JSFiddle - React, Tailwind, and code Playground

HTML

<div id="box1">
    
    <div id="box2">div 2</div>
   
</div>

CSS

#box1{
    display:block;
    width: 300px;
    height: 300px; 
    background-color:#0000FF;
    border:solid transparent 1px;    
}

#box2{
    margin-top: 30px;
    background-color:#008000;
    
}