Understanding Z – Index Property in CSS Part 2

In part 1 of this series, we have get some fundamental understanding of Z-index concept and about natural stacking order in software programming. I would like to clarify that z-index is only applicable where position properties exist otherwise the element without position property will render first and will remain at the bottom of staking order even though it is top in HTML hierarchy.

Stacking order without Z-index
Stacking order without Z-index

For instance, in above image, we have taken DIV elements in their natural order in HTML like from DIV #1 to DIV #5. In this example Div #1 to Div #4 are appearing in natural HTML stacking order though they don’t have rendered any Z-index in below example code. Whereas Div #5 is seating at the bottom/behind the stacking order because it doesn’t have any position property allocated.

div {
font: 12px Arial;
}
span.bold { font-weight: bold; }
#normdiv {
height: 70px;
border: 1px dashed #999966;
background-color: #ffffcc;
margin: 0px 50px 0px 50px;
text-align: center;
}
#reldiv1 {
opacity: 0.7;
height: 100px;
position: relative;
top: 30px;
border: 1px dashed #669966;
background-color: #ccffcc;
margin: 0px 50px 0px 50px;
text-align: center;
}
#reldiv2 {
opacity: 0.7;
height: 100px;
position: relative;
top: 15px;
left: 20px;
border: 1px dashed #669966;
background-color: #ccffcc;
margin: 0px 50px 0px 50px;
text-align: center;
}
#absdiv1 {
opacity: 0.7;
position: absolute;
width: 150px;
height: 350px;
top: 10px;
left: 10px;
border: 1px dashed #990000;
background-color: #ffdddd;
text-align: center;
}
#absdiv2 {
opacity: 0.7;
position: absolute;
width: 150px;
height: 350px;
top: 10px;
right: 10px;
border: 1px dashed #990000;
background-color: #ffdddd;
text-align: center;
}
</style></head>
<body>
<br /><br />
<div id="absdiv1">
<br /><span class="bold">DIV #1</span>
<br />position: absolute;
</div>
<div id="reldiv1">
<br /><span class="bold">DIV #2</span>
<br />position: relative;
</div>
<div id="reldiv2">
<br /><span class="bold">DIV #3</span>
<br />position: relative;
</div>
<div id="absdiv2">
<br /><span class="bold">DIV #4</span>
<br />position: absolute;
</div>
<div id="normdiv">
<br /><span class="bold">DIV #5</span>
<br />no positioning
</div>

 

Stacking order with Z-index
Stacking order with Z-index

Now, if we apply z-index to each Div element and try to change their natural stacking orders in our customized ways, we would have definite success when the Div element has position property applied before. Otherwise, as in case of Div #5 it will remain at the bottom of visual hierarchy even though it has z-index 8, and at the top in HTML code hierarchy.

This becomes obvious in the example below.

div {
opacity: 0.7;
font: 12px Arial;
}
span.bold { font-weight: bold; }
#normdiv {
z-index: 8;
height: 70px;
border: 1px dashed #999966;
background-color: #ffffcc;
margin: 0px 50px 0px 50px;
text-align: center;
}
#reldiv1 {
z-index: 3;
height: 100px;
position: relative;
top: 30px;
border: 1px dashed #669966;
background-color: #ccffcc;
margin: 0px 50px 0px 50px;
text-align: center;
}
#reldiv2 {
z-index: 2;
height: 100px;
position: relative;
top: 15px;
left: 20px;
border: 1px dashed #669966;
background-color: #ccffcc;
margin: 0px 50px 0px 50px;
text-align: center;
}
#absdiv1 {
z-index: 5;
position: absolute;
width: 150px;
height: 350px;
top: 10px;
left: 10px;
border: 1px dashed #990000;
background-color: #ffdddd;
text-align: center;
}
#absdiv2 {
z-index: 1;
position: absolute;
width: 150px;
height: 350px;
top: 10px;
right: 10px;
border: 1px dashed #990000;
background-color: #ffdddd;
text-align: center;
}
</style></head>
<body>
<br /><br />
<div id="absdiv1">
<br /><span class="bold">DIV #1</span>
<br />position: absolute;
<br />z-index: 5;
</div>
<div id="reldiv1">
<br /><span class="bold">DIV #2</span>
<br />position: relative;
<br />z-index: 3;
</div>
<div id="reldiv2">
<br /><span class="bold">DIV #3</span>
<br />position: relative;
<br />z-index: 2;
</div>
<div id="absdiv2">
<br /><span class="bold">DIV #4</span>
<br />position: absolute;
<br />z-index: 1;
</div>
<div id="normdiv">
<br /><span class="bold">DIV #5</span>
<br />no positioning
<br />z-index: 8;
</div>

Therefore, web developers at Lujayn has concluded that z-index will work only when an element has a position property explicitly set to absolute, relative, or fixed.

 

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