Stacking context example 1 - CSS: Cascading Style Sheets 编辑

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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.

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

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.

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 #4, belongs to the same root stacking context.

In terms of stacking contexts, DIV #1 and DIV #3 are 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

HTML

<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;
   </div>
</div>

<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;
   </div>
</div>

</body></html>

CSS

.bold {
    font-weight: bold;
    font: 12px Arial;
}
#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;
}

Result

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.
• Using z-index: How to use z-index to change default stacking.
• The stacking context: Notes on the stacking context.
• 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: July 9, 2005