CSS
CSS z-index Property Explained: Controlling Element Stack Order
When designing web interfaces, elements often overlap — think modals, dropdowns, tooltips, or sticky headers. To control which element appears on top, we use the CSS z-index
property.
In this article, you’ll learn:
- What
z-index
is and how it works - When and how to use it
- Common mistakes and best practices
🧱 What Is z-index
in CSS?
The z-index
property controls the vertical stacking order of elements that overlap each other. Elements with a higher z-index
appear on top of those with a lower z-index
.
📌 Syntax:
.element {
position: relative;
z-index: 10;
}
🔑
z-index
only works on positioned elements (relative
,absolute
,fixed
, orsticky
), and on flex/grid children withz-index
.
🧪 Example Use Case
Let’s say you have a dropdown menu that overlaps a header:
<div class="header">Header</div>
<div class="dropdown">Dropdown Menu</div>
.header {
position: relative;
z-index: 1;
}
.dropdown {
position: absolute;
z-index: 10;
}
The dropdown now correctly appears on top of the header.
🔢 How z-index
Works
- Default
z-index
is auto (typically 0). - Higher values appear on top.
- Negative values (
z-index: -1
) push elements behind others.
z-index Value | Effect |
---|---|
auto | Uses default stacking |
0 | Neutral |
1 or more | Higher = in front |
Negative | Lower = behind |
📦 Stack Context Matters
z-index
works within stacking contexts. A new stacking context is created when:
- An element has a
position
andz-index
other thanauto
- An element has
opacity
less than1
- Some CSS properties like
transform
,filter
, orwill-change
are applied
🧠 This means a child with a high
z-index
won’t escape its parent’s stacking context if the parent has its ownz-index
.
🧭 Common Mistakes
❌ z-index
not working?
Solution: Check if the element has a position
set.
/* Won't work */
.box {
z-index: 5;
}
/* Will work */
.box {
position: relative;
z-index: 5;
}
✅ Best Practices
- Keep
z-index
values low and meaningful (avoid huge jumps like 9999 unless necessary) - Group related components within the same stacking context
- Use logical naming and layering for maintainability
- Be cautious when combining multiple stacking contexts
📝 Conclusion
The z-index
property is a powerful tool to control the visual stacking order of elements in a webpage. When used correctly, it ensures that important UI elements—like modals, dropdowns, and sticky headers—display as intended.
Mastering z-index
can help you debug layering issues faster and build interfaces that feel crisp and intentional.
🔑 Quick Reference
Property | Required? |
---|---|
position | Yes (relative , absolute , fixed , or sticky ) |
z-index value | Higher = in front |
Stacking context | May isolate z-index |