JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div id="left">fixed width</div>
    <div id="right">rest of space width</div>
</div>

CSS

#left
{
width:20%;float:left;position:fixed;left:0px;
border:1px solid yellow;
}
#right
{
width:80%;float:right;
    border:1px solid red;
}