JSFiddle - React, Tailwind, and code Playground

HTML

<div class="progress">
</div>

CSS

.progress {
    width:200px;
    height:50px;
    border:1px solid black;
    position:relative;
}
.progress:after {
    content:'\A';
    position:absolute;
    background:black;
    top:0; bottom:0;
    left:0; 
    width:50%;
}

JavaScript

$(function(){
var a=20;
$(".progress:after").width(a+"%");
});