When do you need ARIA, and what is the first rule of ARIA?

First rule: if a native element does the job, don't use ARIA. <button> already has a role, focus behavior and Enter/Space handling; <div role="button"> must reimplement all of it.

<!-- Good -->
<button aria-expanded="false" aria-controls="menu">Menu</button>

<!-- Bad -->
<div class="btn" onclick="…">Menu</div>

Reach for ARIA when HTML can't express state: aria-expanded, aria-current, aria-live, aria-describedby, aria-label for icon-only buttons.

ARIA only changes what assistive tech is told - it adds no keyboard behavior; you still have to write that.