Because Z-INDEX has defined the order which piles up one on top of another,was similar chart level which used in the graph design,Therefore had the Z-INDEX attribute block element to be called vividly the layer.

• 因为Z-INDEX定义了堆叠的顺序,类似于图形设计中使用的图层,所以拥有了Z-INDEX属性的块元素被形象地称为层。