Можете посмотреть во вкладке Result;
span{
position: relative;
cursor: pointer;
color: blue;
}
span:before{
content: "";
position: absolute;
width: 100%;
height: 1px;
bottom: 0;
left: 0;
background-color: blue;
visibility: hidden;
-webkit-transform: scaleX(0);
transform: scaleX(0);
-webkit-transition: all 0.3s ease-in-out 0s;
transition: all 0.3s ease-in-out 0s;
}
span:hover:before{
visibility: visible;
-webkit-transform: scaleX(1);
transform: scaleX(1);
}