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CSS z-index property (CSS Layout)

The z-index property specifies the stack order of an element.

The Stack order is the order in which the elements are positioned one above another i.e. in dialog, menus etc.

The z-index Property

When elements are positioned, they can overlap other elements.

The z-index property specifies the stack order of an element (which element should be placed in front of, or behind, the others).

An element can have a positive or negative stack order:

This is a heading

Tree

Because the image has a z-index of -1, it will be placed behind the text.

Example

img {
position: absolute;
left: 0px;
top: 0px;
z-index: -1;
}

Note: z-index only works on positioned elements (position: absolute, position: relative, position: fixed, or position: sticky) and flex items (elements that are direct children of display: flex elements).

Another z-index Example

Example

Here we see that an element with greater stack order is always above an element with a lower stack order:

<html>
<head>
<style>
.container {
position: relative;
}

.black-box {
position: relative;
z-index: 1;
border: 2px solid black;
height: 100px;
margin: 30px;
}

.gray-box {
position: absolute;
z-index: 3;
background: lightgray;
height: 60px;
width: 70%;
left: 50px;
top: 50px;
}

.green-box {
position: absolute;
z-index: 2;
background: lightgreen;
width: 35%;
left: 270px;
top: -15px;
height: 100px;
}
</style>
</head>
<body>

<div class="container">
<div class="black-box">Black box</div>
<div class="gray-box">Gray box</div>
<div class="green-box">Green box</div>
</div>

</body>
</html>

Without z-index

If two positioned elements overlap each other without a z-index specified, the element defined last in the HTML code will be shown on top.

Example

Same example as above, but here with no z-index specified:

<html>
<head>
<style>
.container {
position: relative;
}

.black-box {
position: relative;
border: 2px solid black;
height: 100px;
margin: 30px;
}

.gray-box {
position: absolute;
background: lightgray;
height: 60px;
width: 70%;
left: 50px;
top: 50px;
}

.green-box {
position: absolute;
background: lightgreen;
width: 35%;
left: 270px;
top: -15px;
height: 100px;
}
</style>
</head>
<body>

<div class="container">
<div class="black-box">Black box</div>
<div class="gray-box">Gray box</div>
<div class="green-box">Green box</div>
</div>

</body>
</html>

 CSS Property

Property Description
z-index Sets the stack order of an element
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