Using z-index - CSS: Cascading Style Sheets 编辑

The first part of this article, Stacking without the z-index property, explains how stacking is arranged by default. If you want to create a custom stacking order, you can use the z-index property on a positioned element.

The z-index property can be specified with an integer value (positive, zero, or negative), which represents the position of the element along the z-axis. If you are not familiar with the z-axis, imagine the page as a stack of layers, each one having a number. Layers are rendered in numerical order, with larger numbers above smaller numbers.

• bottom layer (farthest from the observer)
• ...
• Layer -3
• Layer -2
• Layer -1
• Layer 0 (default rendering layer)
• Layer 1
• Layer 2
• Layer 3
• ...
• top layer (closest to the observer)

Notes:

• When no z-index property is specified, elements are rendered on the default rendering layer 0 (zero).
• If several elements share the same z-index value (i.e., they are placed on the same layer), stacking rules explained in the section Stacking without the z-index property apply.

In the following example, the layers' stacking order is rearranged using z-index. The z-index of element #5 has no effect since it is not a positioned element.

Source code for the example

HTML

<div id="abs1">
  <b>DIV #1</b>
  <br />position: absolute;
  <br />z-index: 5;
</div>

<div id="rel1">
  <b>DIV #2</b>
  <br />position: relative;
  <br />z-index: 3;
</div>

<div id="rel2">
  <b>DIV #3</b>
  <br />position: relative;
  <br />z-index: 2;
</div>

<div id="abs2">
  <b>DIV #4</b>
  <br />position: absolute;
  <br />z-index: 1;
</div>

<div id="sta1">
  <b>DIV #5</b>
  <br />no positioning
  <br />z-index: 8;
</div>

CSS

div {
  padding: 10px;
  opacity: 0.7;
  text-align: center;
}

b {
  font-family: sans-serif;
}

#abs1 {
  z-index: 5;
  position: absolute;
  width: 150px;
  height: 350px;
  top: 10px;
  left: 10px;
  border: 1px dashed #900;
  background-color: #fdd;
}

#rel1 {
  z-index: 3;
  height: 100px;
  position: relative;
  top: 30px;
  border: 1px dashed #696;
  background-color: #cfc;
  margin: 0px 50px 0px 50px;
}

#rel2 {
  z-index: 2;
  height: 100px;
  position: relative;
  top: 15px;
  left: 20px;
  border: 1px dashed #696;
  background-color: #cfc;
  margin: 0px 50px 0px 50px;
}

#abs2 {
  z-index: 1;
  position: absolute;
  width: 150px;
  height: 350px;
  top: 10px;
  right: 10px;
  border: 1px dashed #900;
  background-color: #fdd;
}

#sta1 {
  z-index: 8;
  height: 70px;
  border: 1px dashed #996;
  background-color: #ffc;
  margin: 0px 50px 0px 50px;
}

See also

• Stacking without the z-index property: The stacking rules that apply when z-index is not used.
• Stacking with floated blocks: How floating elements are handled with stacking.
• The stacking context: Notes on the stacking context.
• Stacking context example 1: 2-level HTML hierarchy, z-index on the last level
• Stacking context example 2: 2-level HTML hierarchy, z-index on all levels
• Stacking context example 3: 3-level HTML hierarchy, z-index on the second level

Original Document Information

• Author(s): Paolo Lombardi
• This article is the English translation of an article I wrote in Italian for YappY. I grant the right to share all the content under the Creative Commons: Attribution-Sharealike license.
• Last Updated Date: November 3, 2014