« CSS « Understanding CSS z-index
Stacking context example 2
This is a very simple example, but it is the key for understanding the concept of stacking context. There are the same four DIVs of the previous example, but now z-index properties are assigned on both levels of the hierarchy.
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You can see that DIV #2 (z-index: 2) is above DIV #3 (z-index: 1), because they both belong to the same stacking context (the root one), so z-index values rule how elements are stacked.
What can be considered strange is that DIV #2 (z-index: 2) is above DIV #4 (z-index: 10), despite their z-index values. The reason is that they do not belong to the same stacking context. DIV #4 belongs to the stacking context created by DIV #3, and as explained previously DIV #3 (and all its content) is under DIV #2.
To better understand the situation, this is the stacking context hierarchy:
- root stacking context
- DIV #2 (z-index 2)
- DIV #3 (z-index 1)
- DIV #4 (z-index 10)
Example source code
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"https://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head><style type="text/css">
div { font: 12px Arial; }
span.bold { font-weight: bold; }
#div2 { z-index: 2; }
#div3 { z-index: 1; }
#div4 { z-index: 10; }
#div1,#div3 {
height: 80px;
position: relative;
border: 1px dashed #669966;
background-color: #ccffcc;
padding-left: 5px;
}
#div2 {
opacity: 0.8;
position: absolute;
width: 150px;
height: 200px;
top: 20px;
left: 170px;
border: 1px dashed #990000;
background-color: #ffdddd;
text-align: center;
}
#div4 {
opacity: 0.8;
position: absolute;
width: 200px;
height: 70px;
top: 65px;
left: 50px;
border: 1px dashed #000099;
background-color: #ddddff;
text-align: left;
padding-left: 10px;
}
</style></head>
<body>
<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: 2;
</div>
</div>
<br />
<div id="div3">
<br /><span class="bold">DIV #3</span>
<br />position: relative;
<br />z-index: 1;
<div id="div4">
<br /><span class="bold">DIV #4</span>
<br />position: absolute;
<br />z-index: 10;
</div>
</div>
</body></html>
See also
- Stacking without z-index : Default stacking rules
- Stacking and float : How floating elements are handled
- Adding z-index : Using z-index to change default 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 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 Creative Commons: Attribution-Sharealike license
- Last Updated Date: July 9th, 2005