What Does z-index: 0 Mean in CSS?

When building web pages, it’s common to have elements overlap each other — like dropdowns, modals, sticky headers, or tooltips. In such scenarios, the z-index property becomes essential in controlling which elements appear on top of others.

In this article, we’ll focus on a frequently used but often misunderstood value: z-index: 0. What does it mean, and how does it affect element stacking?

Table of Contents

🧠 What Is `z-index` in CSS?

The z-index property determines the stacking order of positioned elements (those with position set to relative, absolute, fixed, or sticky).

Elements with higher z-index values appear in front of those with lower values.
Elements without a z-index value follow the default HTML stacking order.

🔢 So, What Does `z-index: 0` Mean?

Setting z-index: 0 means the element:

Has a defined stacking context (if it also has a positioning context),
Will appear behind elements with higher z-index values, but above elements with negative z-index values.

Example:

.box {
  position: relative;
  z-index: 0;
}

In this case:

.box is now part of a stacking context,
It will be rendered behind any sibling element with z-index: 1 or higher,
It will be rendered in front of any element with z-index: -1.

⚠️ Important: `z-index` Only Works on Positioned Elements

The z-index property has no effect unless the element is positioned, i.e., it has a position value other than static (which is the default).

Doesn’t work:

.box {
  z-index: 0; /* ignored if position is static */
}

Works:

.box {
  position: relative;
  z-index: 0;
}

✅ When to Use `z-index: 0`

Here are some common scenarios where z-index: 0 is helpful:

Resetting Stacking Order
- If an element has an inherited or higher z-index, you can reset it to 0 to place it beneath overlapping elements.
Creating a Local Stacking Context
- Using z-index: 0 along with position can create an isolated stacking context to avoid interference from outside layers.
Organizing Layered UI
- In UI components like tooltips, modals, and sidebars, z-index: 0 can serve as the base layer, ensuring higher-index elements stack properly above it.

🧾 Conclusion

z-index: 0 in CSS simply places the element at a base stacking level — above negative z-index elements but below any positive ones — as long as the element is positioned. It also helps create local stacking contexts for better control over element layering.

Key Takeaways:

z-index: 0 sets a baseline stacking level.
It only works on positioned elements.
Use it to manage stacking order in complex UIs without always resorting to high values like z-index: 999.

🧠 What Is z-index in CSS?

🔢 So, What Does z-index: 0 Mean?