JSFiddle - React, Tailwind, and code Playground

HTML

<div class="relative black blue">
  <div class="relative">
    <h1>sds</h1>
    <h1>sdsds</h1>
    <h1>sdsds</h1>
    <h1>sdsds</h1>
    <div class="block red absolute"></div>
  </div>
  
</div>

CSS

.block {
  width: 100px;
  height: 100%;
}

.black {
  background-color: black; 
}
.red {
  background-color: red;
}
.blue {
  color: blue;
}

.relative {
  position: relative;
  width: auto;
  padding: 10px;
}
.absolute {
  position: absolute;
}
.block.red.absolute {
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  z-index: -1;
}
.relative.black.blue {
  z-index: 0;
}