Z-Index and Pseudo-Elements: A Case Study

In CSS, the z-index property specifies the stacking order of elements on a page, determining which elements appear "in front" or "behind" others. However, when it comes to pseudo-elements, such as ::before or ::after, their interaction with z-index can sometimes be less than straightforward.

Consider a scenario where we create a header element with a ::before pseudo-element, as described in the problem statement. We intend to keep the pseudo-element behind the header element, but when we apply a z-index to the header, the pseudo-element unexpectedly comes to the foreground.

The explanation lies in the nature of pseudo-elements themselves. As stated in the CSS specification, ::before and ::after pseudo-elements "interact with other boxes as if they were real elements inserted just inside their associated element." This means that the pseudo-element is effectively nested within the parent element.

When we apply a z-index to the header element, it creates a new stacking context. A stacking context is a 3D space where elements are rendered in order of their z-index values. Elements within the same stacking context cannot overlap one another.

In this case, the pseudo-element is positioned within the same stacking context as the header element. Since it is considered a nested element, it cannot float behind the header because that would require it to escape the stacking context.

To resolve this issue, one solution is to create a separate element before the header, as suggested in the answer. This element will act as a container for the header and the pseudo-element, and its z-index will correctly stack the header in front.