Stacking context example 2
Description
This is a basic 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)
- DIV #4 (
- DIV #2 (
Note: It is worth remembering that 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
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: 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>
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;
}