.button {
  position: relative;
  appearance: none;
  background: #337ab7;
  padding: 10px 20px;
  border: none;
  color: white;
  cursor: pointer;
  outline: none;
  overflow: hidden;
  border-radius: 10px;
  text-align: center;
  width: 180px;
  height: 40px;
  line-height: 20px;
  box-sizing: border-box;
  box-shadow: inset 0 -2px 0 #23527c;
  margin: 20px auto 40px;
  transform: scale(2);
}
.button span {
  position: relative;
  letter-spacing: 8px;
}
.button::before {
  --size: 0;
  content: '';
  position: absolute;
  left: var(--x);
  top: var(--y);
  width: var(--size);
  height: var(--size);
  background: radial-gradient(circle closest-side, #4405f7, transparent);
  transform: translate(-50%, -50%);
  transition: width 0.2s ease, height 0.2s ease;
}
.button:hover::before {
  --size: 400px;
}