title: Bootstrap: what does class="collapsed" do in <button class="collapsed">?
body: <p>It might sound like a shallow and pedantic question, but I can't find any explanation as to why this class is used or how it works. Obviously for collapsing links in the nav bar when viewport is reduced in size at mobile breakpoint, but I thought this was already achieved through data-toggle and data-target. </p>
  
  <pre><code>&lt;button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1"&gt;
  </code></pre>
  
  <p>The <a href="http://getbootstrap.com/components/#navbar-default" rel="nofollow">Bootstrap navbar explanation</a> doesn't explain why they're using it, the class doesn't appear in bootstrap.css and tutorial sites don't include the class in their exercises.</p>