title: Why doesn't margin get filled with background color in CSS?
body: <p>I have this simple piece of HTML.</p>
  <p>Why doesn't the background color fill the entire <code>#footer</code> element's background - specifically, including the margin?</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;!doctype html&gt;
  &lt;html&gt;
  
  &lt;head&gt;
    &lt;style&gt;
      #footer {
        background: #263238;
        margin: 100px;
        padding: 0;
      }
      .footer-text {
        margin: 0;
        color: #fff;
      }
    &lt;/style&gt;
  &lt;/head&gt;
  
  &lt;body&gt;
    &lt;div id="footer"&gt;
      &lt;div class="footer-text"&gt;All Rights Reserved © 2016&lt;/div&gt;
    &lt;/div&gt;
  &lt;/body&gt;
  
  &lt;/html&gt;</code></pre>
  </div>
  </div>
  </p>