<aside>
Content tangentially related to what surrounds it: pull quotes, related links, sidebars, callouts.
Baseline: Widely available · Sectioning & landmarks
When to use it
When the page would still make complete sense with the block removed.
When not to
Don't use it for content the flow depends on, and label it (aria-labelledby) when it has a heading.
Key attributes
aria-labelledby- points at the aside's heading
Example
<aside aria-labelledby='related-h'>
<h2 id='related-h'>Related reading</h2>
<ul><li><a href='/tags/section.html'>section vs article</a></li></ul>
</aside>