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Stacking context example 2 - CSS: Cascading Style Sheets 编辑

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.

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)

Note: It is worth remembering that in general the HTML hierarchy is different from the stacking context hierarchy. In the stacking context hierarchy, elements that do not create a stacking context are collapsed on their parent.

Example source code

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://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>

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: July 9, 2005

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