title: Why isn't the height of my div changing when I use a percent value?
body: <p>I'm trying to make the height of my div 75% of the viewport size. When I use a pixel value it adds height to it but when I use a percentage, it appears as a very thin line and doesn't change.</p>
  
  <p>This is my HTML:</p>
  
  <pre><code> &lt;body&gt;
        &lt;div class="container-fluid"&gt;
             &lt;div class="row"&gt;
                  &lt;div class="col-md-8 col-md-offset-2" id="main-wrapper"&gt; &lt;/div&gt;
             &lt;/div&gt;
        &lt;/div&gt;
   &lt;/body&gt;
  </code></pre>
  
  <p>This is my CSS:</p>
  
  <pre><code> #main-wrapper {
        background-color: blue;
        height: 75%;
   }
  </code></pre>