#mobileMenu li{
  color: #f80;
  position: relative;
}
#mobileMenu li a:after {
  content: "";
  position: absolute;
  width: 0%;
  height: 3px;
  background: red;
  bottom: 0;
  top: 98%;
  transition: 0.5s;
  transform-origin: center;
}
#mobileMenu li:hover a:after {
  width: 50%;
}