Why does my ::before pseudo-element not appear behind its parent element when I apply a z-index to the parent element?

Z-index and Pseudo-Elements: An In-Depth Exploration

In this article, we will delve into the fascinating world of CSS stacking contexts and how they interact with pseudo-elements, specifically the ::before pseudo-element. We will explore a common problem encountered when using z-index with ::before and provide a comprehensive solution that ensures the correct stacking behavior.

The Problem

Consider the following CSS:

<code class="css">header { 
    background: yellow;
    position:relative;
    height: 100px;
    width: 100px;
    z-index:30;
}

header::before { 
    content:"Hide you behind!";
    background: red;
    position:absolute;
    height: 100px;
    width: 100px;
    top:25px;
    left:25px;
    z-index:-1;
}</code>

In this code, we have created a 'header' element with a ::before pseudo-element. The desired behavior is for the yellow 'header' element to be in the foreground and the red ::before pseudo-element to be in the background. However, when we apply a z-index of 30 to the 'header' element, the ::before pseudo-element no longer appears behind the parent.

Explanation

To understand this behavior, we need to delve into the concept of CSS stacking contexts. A stacking context defines a set of layers where elements are stacked in front of or behind each other based on their z-index values. The creation of a stacking context is triggered by certain CSS properties, including z-index.

In the example above, when we apply a z-index of 30 to the 'header' element, it creates a new stacking context. This new stacking context separates the 'header' element and its children (including the ::before pseudo-element) from the rest of the document flow.

The ::before pseudo-element is considered a child element within the 'header' element's stacking context. Therefore, it is constrained by the stacking context's boundaries and cannot float outside of it to appear behind the 'header' element.

Solution

To solve this problem, we cannot simply increase the z-index of the ::before pseudo-element. Instead, we need to create a new stacking context for the ::before pseudo-element so that it can stack correctly behind the 'header' element.

One way to achieve this is to wrap the 'header' element in another element, as shown below:

<code class="css"><div class="wrapper">
  <header>
    ...
  </header>
</div>

.wrapper { 
    position:relative;
    z-index:30;
}</code>

By creating this new element and assigning it a z-index, we are essentially creating a new stacking context that is separate from the 'header' element's stacking context. This allows the ::before pseudo-element to stack correctly behind the 'header' element within the new stacking context.

Conclusion

Understanding CSS stacking contexts and pseudo-element interactions is essential for creating complex layouts and ensuring proper element positioning. By carefully managing stacking contexts, we can control the layering of elements and achieve the desired visual effects.

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