JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="left"> Left</div>
    <div class="main"> Main </div>
    
</div>

CSS

.container {
    width:90%;
    height:200px;
    border:1px solid;
}
.main {
    width:auto;
    height:200px;
    background:red;
    overflow:hidden;
    
}
.left {
    height:200px;
    width:20%;
    background:blue;
    float:left;
    margin-right: 10px; //or you can add a percentage
}