z-index

數字大的會在上面，數字小的在下面
<div style="position: relative; top: 0px; left: 0px; width: 150px; height: 150px; background-color: yellow; z-index: 1;">
yellow; z-index:1;
</div>
<div style="position: relative; top: -75px; left: 75px; width: 150px; height: 150px; background-color: red; z-index: 2;">
red; z-index:2;
</div>
<div style="position: relative; top: -150px; left: 150px; width: 150px; height: 150px; background-color: pink; z-index: 3;">
pink; z-index:3;
</div>

yellow; z-index:1;

red; z-index:2;

pink; z-index:3;