JSFiddle - React, Tailwind, and code Playground

HTML

<div id="wrap"><div id="inner"></div></div>

CSS

#wrap {
    width: 100%; /* useless */
    background: #ccc;
    overflow:hidden; /* establish a new formatting context */
    min-height: 20px;
}

#inner {
    width:30%;
    background: #000;
    float:right;
    height: 200px
}