浮动布局转化为定位布局为什么要去除margin?

for(var i=0;i<aLi.length;i++){                aLi[i].style.position = 'absolute';                aLi[i].style.left = arr[i][0] + 'px';                aLi[i].style.top = arr[i][1] + 'px';                ***aLi[i].style.margin = 0;***            }