JSFiddle - React, Tailwind, and code Playground

HTML

<div class="side">
    side<br />
    side<br />
    side
</div>

<div class="main">
    <!-- try removing the element -->
    <div style="clear:both"></div>
    main
</div>

CSS

* {
    outline: 1px solid red;
}
.side {
    float: right;
    width: 100px;
}
.main {
    margin-right: 100px;
}