JSFiddle - React, Tailwind, and code Playground

HTML

<header>
    <div class="site-header">
        some stuff + a full width border in the middle!
    </div>
    <nav>
        <ul>
            <li>linka</li>
            <li>link b</li>
        </ul>
    </nav>
</header>

CSS

.site-header, nav {
    max-width: 500px;
    margin: 0 auto;
}

nav {
    border-top: 1px solid red; 
}