让火狐、IE8、IE6都支持这个属性position:fixed

position:fixed; top:200px; _position:  absolute; background-color:White;
_top:       e­xpression(eval(document.compatMode &&
            document.compatMode=='CSS1Compat') ?
            documentElement.scrollTop+15 :
            document.body.scrollTop +
            (document.body.clientHeight
            -this.clientHeight));
            
            }