JSFiddle - React, Tailwind, and code Playground

HTML

<div id="hider">hide you behind</div>
<header>Foreground</header>

CSS

header { 
    background: yellow;
    position:relative;
    height: 100px;
    width: 100px;
    z-index:30;


#hider
{
    background: red;
    position:absolute;
    height: 100px;
    width: 100px;
    top:25px;
    left:25px;
    z-index:-1;   
}