CSS Z-Index

Introduction to CSS Z-Index

The z-index property in CSS controls the vertical stacking order of elements that overlap. It is pivotal in determining which elements appear on top of others on a webpage. Z-index only affects elements whose position is set to something other than the default static positioning.

Basic Usage of Z-Index

The z-index property can be assigned to any positioned element (elements with position values of relative, absolute, fixed, or sticky). A higher z-index value means the element will appear on top of elements with lower z-index values.

Understanding Stacking Context

A stacking context is a 3D conceptualization of HTML elements along an imaginary z-axis relative to the user viewing the webpage. This context is formed by elements that meet specific criteria, such as having a position value other than static and a z-index value other than auto. Each stacking context is isolated from its siblings, meaning that z-index values are only considered within the same context.

Creating a New Stacking Context

New stacking contexts can be formed not only by setting a z-index but also by CSS properties such as opacity, transform, filter, and flex container with a z-index. Elements that create a new stacking context will stack their children independently from the rest of the document.

Practical Example of Z-Index

Consider a scenario with multiple overlapping elements. Using z-index, you can control their rendering order:

In this example, the red box is on top of the blue box due to its higher z-index value.

Common Pitfalls and Tips

• Remember that z-index only works on positioned elements (not static).
• Check for parent elements that might have created a new stacking context affecting the z-index behavior.
• Use z-index values sparingly and document them well in your CSS to avoid confusion.