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;
}