CSS: Demystifying the z-index property
Tags: css, webdev
Published: 2023-04-18 23:39:33

by [Chidera Okechukwu](https://blog.openreplay.com/authors/chidera-okechukwu)

As a web developer, you may have encountered the `z-index` property in CSS when dealing with the layout and stacking of HTML elements. It is an essential tool for controlling the visual order of HTML elements on a web page. However, understanding how it works can be tricky, and many developers find using it difficult. 

However, the `z-index` property can be confusing, and it is often misused or misunderstood by web designers and developers. 

In this article, we will explore the `z-index` property in CSS and explain how it works, what it does, and how to use it effectively. 

## The Stacking Context
Before we dive deep into the `z-index` property, let's look at a term called **Stacking Context**.
The idea of stacking context is used in web development to control how things are presented and layered on a web page. When creating and executing web layouts, particularly ones that have overlapping elements, it's crucial to understand the stacking context.

Each element in a stacking context is given a stacking order, establishing its location along the z-axis. The element's position in the DOM, its CSS values, and whether it has a child stacking context are some criteria that affect the stacking order.

An element is positioned relative to its parent element's stacking order when placed within a stacking context. Higher stacking order components will be shown on top of lower stacking order ones. However, a number of CSS properties, including `z-index`, `opacity`, `transform`, and `mix-blend-mode`, can influence how an element is stacked within a stacking context.

It's critical to remember that stacking contexts can be nested, which enables a child element to have a separate stacking context and stacking order. In this situation, the stacking context of the parent element will determine the child element's stacking order.

The `z-index` property only affects positioned items; it has no bearing on non-positioned elements, it is vital to note. Moreover, the `z-index` property only affects the stacking order of the positioned elements; it does not impact the document flow or the layout of other items on the page.
  
## How does the `z-index` property work?
The `z-index` property is a CSS property that specifies the stacking order of positioned elements. It determines which elements should appear in front of or behind other elements on the screen.

The `z-index` property only works on elements that have been positioned using one of the following CSS positioning properties:

```css
position: relative / absolute / fixed
```

The `z-index` property takes a numeric value as its input, including negative values with higher numbers indicating that an element should be placed on top of other elements. Elements with a higher `z-index` value will appear in front of elements with a lower `z-index` value. If two elements have the same `z-index` value, the element appearing later in the HTML document will be placed on top.

Here's an example of how the `z-index` property can be used:

HTML

```html
<div class="container">
  <div class="box box-1"></div>
  <div class="box box-2"></div>
  <div class="box box-3"></div>
</div>
```

CSS 

```css
.container {
  position: relative;
}

.box {
  position: absolute;
  width: 100px;
  height: 100px;
}

.box-1 {
  background-color: red;
  z-index: 3;
}

.box-2 {
  background-color: green;
  top: 56px;
  left: 20px;
  z-index: 2;
}

.box-3 {
  background-color: blue;
  top: 96px;
  left: 40px;
  z-index: 1;
}
```

  
Your output result will be: 


![-](https://blog.openreplay.com/images/css--demystiying-the-z-property/images/Ymbqu1w.png)


This example shows three boxes with different `z-index` values. The container has a relative position, which allows the absolute-positioned boxes to be positioned inside.

The box with the highest `z-index` (box-1) will be on top of the other boxes, followed by box-2, and then box-3. The `z-index` property determines the stacking order of the boxes.

It's important to note that the `z-index` property only applies to elements positioned using one of the CSS positioning properties mentioned above. If an element is not positioned, then the `z-index` property will have no effect. Static elements will always be displayed behind positioned elements, regardless of their `z-index` value.
  
## Possible `z-index` values 

Some common values for `z-index` are:

- `<integer>`: Elements with higher values appear on top of elements with lower values. While elements with lower values appear on top of elements with higher values. Generally, it is best to use positive numbers for the `z-index` value and avoid using negative values, and negative values can be confusing and may cause unexpected behavior.

- `auto`: The `auto` value in `z-index` indicates that an element's stacking order should be determined automatically based on the default order of the document. This means the element will be placed in the stacking order according to its position in the DOM hierarchy without any explicit or relative values being set.

There are also global values that can be used with the `z-index` property:

- `initial` value: Setting `z-index` to initial is equivalent to not setting it at all. It sets the property to its default value, which is auto. So, if you set the `z-index` initial on an element, it will be positioned in the stacking order based on its position in the document and the default order of the rendering tree.

- `unset` value: The `unset` value in `z-index` is used to reset the `z-index` property back to its default value, allowing the element to follow the natural stacking order based on its position in the HTML structure. It is often used when you want to remove or reset any previously defined `z-index` value on an element.

- `revert` value: In the case of `z-index`, revert can be used to undo any changes made to the stacking order of elements. When the `z-index` of an element is set to revert, it will be placed back in its original position in the stacking order, as if no `z-index` value had been set for it.
  


## How Does `z-index` Property Affect Stacking Contexts?
The `z-index` property creates stacking contexts, determining the order in which elements are stacked. A stacking context is created when an HTML element has a position property value other than static, and one of the following properties is set: `z-index`, `opacity`, or `transform`.
  


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## Practical Examples of `z-index` Property
Now that we've covered the basics of the `z-index` property and how it interacts with other CSS layout properties, let's look at some practical examples of using the `z-index` property.

### Creating Dropdown Menus
Another common use case for the `z-index` property is creating dropdown menus. Dropdown menus typically appear on top of other page elements, so we need to use the `z-index` property to ensure that they appear in front of other content.

To achieve this effect, we can use the following CSS code:


HTML:
```html
<div class="dropdown">
  <button class="dropbutton">Dropdown</button>
  <div class="dropdown-options">
    <a href="#">Option 1</a>
    <a href="#">Option 2</a>
    <a href="#">Option 3</a>
  </div>
</div>
```
CSS:
```css
body {
  background-color: black;
}

.dropbutton {
  background-color: #04AA6D;
  color: white;
  padding: 16px;
  font-size: 16px;
  border: none;
}

.dropdown {
  position: relative;
  display: inline-block;
}

.dropdown-options {
  display: none;
  position: absolute;
  background-color: #f1f1f1;
  min-width: 160px;
  box-shadow: 0px 8px 16px 0px rgba(0, 0, 0, 0.2);
  z-index: 1;
}

.dropdown-options a {
  color: black;
  padding: 12px 16px;
  text-decoration: none;
  display: block;
}

.dropdown-options a:hover {
  background-color: #ddd;
}

.dropdown:hover .dropdown-options {
  display: block;
}

.dropdown:hover .dropbutton {
  background-color: red;
}
```
  
![-](https://blog.openreplay.com/images/css--demystiying-the-z-property/images/emddAtF.gif)



  
### Creating Modal Dialog Boxes
Finally, the` z-index` property is often used to create modal dialog boxes, which are temporary windows that appear on top of other page content.

To create a modal dialog box, we can use the following CSS code:

CSS
```css
.modal {
  position: fixed;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  z-index: 3;
}

.overlay {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-color: rgba(0, 0, 0, 0.5);
  z-index: 2;
}
``` 
  
In this case, the modal dialog box has a `z-index` of 3, which ensures that it appears on top of all other page content. The overlay, a semi-transparent layer covering the entire page, has a lower `z-index` of 2. This ensures the overlay appears on top of all other page content except for the modal dialog box.
  
## Common issues with the z-index property

Several common issues can arise when using the `z-index` property. Here are some of the most common issues:

- Inconsistent behavior across browsers: The `z-index` property can behave differently across different browsers, which can lead to unexpected results. It is important to test the layout on different browsers to ensure that the stacking order is maintained.

- Conflicts with other elements: The `z-index` property can conflict with other elements in the document, especially when used in conjunction with absolute or fixed positioning. It is important to carefully consider the layout and ensure that the `z-index` values are assigned appropriately.

- Overlapping elements: Overlapping elements can occur when the `z-index` values are not assigned correctly. This can lead to elements appearing on top of each other, making it difficult to interact with the underlying elements.

- `z-index` stacking context: The `z-index` property can create a stacking context, which can affect the stacking order of elements within the context. It is important to understand how the stacking context works and how it can affect the layout. `z-index` conflicts can occur when two or more elements have the same `z-index` value or when the position and parent elements of two elements conflict. In such cases, the element that appears later in the HTML code will be placed on top. To avoid `z-index` conflicts, you should always use unique `z-index` values for each element.

  
## Best practices for using `z-index`

Now that we understand how the `z-index` property works and how it can be affected by stacking contexts, let's take a look at some best practices for using `z-index` effectively.

- You must always specify a `z-index` value for positioned elements: As mentioned earlier, it is a good practice to always specify a `z-index` value for positioned elements, even if the value is simply 0. This can help prevent unexpected behavior and ensure that elements are displayed in the intended order.

- Use meaningful values for `z-index`: When assigning values to the `z-index` property, it is important to use meaningful values that are easy to understand and remember. For example, you might use a value of 1 for the header of a page, a value of 2 for the content, and a value of 3 for the footer.

- Avoid using extremely high or low values for z-index: While it is possible to use very high or low values for the `z-index` property, it is generally not recommended. Extremely high values can cause performance issues and make it difficult to maintain the code, while extremely low values may be overwritten by other elements or not display as expected.

- Be aware of stacking contexts: As we have seen, stacking contexts can affect the stacking order of elements on a web page. When working with `z-index`, it is important to be aware of stacking contexts and how they can impact the display of elements. In general, it is best to avoid creating unnecessary stacking contexts and to ensure that elements are placed in the correct stacking order.

## Conclusion
The `z-Index` property in CSS is a powerful tool that can be used to enhance your web design, but it's important to use it correctly to avoid layout issues. By understanding how the `z-Index` property works and by following the tips outlined in this article, you can use the `z-Index` property effectively to create visually appealing designs.