style.css

body {
  margin: 0;
  display: flex;
  justify-content: center;
  height: 100vh;
  align-items: center;
  background-color: #13e695;
  font-family: Verdana, Geneva, Tahoma, sans-serif;
}

.btn {
  background-color: #00339b;
  box-shadow: 0 4px 8px rgba(0, 0, 0, 0.3);
  text-decoration: none;
  color: beige;
  padding: 20px 40px;
  border-radius: 5px;
  position: relative;
  overflow: hidden;
}

.btn::before {
  content: "";
  position: absolute;
  background-color: #ff0081;
  width: 0px;
  height: 0px;
  left: var(--xPos);
  top: var(--yPos);
  transform: translate(-50%, -50%);
  /* The reason why negative values are used in this case is because the translate() function is relative to the element's own dimensions. That is, a positive value would move the element away from the center, while a negative value would move it closer to the center. */
  border-radius: 50%;
  transition: width 0.8s, height 0.8s;
}

.btn:hover::before {
  width: 300px;
  height: 300px;
}

.btn span {
  position: relative;
  z-index: 1;
}