How Can You Position a Pseudo-Element Behind Its Parent When The Parent Has a Z-Index?

Manipulating Z-Index for Pseudo-Elements

In CSS, applying a z-index to parent elements can impact the stacking behavior of pseudo-elements. This article explores the issue and provides a solution.

The Problem

When a parent element is given a z-index, the pseudo-element it contains becomes confined within the parent's stacking context. The pseudo-element can no longer extend outside this context to appear behind the parent.

Understanding Stacking Contexts

Creating a new Stacking Context essentially creates a separate layer for the element in question. Elements within that layer are isolated from elements outside the context. In this case, the pseudo-element, which typically exists inside the parent's layer, is now trapped within the parent's new stacking context.

The Solution

To resolve the issue, CSS allows one to break out of a stacking context. This is typically achieved by introducing a new element before the parent element that requires a specific stacking order. This "wrapper" element forms its own stacking context, allowing the pseudo-element to remain behind it while still affecting the stacking order of subsequent elements.

Code Example

Consider the following code:

<code class="css"><div id="wrapper">
  <header>
    <span class="pseudo">Hide me!</span>
  </header>
</div>

#wrapper {
  position: relative;
  z-index: 30;
}

header {
  position: relative;
  height: 100px;
  width: 100px;
  background-color: yellow;
}

.pseudo {
  position: absolute;
  top: 25px;
  left: 25px;
  height: 100px;
  width: 100px;
  background-color: red;
  z-index: -1;
}</code>

By creating the "wrapper" div and setting its position to relative and z-index to 30, we establish a new stacking context outside which the pseudo-element can exist. The pseudo-element can now appear behind the yellow "header" element, as intended.

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