how to animate underline in css code example

Example: css + underline + animation

a.middle
{
    position: relative;
}

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