//= require modernizr

https://developer.mozilla.org/en/CSS/background-position
http://www.w3.org/TR/css3-background/#background-position

Example: http://jsfiddle.net/Blink/bBXvt/

(function() {

var elem = document.createElement('a'),
    eStyle = elem.style,
    val = "right 10px bottom 10px";

Modernizr.addTest('bgpositionshorthand', function(){
    eStyle.cssText = "background-position: " + val + ";";
    return (eStyle.backgroundPosition === val);
});

}());