JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
<div id="footer">
</div>
</div>

CSS

#footer {    
    padding: 20px;
    width: 100%;
    height: 20px;
    background: red;
    -webkit-box-sizing: border-box; /* Safari/Chrome, other WebKit */
    -moz-box-sizing: border-box;    /* Firefox, other Gecko */
    box-sizing: border-box;         /* Opera/IE 8+ */
}
#container { width:500px; background: #333; padding: 20px; }