JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="absolute-background" />
    <div class="where-is-this" />
  </div>

CSS

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

* {
  box-sizing: border-box;
}


.container {
  background: transparent;
  position: relative;
  overflow: hidden;
  height: 100%;
  width: 100%;
}

.absolute-background {
  position: absolute;
  height: 100%;
  width: 100%;
  left: 0;
  top: 0;
  background: blue;
  z-index: 0;
}

.where-is-this {
  height: 100px;
  width: 100%;
  z-index: 1000000;
  background: red;
}