styles.css

*,
*::before,
*::after {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
  background: aliceblue;
  font-family: sans-serif;
  font-size: 62.5%;
}

.btn {
  padding: 0.6rem 1.4rem;
  background: pink;
  font-size: 1.2rem;
  text-decoration: none;
  color: black;
  border-radius: 1rem;
  box-shadow: 0px 3px 4px rgba(0, 0, 0, 0.3);
  cursor: pointer;
  position: relative;
  overflow: hidden;
  /* transition: height 2000ms ease-in-out; */
}

#btn {
  z-index: 1;
  position: relative;
}

.btn::before {
  content: "";
  background: orangered;
  width: 0;
  height: 0;
  position: absolute;
  top: var(--yPos);
  left: var(--xPos);
  transform: translate(-50%, -50%);
  border-radius: 10px;
  transition: width 0.5s, height 0.5s;
}

.btn:hover::before {
  width: 12rem;
  height: 12rem;
}