title: Bootstrap 4 responsive spacing not working
body: <p>I'm using the classes as defined</p>
  
  <p>Applies margin to an element using responsive breakpoints {property}{sides}-{breakpoint}-{size} 
  m(t,b,r,l,x,y)-(sm, md, lg, xl)-(0, 1, 2, 3, 4, 5)</p>
  
  <p>Meaning that</p>
  
  <pre><code>&lt;div class="col-12 mb-sm-3"&gt;&lt;/div&gt;
  </code></pre>
  
  <p>Should produce a div with margin bottom on small devices, but it doesn't.</p>