JSFiddle - React, Tailwind, and code Playground

HTML

<div class="flex-container">
    <div id="left">
        The floating <br/>column<br />
    </div>
    
    <div id="right">
        This one fill the rest
    </div>
</div>

CSS

.flex-container {
  display: flex;
  flex-flow: row no-wrap;
  justify-content: space-around;
  background: yellow;
  width: 500px;
}

#left {
    width:20%;
    background-color: green; 
}

#right {
    width:80%;
    background-color: red; 
}