Problem with css("margin")

Problem with css("margin")

Hi,
it seems that <span style="font-weight: bold;">$(elem).css("margin")</span> works correctly if margin is defined directly via the "style" attribute in the concerned markup.
But if margin is defined via a stylesheet, it returns undefined on Firefox (and even js error on IE6, not tested on IE7).
Here is an example : <a href="http://zavatta35.free.fr/testMargin.html" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">http://zavatta35.free.fr/testMargin.html</a><font size="1"></font>
Not very normal, isn't it ?
Xavier