Bootstrap-4 spacing class mb-2 doesn't work
<p>The Bootstrap-4 spacing class mb-2 doesn't work while ml-2 does work. Here's the code,</p>

<pre><code>.mb-2 { 
   margin-bottom: 1.5rem !important;
  }
</code></pre>

<p>!important should override other margin-bottom definitions. Also what does the 1.5rem reference, that is how many pixels?</p>