To change the default stack order positioned elements (position property set to relative, absolute or fixed), use the z-index property.

The higher the z-index, the higher up in the stacking context (on the z-axis) it is placed.

Example

In the example below, a z-index value of 3 puts green on top, a z-index of 2 puts red just under it, and a z-index of 1 puts blue under that.

HTML

<div id="div1"></div>
<div id="div2"></div>
<div id="div3"></div>

CSS

div {
    position: absolute;
    height: 200px;
    width: 200px;
}
div#div1 {
    z-index: 1;
    left: 0px;
    top: 0px;
    background-color: blue;
}
div#div2 {
    z-index: 3;
    left: 100px;
    top: 100px;
    background-color: green;
}
div#div3 {
    z-index: 2;
    left: 50px;
    top: 150px;
    background-color: red;
}

This creates the following effect:

See a working example at JSFiddle.

Syntax

z-index: [ number ] | auto;

Parameter | Details | — | — |

number | An integer value. A higher number is higher on the z-index stack. 0 is the default value. Negative values are allowed. |

auto | Gives the element the same stacking context as its parent. (Default) |

Remarks

All elements are laid out in a 3D axis in CSS, including a depth axis, measured by the z-index property. z-index only works on positioned elements: (see: Why does z-index need a defined position to work?). The only value where it is ignored is the default value, static.

Read about the z-index property and Stacking Contexts in the CSS Specification on layered presentation and at the Mozilla Developer Network.