title: How do I convert a CSS background-position property from percentages into pixels?
body: <p>I'm centering a background right now (using <code>background-position: 50% 50%;</code>), that I'd like to animate using Javascript. But, I want to be able to animate this using pixel measurements. Effectively I want to set the background-position to something like "50% + 10px". Is this possible using Javascript?</p>
  
  <p>I can possibly do it by finding the height of the background image and the height of the browser screen and do some arithmetic, but this seems like a convoluted solution.</p>