background image link

.link {
  background-size: 0 3px;
  background-repeat: repeat-x;
  background-position: 0 95%, 100% 95%, 0 95%;

  &:hover {
    background-size: 100% 3px;
    background-image: linear-gradient(to right, color-one, color-two);
  }
}

box shadow link

a {
  box-shadow: inset 0 -.1rem 0 0 #297fdd;
  -moz-transition: color .3s ease,box-shadow .3s ease;
  transition: color .3s ease,box-shadow .3s ease;
  
  &:hover {
    color: #fff;
    box-shadow: inset 0 -1.3rem 0 0 #297fdd;
  }
}