Z-index doesn't work on empty element in IE9

The z-index property will not work on empty elements in IE8 and IE9 unless they have a background. The solution is to give them one, and then hide it using the opacity property.

background: white;
filter: alpha(opacity=1);

REFERENCES
http://stackoverflow.com/questions/6480774/z-index-problem-in-ie-with-transparent-div