title: Background cover while page resize
body: <p>My page has some animations that sometimes change the page height (on purpose), so initialy when the page is loaded and page height is 100%, the background image covers the page. 
  But when it runs the animations, the page height can go beyond 100%. The only way to effectively cover all the background is to change background height from auto to a bigger value (e.g. 200%), but doing so i am also changing the page height. In other words, is it possible to cover (dynamically) the page when the animations are running while keeping page height at 100% when animations are not running?</p>
  
  <p>my css code for the background:</p>
  
  <pre><code>background: url(background2.jpg)repeat 5% 5%;
  position: absolute;
  background-size:cover;
  top: 0; left: 0; right: 0; bottom: 0;
  z-index: 400;
  height: auto;
  width: auto;
  </code></pre>