title: Functionality of "aria-labelledby" attribute
body: <p>When I was doing some web development, an attribute named <em>aria-labelledby</em> has cropped up frequently. For example in bootstrap offical example:</p>
  
  <blockquote>
    <p><code>&lt;ul class="dropdown-menu" role="menu"</code> <strong>aria-labelledby</strong>=<code>"dLabel"&gt;</code></p>
  </blockquote>
  
  <p>After doing some research I know it is accessibility related, but not many clues of the exact functionality of the attribute. </p>
  
  <p>Many thanks. </p>