.css method return wrong value for margin on an hidden element if it is specified as negative percent.
the value of width is returned instead.

examples:
$("<div/>").css({'margin': '-2%'}).css("marginTop"); // return "auto" - WRONG: expected "-2%"

$("<div/>").css({'margin': '-2%', 'width': '60%'}).css("marginTop"); // return "60%" - WRONG: expected "-2%"

$("<div/>").css({'margin': '2%'}).css("marginTop"); // return "2%" - CORRECT

$("<div/>").css({'margin': '-2px'}).css("marginTop"); // return "-2px" - CORRECT