a::after {
  position: absolute;
  bottom: -4px;
  left: 0;
  content: '';
  width: 100%;
  height: 2px;
  background: #FFF;
  transform: scale(0, 1);
  transform-origin: left top;
  transition: transform .3s;
}

a:hover::after {
  transform: scale(1, 1);
}