How Does z-index Affect Element Stacking Order in CSS?

Understanding the z-index Stacking Order

Determining element stacking order using z-index can be confusing, especially when mixing elements with various position properties. To clarify, let's dive into the basics and explore specific scenarios.

z-index Basics

Z-index is a CSS property used to establish the depth of positioned elements (absolute, relative, fixed, sticky). Elements with higher z-index values appear in front of those with lower values on the z-axis.

Positioning and Stacking Contexts

Z-index requires an element to be positioned for it to take effect. Non-positioned elements follow the default stacking order based on their appearance in the markup.

Positioning an element creates a stacking context, isolating it and its descendants from elements in other stacking contexts. This means an element's z-index will only affect elements within its own stacking context.

Stacking Order

When no z-index is specified, the stacking order is determined as follows:

• Backgrounds and borders of the root element
• Non-positioned block elements in source code order
• Non-positioned floating elements in source code order
• Inline elements
• Positioned elements in source code order

If z-index is present, the order is modified by considering positioned elements with z-index values less than 0, greater than 0, and finally positioned elements with no z-index value.

Mixed Elements with and Without Positions

Mixed Sibling Elements:

• Elements without position follow the default stacking order.
• Elements with position and higher z-index values will overlay others, regardless of position properties.

Nested and Mixed Sibling Elements:

• Elements within a stacking context (nested or siblings with position) have their stacking order determined by their own z-index values.
• Elements outside this stacking context cannot affect the order within it, even with a higher z-index.

Conclusion

Grasping z-index requires understanding stacking contexts. Once that is clear, manipulating the stacking order becomes straightforward. For further clarity, consult the detailed resources provided at the bottom of this answer.

The above is the detailed content of How Does z-index Affect Element Stacking Order in CSS?. For more information, please follow other related articles on the PHP Chinese website!