JSFiddle - React, Tailwind, and code Playground

HTML

<div id="left_column">250px fixed width</div>
<div id="right_column">Remaining 100% width</div>

CSS

html,body{
    height:100%;
}
* {
    padding: 0px;
    margin: 0px
}
#left_column {
    width: 250px;
    background-color: orange;
    float: left;
    height:100%;
}
#left_column + #right_column {
    margin-left: 250px;
}
#right_column {
    background-color: gold;
    height:100%;
}