<div class="container">
  <a class="btn-loading" href="#">
    <span>PUSH ME</span>
  </a>
</div>
/* ここはコピーしなくてOK */
.container {
  @import url('https://fonts.googleapis.com/css2?family=Noto+Sans+JP:wght@700&display=swap');
  font-family: 'Noto Sans JP', sans-serif;
  text-align: center;
  padding-top: 40px;
}

/* ここから下がボタンのCSS */
.btn-loading {
  display: inline-flex;
  text-decoration: none;
  font-weight: 700;
  color: #111;
  width: 100px;
  height: 100px;
  font-size: 14px;
  position: relative;
  opacity: 1 !important;
  transition: all 0.3s;
  background-color: #CCC;
  border-radius: 100%;
  transition: color 0.4s;
  overflow: hidden;
}
.btn-loading:before,
.btn-loading:after {
  content: "";
  display: block;
  position: absolute;
  width: 50%;
  height: 0;
  background-color: #65A04D;
  pointer-events: none;
}
.btn-loading:before {
  bottom: 0;
  left: 0;
}
.btn-loading:after {
  top: 0;
  right: 0;
}
.btn-loading:hover {
  color: #65A04D;
}
.btn-loading:hover:before,
.btn-loading:hover:after {
  height: 100%;
}
.btn-loading:hover:before {
  transition: height 0.2s 0.2s linear;
}
.btn-loading:hover:after {
  transition: height 0.2s linear;
}
.btn-loading span {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: calc(100% - 4px);
  height: calc(100% - 4px);
  background-color: #FFF;
  border-radius: 100%;
  display: flex;
  align-items: center;
  justify-content: center;
  z-index: 99;
}

External CSS

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External JavaScript

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