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Stacking context example 1 Redirect 2

« CSS « Understanding CSS z-index

Stacking context example 1

Let's start with a basic example. In the root stacking context we have two DIVs (DIV #1 and DIV #3), both relatively positioned, but without z-index properties. Inside DIV #1 there is an absolutely positioned DIV #2, while in DIV #3 there is an absolutely positioned DIV #4, both without z-index properties.

The only stacking context is the root context. Without z-indexes, elements are stacked in order of occurrence.

Stacking context example 1

If DIV #2 is assigned a positive (non-zero and non-auto) z-index value, it is rendered above all the other DIVs.

Stacking context example 1

Then if DIV #4 is also assigned a positive z-index greater than DIV #2's z-index, it is rendered above all the other DIVs including DIV #2.

Stacking context example 1

In this last example you can see that DIV #2 and DIV #4 are not siblings, because they belong to different parents in the HTML elements' hierarchy. Even so, stacking of DIV #4 with respect of DIV #2 can be controlled through z-index. It happens that, since DIV #1 and DIV #3 are not assigned any z-index value, they do not create a stacking context. This means that all their content, including DIV #2 and DIV #3, belongs to the same root stacking context.

In terms of stacking contexts, DIV #1 and DIV #3 are simply assimilated into the root element, and the resulting hierarchy is the following:

  • root stacking context
    • DIV #2 (z-index 1)
    • DIV #4 (z-index 2)
Note: DIV #1 and DIV #3 are not translucent. It is important to remember that assigning an opacity less than 1 to a positioned element implicitly creates a stacking context, just like adding a z-index value. And this example shows what happens when a parent element does not create a stacking context.

Example source code

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
<head><style type="text/css">

div { font: 12px Arial; }

span.bold { font-weight: bold; }

#div1,#div3 {
   height: 80px;
   position: relative;
   border: 1px dashed #669966;
   background-color: #ccffcc;
   padding-left: 5px;

#div2 {
   opacity: 0.8;
   z-index: 1;
   position: absolute;
   width: 150px;
   height: 200px;
   top: 20px;
   left: 170px;
   border: 1px dashed #990000;
   background-color: #ffdddd;
   text-align: center;

#div4 {
   opacity: 0.8;
   z-index: 2;
   position: absolute;
   width: 200px;
   height: 70px;
   top: 65px;
   left: 50px;
   border: 1px dashed #000099;
   background-color: #ddddff;
   text-align: left;
   padding-left: 10px;



<br />

<div id="div1">
<br /><span class="bold">DIV #1</span>
<br />position: relative;
   <div id="div2">
   <br /><span class="bold">DIV #2</span>
   <br />position: absolute;
   <br />z-index: 1;

<br />

<div id="div3">
<br /><span class="bold">DIV #3</span>
<br />position: relative;
   <div id="div4">
   <br /><span class="bold">DIV #4</span>
   <br />position: absolute;
   <br />z-index: 2;


See also

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Last updated by: Sheppy,