title: CSS: invisible background on margin
body: <p>this is something rather simple I think, so I'll just paste the code.</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
  &lt;head&gt;
      &lt;title&gt;&lt;/title&gt;
      &lt;style&gt;
          *{
              margin:0;
              padding:0;
          }
  
          .c1 {
              background: yellow;
              width: 800px;
          }
  
          .c2 {
              background: blue;
              margin: 50px;
          }
      &lt;/style&gt;
  &lt;/head&gt;
  &lt;body&gt;
  
  &lt;div class="c1"&gt;
      &lt;div class="c2"&gt;  
          1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;1&lt;br&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>The problem is, why there isn't a yellow strip at the top or bottom of the page? I think it should be in controlled space of c1 but the background color is just invisible.</p>