What Does z-index: 0 Mean in CSS?

The z-index property in CSS is used to control the stacking order of elements on a web page. It’s especially useful when working with positioned elements that might overlap, such as modals, dropdowns, or layers in complex UIs.

In this article, we’ll focus on a common but often misunderstood value:

What does z-index: 0 actually mean?

Let’s break it down with examples and best practices.

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🧠 Understanding z-index Basics

The z-index value controls how elements are layered on the z-axis (from front to back).

• Higher values = in front
• Lower values = behind
• Only works on positioned elements (position: relative, absolute, fixed, or sticky)

Example:

.box1 {
  position: absolute;
  z-index: 1;
}
.box2 {
  position: absolute;
  z-index: 2; /* Appears above .box1 */
}

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🟡 So, What Does z-index: 0 Do?

When you assign z-index: 0 to a positioned element, it sets the element’s stacking level at 0—which is:

• Above elements with negative z-index values
• Below elements with positive z-index values
• Above elements with z-index: auto only if stacking context prioritizes it

Example:

.modal {
  position: fixed;
  z-index: 0;
}

This means the modal will:

• Be stacked behind other elements with z-index: 1 or higher
• Be stacked above any z-index: -1 elements
• Not automatically appear on top unless manually adjusted

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🎯 When to Use z-index: 0

Use z-index: 0 when you want to:

• Create a new stacking context without changing the default layering much
• Ensure layering priority over negative values, but under any UI overlays
• Reset z-index to neutral if previously set higher

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⚠️ Common Mistakes with z-index: 0

1. Forgetting to Set position

Without a position value, z-index doesn’t work.

.bad-example {
  z-index: 0; /* This does nothing */
}

✅ Fix:

.good-example {
  position: relative;
  z-index: 0;
}

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2. Assuming z-index: 0 Is the Default

Elements without any z-index specified are treated as auto, and are stacked in DOM order unless explicitly styled.

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3. Overlapping Elements Not Responding to z-index

You may need to inspect parent stacking contexts. A parent element with its own z-index creates a new stacking context, which limits the effectiveness of child z-index values.

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✅ Summary: What Does z-index: 0 Do?

Rule	Effect
Applies only to positioned elements	Must use position: relative, absolute, fixed, or sticky
Creates a stacking context	Even z-index: 0 establishes a new context
Sits above negative z-index elements	And below positive z-index elements
Helps normalize or reset stacking levels	Useful in modular CSS components

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📝 Conclusion

z-index: 0 in CSS sets an element’s stacking order to neutral—above negative layers, below positive ones. While it might seem insignificant, it’s a valuable tool when you need precise control over layering, especially in complex interfaces.

Remember: for z-index to take effect, the element must be positioned. Mastering this property helps you create cleaner, more predictable layouts in modern CSS.