title: aria-expanded html attribute
body: <p>I am currently making a navbar and came accross the <code>aria-expanded</code> html attribute. I know of course that the element is used to let an element either expand or collapse. I could find very little information with regard to this and the <a href="https://www.w3.org/WAI/GL/wiki/Using_aria-expanded_to_indicate_the_state_of_a_collapsible_element" rel="noreferrer">W3C docs</a> aren't that clear IMO.</p>
  
  <h2>Question</h2>
  
  <p>How does this attribute work exactly?</p>