What Does the z-index Property Control in CSS?

When designing web pages, you often deal with elements that overlap — like modals, dropdowns, sticky headers, or tooltips. But how do you control which element appears on top of others?

The answer is simple: CSS z-index.

In this blog post, you’ll learn:

✅ What the z-index property does
🧠 How stacking order works
💡 When and how to use z-index effectively
🚫 Common mistakes to avoid

Table of Contents

🧱 What is the `z-index` Property?

The z-index CSS property controls the vertical stacking order of overlapping elements along the z-axis (the axis coming “out of” the screen).

🔤 In simple terms:

Elements with a higher z-index value appear on top of elements with a lower z-index value — when they overlap.

🛠️ Syntax

selector {
  position: relative; /* or absolute, fixed, sticky */
  z-index: 10;
}

z-index: auto — default; follows HTML order
z-index: [number] — sets stacking level

⚠️ z-index only works on elements that are positioned (relative, absolute, fixed, or sticky). If you use z-index without setting position, it won’t do anything.

📐 How Does `z-index` Work?

The browser renders HTML elements in stacking order. By default:

Background and non-positioned elements
Positioned elements (relative, absolute, fixed, or sticky)
Elements with higher z-index values

🧠 Example:

<div class="box1">Box 1</div>
<div class="box2">Box 2</div>

.box1 {
  position: absolute;
  background: red;
  z-index: 1;
}

.box2 {
  position: absolute;
  background: blue;
  z-index: 2;
}

➡️ Result: Box 2 appears on top of Box 1.

💡 Real-World Use Cases

Use Case	Why `z-index` Matters
Dropdown menus	Ensure they float above other UI
Modals & popups	Appear above page content
Tooltips	Hover tooltips over other text
Sticky headers	Stay above scrolling content

🚫 Common Mistakes to Avoid

❌ 1. Using `z-index` Without Positioning

/* This won't work */
.element {
  z-index: 10;
}

✔️ Fix: Add a positioning property like relative or absolute.

❌ 2. Overusing High Values

Using z-index: 99999 everywhere can cause layering conflicts. Instead, define a layering system for your project.

📦 Pro Tip: Understand Stacking Contexts

Each positioned element with a z-index forms a new stacking context. Children cannot break out of their parent’s context, even with a higher z-index.

✅ Summary

Property	Controls
`z-index`	The stack order of overlapping elements
Requires	A `position` other than `static`
Higher Value	Appears on top
Default	`auto` (follows document order)

🚀 Conclusion

The z-index property is a powerful CSS tool that controls which elements appear above or below others when they overlap. When used properly, it helps you build polished UIs — from modals to dropdowns — with perfect layering.

🧱 What is the z-index Property?