JSFiddle - React, Tailwind, and code Playground

HTML

<div id="inner_fixed">
    I have a fixed height
</div>

<div id="inner_remaining">
    I take up the remaining height
</div>

CSS

html, body {
    height: 100%;
    width: 100%;
    margin: 0;
}

#inner_fixed {
    height: 100px;
    background-color: grey;
}

#inner_remaining {
    background-color: #DDDDDD;
    
    height: calc(100% - 100px);    
}