title: Does aria-haspopup require a true/false value?
body: <p>According to the <a href="https://www.w3.org/WAI/PF/aria-1.1/states_and_properties#aria-haspopup" rel="nofollow noreferrer">spec</a>, the value should be a boolean. However, one of the <a href="https://www.w3.org/TR/wai-aria-practices/examples/combobox/aria1.1pattern/listbox-combo.html" rel="nofollow noreferrer">spec examples of proper implementation</a> uses a string value other than true/false - "listbox", which is the value of another element's role attribute. Is this a flaw in the implementation example or do screen readers, in practice, gain additional information from values other than true?</p>
  
  <p>Relevant code from linked example:</p>
  
  <pre><code>&lt;div role="combobox" aria-expanded="false" aria-owns="ex1-listbox" aria-haspopup="listbox" id="ex1-combobox"&gt;
      &lt;input type="text" aria-autocomplete="list" aria-controls="ex1-listbox" id="ex1-input" aria-activedescendant=""&gt;
  &lt;/div&gt;
  </code></pre>