Is it legitimate to use the HTML5 semantic "nav" element for a vertical side navigation menu?
Some of the stuff I've come across seems to suggest that it is meant to be horizontal.
Yes, it is legitimate to use the HTML5 semantic "nav" element for a vertical side navigation menu. Feel free to use the "nav" element for a vertical side navigation menu. Just be sure to use it appropriately to help users understand the structure and purpose of your content. Example:
<body>
<nav>
<ul>
<li><a href="#section-1">Section 1</a></li>
<li><a href="#section-2">Section 2</a></li>
<li><a href="#section-3">Section 3</a></li>
<li><a href="#section-4">Section 4</a></li>
</ul>
</nav>
<main>
<section id="section-1">
<h2>Section 1</h2>
<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>
</section>
<section id="section-2">
<h2>Section 2</h2>
<p>Nullam consectetur mauris euismod dolor consectetur, vel bibendum ante rutrum.</p>
</section>
<section id="section-3">
<h2>Section 3</h2>
<p>Donec vehicula nisi vel ex vestibulum, sed tempus sapien faucibus.</p>
</section>
<section id="section-4">
<h2>Section 4</h2>
<p>Etiam vulputate libero in elit finibus, sit amet cursus elit pellentesque.</p>
</section>
</main>
</body>