HTML5 elements such as main, nav, and aside act as landmarks,
or special regions on the page to which
screen readers and other assistive technologies can jump.
By using landmark elements,
you can dramatically improve the navigation experience on your site
for users of assistive technology.
Learn more in Deque University's
HTML 5 and ARIA Landmarks.
How to manually check landmarks
Use the W3C's list of landmark elements to check that each major section of your page is contained by a landmark element. For example:
<header>
<p>Put product name and logo here</p>
</header>
<nav>
<ul>
<li>Put navigation here</li>
</ul>
</nav>
<main>
<p>Put main content here</p>
</main>
<footer>
<p>Put copyright info, supplemental links, etc. here</p>
</footer>
You can also use tools like Microsoft's Accessibility Insights extension to visualize your page structure and catch sections that aren't contained in landmarks: