ul {
  position: relative;
  display: table;
  table-layout: fixed;
  width: 100%;
  border-bottom: 1px solid gray;
}

ul::after {
  content: '';
  position: absolute;
  bottom: 0;
  width: 66px;
  margin-left: -33px;
  border-bottom: 2px solid blue;
  transition: left 1s
}

ul.foo::after {
  left: 25%;
}

ul.bar::after {
  left: 75%;
}

li {
  display: table-cell;
  width: 50%;
  text-align: center;
  line-height: 3;
  transition: left 1s
}

ul.foo li.foo,
ul.bar li.bar {
  color: blue;
}