backgroundSizeExtended.js

    tests['backgroundsize'] = function() {
        var bool = test_props_all( 'backgroundSize' );

        if (bool){  
            bool = new Boolean(bool);  
            bool.contain = !!(function() {
              set_css('background-size: contain');
              return contains(m_style['backgroundSize'], 'contain');
            });
            bool.cover = !!(function() {
              set_css('background-size: cover');
              return contains(m_style['backgroundSize'], 'cover');
            });
        }
        return bool;
    };

Regarding Modernizr issue:
http://github.com/Modernizr/Modernizr/issues/issue/109

Adding support for backgroundSize: contain and backgroundSize: cover.

Oops, was returning a function instead of a boolean.

Fixed that, BUT it's worth noting that going down this rabbit hole breaks the existing feature in test evaluation loop and does not provide a css flag for these properties. That css flag is probably the most valuable contribution from adding these contributions. The use case that I could think of for this is primarily css related: if there's no .backgroundsize-contain support, for example, provide a sensible work around in another css block.