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.
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