How Does CSS z-index Control Element Stacking Order?

Understanding z-index Stacking Order

Stacking order in CSS refers to the placement of elements on a web page. The z-index property controls the order in which elements are stacked, with higher values being placed in front of lower values along the z-axis.

Rules for Stacking Elements

The rules for determining the stack order are based on the following:

• Elements without a position property are stacked in the order they appear in the source code.
• Elements with a position set to static are also stacked in the order they appear.
• Elements with a position set to absolute, relative, fixed, or sticky can have their stack order controlled with the z-index property.

Mixed Sibling and Nested Elements

• When sibling elements have different positions, positioned elements will be stacked in front of non-positioned elements, regardless of their appearance in the source code.
• Nested elements follow the same rule, with positioned elements being stacked in front of their non-positioned parent elements.

Stacking Contexts

When an element is positioned (e.g., with position: absolute), it creates a stacking context. Elements within a stacking context are stacked independently of elements in other stacking contexts. This means it's impossible for an element in one stacking context to overlay an element in a different stacking context, even with a higher z-index.

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