JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box"></div>

CSS

.box { 
    width: 300px;
    height: 300px;
    background: #ddd;
    display: block;        
}

.box.red { background: #f00; }

JavaScript

$(".box").mouseover(function() {
    $("<div class='box red' />").css('display','none').appendTo($(this)).slideDown();
});