li {
    margin-bottom: 10px;
}

.cool-link {
    display: inline-block;
   
    text-decoration: none;
}

.cool-link::after {
    content: '';
    display: block;
    width: 0;
    height: 2px;
    background: brown;
    transition: width .3s;
}

.cool-link:hover::after {
    width: 100%;
  
}