Understanding Z-Index and Stacking Context in CSS

In the world of web and page design, there is a concept that works with CSS properties, which is very important and is called z-index. This property helps us manage overlapping elements on top of each other. One thing to understand is that z-index only works on elements that are positioned. That is, their position must be other than static, like absolute, relative, or fixed.

When we want to position several elements on a page, the z-index values can help us specify which elements should be on top of which and which ones should be behind. This feature is particularly useful in structures that require displaying texts, windows, or anything related to graphical representation, greatly enhancing user experience. Correctly applied styles to elements create a stacking context that is of great importance to designers.

Now you might ask, what is a stacking context? It is an environment that controls the layering of a group of elements, allowing us to manage their display order. Any element with a position type other than static, accompanied by a specified z-index, creates a new context that allows those elements to stack based on z-index.

For example, let’s assume we want a background image overlaid with some text. To do this correctly, we need to use z-index effectively so that the user can clearly see the content without any interruption from the background. Below is a sample code style to make it easier to understand:

<style>
  .background {
    position: absolute;
    z-index: 1;
  }
  .foreground {
    position: absolute;
    z-index: 2;
  }
</style>
<div class="background">Background text</div>
<div class="foreground">Foreground text</div>

And now we get to the sequential explanations:

First, in <style> we define the first segment of the style that creates an element with the class background which is set to position: absolute and z-index equals one. This means this element will be at a specific position compared to other elements, and in the first layer of the stacking context.

Second, the second section code defines an element with class foreground which has a higher z-index value of two. This implies that this element is displayed above the background, signifying a correct and appropriate layering style.