title: set background-image without set height
body: <p>how to set image background without set height ?</p>
  
  <pre><code>#Header {
      background: #eee url("../Images/Banner.gif") repeat-x;
   }
  
  &lt;div id="Header"&gt;
  
  &lt;/div&gt;
  </code></pre>
  
  <p>But the image is not displayed.However, the the following code picture will be displayed.</p>
  
  <pre><code>    #Header {
          background: #eee url("../Images/Banner.gif") repeat-x;
  height=150px;
       }
  </code></pre>
  
  <p>Why do I need to adjust the height?How to measure height automatically?</p>