Edit in JSFiddle

<header>
    <section>Section</section>
    <nav>Nav</nav>
</header>

header{
    width: 20em;
    height: 10em;
    background: yellow;
}

section{
    background: red;
    float: left;
}

nav{
    background: green;
    float: right;
}

section, nav{
    width: 7.5em;
    height: 5em;
    text-align: center;
    line-height: 5em;
    color: white;
    font-weight: bold;
}