This isn't a "Today I learned" so much as "today I remembered that more people should know this".
In accessibility testing I’ve done lately on various pages, a common issue is that folks are using
<aside> when it’s not appropriate. Basically, if you’re inside
<main>, you can’t use
<aside>. (Maybe you want
It’s kind of counter-intuitive, because you’d think you could do, say,
<aside> side-by-side. But unless your
<aside> is outside
<main>, you shouldn’t.
Learn more from Deque at the Axe rule "Complementary landmarks and/or asides are top level".