JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    Content
</div>

CSS

.container{
    position: relative;        
    margin: 0 auto;
    padding: 0 0 0 30px;
    width: 300px;
    height: 100px;        
    background-color: green;
}
.container:before{
    content: "";
    position: absolute;
    left: 0;
    width: 30px;
    height: 100%;
    background-color: blue;
}