[рони]

Vipz,

<style>
.bomba li{
    border-bottom: 2px solid transparent;
    padding-bottom: 4px;
}
.bomba li:hover{
    border-bottom: 2px solid gray;
    padding-bottom: 4px;
}
li.active{
    border-bottom: 2px solid #d04;
}
</style>

<ul class="bomba">
   <li id="1_1">1</li>
   <li id="1_2">2</li>
   <li id="1_3">3</li>
</ul>
<script>
document.getElementById('1_1').classList.add('active')
</script>