Detailed Explanation of the z-index Property in CSS

Detailed Explanation of the z-index Property in CSS

Description
The z-index property in CSS controls the stacking order of elements along the axis perpendicular to the screen (the Z-axis). It determines which element appears in front and which is obscured behind when elements overlap.

Core Concepts

  1. Stacking Context: z-index only takes effect within a specific "stacking context."
  2. Default Order: Elements that appear later in the HTML will overlay those that appear earlier.
  3. Numerical Property: A higher z-index value brings the element to the front; values can be positive, negative, or auto.

Basic Usage of z-index

.element1 {
    position: relative;
    z-index: 10;  /* Appears in front */
}

.element2 {
    position: relative;  
    z-index: 5;   /* Appears behind */
}

Detailed Explanation of Key Points

1. Prerequisites for z-index to Take Effect
z-index only works on positioned elements (where the position value is not static):

  • position: relative/absolute/fixed/sticky
  • Children of flex containers (flex)
  • Children of grid containers (grid)

2. Creation of a Stacking Context
A new stacking context is created under the following conditions:

  • The root element (html)
  • Elements with position set to absolute/relative and z-index not set to auto
  • Elements with position set to fixed/sticky
  • Children of flex/grid containers with z-index not set to auto
  • Elements with opacity less than 1
  • Elements with properties like transform, filter, perspective, clip-path not set to none

3. Stacking Rules (from back to front)

  1. Background and borders of the stacking context
  2. Child stacking contexts with negative z-index
  3. Non-positioned block-level elements in normal flow
  4. Non-positioned floated elements
  5. Non-positioned inline elements in normal flow
  6. Positioned elements with z-index set to auto or 0
  7. Child stacking contexts with positive z-index

4. Practical Application Example

<div class="parent">
    <div class="child1">Child Element 1</div>
    <div class="child2">Child Element 2</div>
</div>

.parent {
    position: relative;
    z-index: 1; /* Creates a stacking context */
}

.child1 {
    position: absolute;
    z-index: 100; /* Compares within the parent's stacking context */
}

.child2 {
    position: absolute;  
    z-index: 50; /* Still appears behind child1 */
}

5. Solutions to Common Issues

Issue 1: z-index Not Taking Effect
Cause: The element is not positioned or is in a different stacking context.
Solution: Ensure the element has a positioning property and is compared within the same stacking context.

Issue 2: Parent Element Obscuring Child Element
Cause: The parent element creates a stacking context with a relatively low z-index.
Solution: Adjust the parent element's z-index or reorganize the HTML structure.

6. Best Practices

  1. Avoid excessive use of high z-index values; use meaningful numerical ranges instead.
  2. Use CSS variables to manage z-index layers.
  3. Pay attention to the creation of stacking contexts to avoid unexpected layering relationships.

By understanding how z-index works and the concept of stacking contexts, you can better control the display layering of page elements.