How Does CSS z-index Affect Element Stacking Order?

Understanding the Stacking Order of CSS Elements

In CSS, the z-index property plays a crucial role in determining the stacking order of elements, but it can be confusing to understand how it interacts with different positioning properties.

How the Stacking Order Works

The stacking order refers to the relative depth of overlapped elements on a web page. Elements with higher z-index values appear in front of elements with lower values. However, the z-index only applies to positioned elements (i.e., elements with position: absolute, relative, fixed, or sticky).

Interaction with Positioned and Non-Positioned Elements

• Non-positioned elements (position: static) are always stacked below positioned elements.
• If no z-index is specified for positioned elements, they follow the default stacking order based on the source code order.
• If z-index values are applied, positioned elements are stacked based on those values, regardless of their source code order.
• Elements with negative z-index values appear behind the background and borders of the root element.

Stacking Contexts

A stacking context is a rectangular area in the document that serves as a container for positioned elements. Once an element is positioned and a z-index is applied, it creates a new stacking context that separates its child elements from other stacking contexts.

Implications for Mixed Element Types

1. Mixed Sibling Elements:

• If sibling elements have different positions (mixed positioned and non-positioned), the positioned elements with z-index values will always be stacked in front of non-positioned elements.
• If both sibling elements are positioned with z-index values, the element with the higher value will appear in front.

2. Nested and Mixed Sibling Elements:

• If a positioned parent element contains both positioned and non-positioned child elements:

  • The positioned child elements are stacked within the parent's stacking context.
  • The non-positioned child elements are stacked below the positioned parent element, regardless of any z-index values.
• If nested positioned elements have conflicting z-index values, the element with the higher value within the innermost stacking context will appear in front.

Example

Consider the following HTML code:

<div>
  <div>

In this example, Box 1 will appear in front of Box 2 because it has a higher z-index within the stacking context created by its parent div. Box 2, being both positioned and having a lower z-index, will appear behind Box 1.

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