a {
  position: relative;
  color: #000;
  text-decoration: none;
}

a:hover {
  color: #000;
}

a::before {
  content: "";
  position: absolute;
  width: 100%;
  height: 2px;
  bottom: 0;
  left: 0;
  background-color: #000;
  visibility: hidden;
  transform: scaleX(0);
  transition: all 0.3s ease-in-out 0s;
}

a:hover::before {
  visibility: visible;
  transform: scaleX(1);
}

출처: tobiasahlin.com/blog/css-trick-animating-link-underlines/