HTML ソース

<div id="zindex2">
z-index の値が 2<br />
要素の順番は 1

<div id="zindex1">
z-index の値が 1<br />
要素の順番は 2

<div id="zindex0">
z-index の値が 0<br />
要素の順番は 3

</div>

</div>

</div>

CSS ソース

div#zindex0 {
position: absolute;
top: 50px;
left: 180px;
z-index: -1;
width: 200px;
height: 200px;
background: yellow;
}

div#zindex1 {
position: absolute;
top: 80px;
left: 210px;
z-index: 1;
width: 200px;
height: 200px;
background: pink;
}

div#zindex2 {
position: absolute;
top: 110px;
left: 240px;
z-index: 2;
width: 200px;
height: 200px;
background: lime;
}