JSFiddle - React, Tailwind, and code Playground

HTML

<div id="fillX">
    <div id="childXFill">
    </div>
</div>

CSS

#fillX
{
    background:red;
    width:100%;
    opacity:0.5;
}

#childXFill
{
    background:blue;
    width:100%;
    padding:10px;
    opacity:0.5;
}