JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block">
    
</div>

CSS

.block{
    background: #ddd;
    height: 50px;
    position: relative;
}

.block:after{
    background: red;
    content: "";
    display: block;
    width: 50%;
    height: 50px;  
    z-index: 100;
}