JSFiddle - React, Tailwind, and code Playground

HTML

<div class="aaa">

    <div class="bbb">
        
    </div>

</div>

CSS

*{
    font-size:12px; 
}

.aaa {
    font-size: .5em;
    
    width: 30em;
    height: 30em;
    
    background: red;
}

.bbb {
    font-size: 1em;
    
    width: 15em;
    height: 15em;

    background: green;
}