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 <section> instead?)

It’s kind of counter-intuitive, because you’d think you could do, say, <article> and <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".