title: DOCTYPE declaration removes my background image
body: <p>I have a html document where i would like to have an image as a background. For the next example eveything works great:</p>
  
  <pre><code>&lt;html&gt;&lt;head&gt;&lt;/head&gt;&lt;body&gt;&lt;div id="principal"&gt;&lt;/div&gt;&lt;/body&gt;&lt;/html&gt;
  </code></pre>
  
  <p>css:</p>
  
  <pre><code>#principal 
  {
      background: url(../Img/org/world.png) no-repeat;
      width:100%;
      height:100%;
      background-size: 100% 100%;
  }
  </code></pre>
  
  <p>but if i rewrite the html file to :</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;&lt;html&gt;&lt;head&gt;&lt;/head&gt;&lt;body&gt;&lt;div id="principal"&gt;&lt;/div&gt;&lt;/body&gt;&lt;/html&gt;
  </code></pre>
  
  <p>the image from css ( background: url(../Img/org/world.png) no-repeat; ) is not displayed anymore. Why?</p>