JSFiddle - React, Tailwind, and code Playground

HTML

<div id="test">
</div>

CSS

#test{
    position:absolute;
    height: 100px;
    width: 100px;
    background-color: #A8A8A8;
    
    -o-transition: all 0.5s ease;
    -ms-transition: all 0.5s ease;
    -moz-transition: all 0.5s ease;
    -webkit-transition: all 0.5s ease;
    transition: all 0.5s ease;
}

#test:hover{
    border-bottom: 10px solid black;
}