JSFiddle - React, Tailwind, and code Playground

HTML

<header>
    <ul>
        <li>Item</li>
        <li>Item</li>
        <li>Item</li>
        <li>Item</li>
        <li class="clear"></li>
    </ul>
</header>
<section>section</section>

CSS

BODY, HTML{
    margin: 0;
}
header{
    margin-bottom: 100px;
}
UL{
    list-style-type: none;
}
UL LI{
    float: left;
    background: green;
}
.clear{
    clear: both;
    float: none;
}
section{
    background: red;
}