title: Setting a div's height to 70%
body: <p>I have a super simple page:</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html lang="en"&gt;
  &lt;head&gt;
      &lt;meta charset="utf-8" /&gt;   
      &lt;link rel="stylesheet" href="@Url.Content("~/ui/all.min.css")" type="text/css" /&gt;
  &lt;/head&gt;
  &lt;body&gt;   
      &lt;div class="header"&gt;
  
  &lt;/div&gt;
  
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>and the css:</p>
  
  <pre><code>.header
  {
      height:70%;
      width:100%;
      background:#990663;
  }
  
  body
  {
      margin:0px;
      padding: 0px;
      height:100%;
      width:100%;
  }
  
  *
  {
      margin: 0px auto;
  }
  </code></pre>
  
  <p>I want the div height to be 70% of screen. How can I achieve that? If I put 400px in height, it works. So why not 70%? Any ideas how to achieve this?</p>