title: Do I need role="button" on a <button>?
body: <p>I've noticed in all of Bootstrap's examples using <code>button</code> elements, they include <code>role="button"</code> (and <code>type="button"</code>), such as:</p>
  
  <pre><code>&lt;div class="dropdown"&gt;
      &lt;button id="dLabel" type="button" role="button" data-toggle="dropdown" 
       aria-haspopup="true" aria-expanded="false"&gt;
          Dropdown trigger &lt;span class="caret"&gt;&lt;/span&gt;
      &lt;/button&gt;
      &lt;ul class="dropdown-menu" role="menu" aria-labelledby="dLabel"&gt;
      ...
      &lt;/ul&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Won't accessibility software already know that a <code>button</code> element is meant to act as a button? Is there any reason I should include <code>role="button"</code> and/or <code>type="button"</code> in my code?</p>