before {
  content: "";
  position: absolute;
  z-index: -2;
  top: -100%;
  left: -100%;
  width: 300%;
  height: 300%;
  display: block;
  background-color: #1a232a;
  background-image: conic-gradient(from -0.25turn, transparent, rgba(168, 239, 255, 1), transparent 50%);
}
.btn::after {
  content: "";
  position: absolute;
  z-index: -1;
  display: block;
  top: 6px;
  left: 6px;
  width: calc(100% - 12px);
  height: calc(100% - 12px);
  background: #fff;
}
.btn:hover::before {
  animation: rotate 1s linear infinite;
}
@keyframes rotate {
  100% {
    transform: rotate(1turn);
  }
}

External CSS

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External JavaScript

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