CSS Z-index

The CSS z-index property controls the stacking order of elements on a webpage. It is a powerful tool for handling overlapping elements and ensuring that some elements appear above others. This article will provide a detailed understanding of z-index, its usage, and practical examples to make it easy to implement in your projects.

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01. Overview of the z-index Property

The z-index property is used to specify the stacking order of elements that overlap. It only affects elements that have a position value of relative, absolute, fixed, or sticky. The higher the z-index value, the closer the element will be to the viewer, making it appear on top of elements with lower z-index values.

Syntax:

element {
  position: value;  /* relative, absolute, fixed, sticky */
  z-index: value;
}

Value: The z-index value can be a positive or negative integer or zero. The default value is auto, which means the element will follow the stacking order based on the document flow.

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02. Understanding z-index Behavior

2.1 Stacking Context

Each positioned element creates a new stacking context. This means that z-index values only affect elements within the same stacking context. When elements with different stacking contexts overlap, the stacking context's z-index values determine their order.

2.2 Stacking Order

The stacking order is based on the following hierarchy:

• 1. Background and borders of the root element.
• 2. Non-positioned elements (i.e., static positioning).
• 3. Positioned elements in the stacking order defined by z-index.

Elements with a higher z-index will appear above elements with a lower z-index.

2.3 Example

<style>
  .box1 {
    position: absolute;
    top: 50px;
    left: 50px;
    z-index: 1;
    background-color: lightblue;
    width: 100px;
    height: 100px;
  }
  .box2 {
    position: absolute;
    top: 70px;
    left: 70px;
    z-index: 2;
    background-color: lightcoral;
    width: 100px;
    height: 100px;
  }
</style>

<div class="box1">Box 1</div>
<div class="box2">Box 2</div>

In this example, Box 2 will appear on top of Box 1 due to its higher z-index value.

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03. Practical Applications of z-index

• Modals and Popups: Use z-index to ensure that modals and popups appear above the main content.
• Dropdown Menus: Use z-index to ensure that dropdowns are displayed above other elements on the page.
• Tooltips: Use z-index to position tooltips above elements when hovering over them.

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04. Browser Compatibility

The z-index property is widely supported across all modern browsers. However, it's important to remember that z-index only works on elements that have a position value other than static. Below is a table outlining the support for different browsers:

Browser	Support
Chrome	Fully Supported
Firefox	Fully Supported
Safari	Fully Supported
Edge	Fully Supported
Internet Explorer	Fully Supported from IE 7+

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05. Conclusion

The z-index property is essential for managing the stacking order of overlapping elements on a webpage. By mastering its usage, you can create dynamic and layered designs with precision. Always remember that z-index works only on positioned elements, and be aware of how stacking contexts affect element layering.