JSFiddle - React, Tailwind, and code Playground

HTML

<div class="set">
    Parent z-index set
    <div class="child">
        Child
    </div>
</div>

<div class="notset">
    Parent z-index not set
    <div class="child">
        Child
    </div>
</div>

CSS

.set {
    float: left;
    position: relative;
    background: #eaeaea;
    height: 200px;
    width: 200px;
    z-index: 20;
}
.child {
    position: relative;
    z-index: -20;
    background: #d6d6d6;
    height: 100px;
    width: 100px;
}

.notset {
    margin-left: 40px;
    float: left;
    position: relative;
    background: #eaeaea;
    height: 200px;
    width: 200px;
}