JSFiddle - React, Tailwind, and code Playground

HTML

<div id="outer">
    <div id="inner">
        
    </div>
</div>

CSS

#outer{
    width:400px;
    background-color:black;
    height:200px;
}
#inner{
    width:calc(80%-100px);
    width:expression(80%-100px);
    background-color:red;
    height:100%;
    
}