JSFiddle - React, Tailwind, and code Playground

HTML

<div id="wrapper">
    <div id="sideBar">
        left<br />left<br />left
    </div>
    <div id="mainContent">
        right<br />right<br />right<br />right<br />right
    </div>
</div>

cleared!

CSS

#wrapper {
    overflow: hidden; /* clear the float */
}
#sideBar {
    width: 100px;
    float: left;
    background: #f0f
}
#mainContent {
    overflow: hidden;
    background: #ccc
}