title: Bootstrap 4 "hidden" class doesn't work
body: <p>I'm using Bootstrap v4 with it's default CSS and JS. I'm trying to apply the classes <code>hidden</code>, <code>hidden-XX-down</code> and <code>hidden-XX-up</code> to various divs, buttons, etc. But it doesn't seem to affect anything. All other classes work, except this one.</p>
  
  <p>Here's an example:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="row"&gt;
  
    &lt;div class="col col-10 offset-1 hidden-sm-down"&gt;
  
      &lt;p class="text-primary text-center"&gt;Lorem ipsum dolor sit amet.&lt;/p&gt;
  
    &lt;/div&gt;
  
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>I want the paragraph to be invisible when I resize the window to a smaller width (sm-down), yet it remains visible and nothing happens.</p>