Z-index basics in CSS

Introduction

Z-index helps decide which item appears on top when things overlap on a webpage.

You want a popup to appear above the page content.

You have overlapping images and want to control which one is visible on top.

You want a menu to appear above other page elements.

You want to fix a header so it stays visible above scrolling content.

You want to layer text over a background image clearly.

Syntax

CSS

selector {
  position: relative | absolute | fixed | sticky;
  z-index: number;
}

Z-index only works if the element has a position other than static.

Higher z-index numbers appear on top of lower ones.

Examples

This makes the div appear above elements with lower z-index.

CSS

div {
  position: relative;
  z-index: 10;
}

A popup with a high z-index stays on top of most elements.

CSS

.popup {
  position: fixed;
  z-index: 1000;
}

This background stays behind elements with higher z-index.

CSS

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

Sample Program

The blue box overlaps the red box and appears on top because it has a higher z-index.

CSS

<style>
  .box1 {
    position: relative;
    width: 100px;
    height: 100px;
    background-color: red;
    z-index: 1;
  }
  .box2 {
    position: relative;
    width: 100px;
    height: 100px;
    background-color: blue;
    margin-top: -50px;
    margin-left: 50px;
    z-index: 2;
  }
</style>
<div class="box1"></div>
<div class="box2"></div>

Important Notes

If two elements have the same z-index, the one later in the HTML code appears on top.

Negative z-index values place elements behind the page background.

Summary

Z-index controls stacking order of overlapping elements.

Only works on positioned elements (not static).

Higher numbers appear on top.

Practice

1. What does the CSS property z-index control?

easy

A. The stacking order of overlapping elements

B. The font size of text

C. The background color of elements

D. The margin space around elements

Solution

1. Step 1: Understand the purpose of z-index

   The z-index property is used to control which elements appear on top when they overlap.

2. Step 2: Compare with other CSS properties

   Font size, background color, and margin do not affect stacking order.

3. Final Answer:

   The stacking order of overlapping elements -> Option A

4. Quick Check:

   z-index controls stacking order [OK]

Hint: Remember: z-index = which element is on top [OK]

Common Mistakes:

• Confusing z-index with color or size properties
• Thinking z-index changes element size
• Assuming z-index works without positioning

2. Which of the following is the correct way to apply z-index to an element?

easy

A. position: relative; z-index: 10;

B. display: block; z-index: 10;

C. position: static; z-index: 10;

D. margin: 10px; z-index: 10;

Solution

1. Step 1: Check which positions allow z-index

   Only positioned elements (relative, absolute, fixed, sticky) respond to z-index.

2. Step 2: Identify correct syntax

   Position must not be static (default). So position: relative with z-index works.

3. Final Answer:

   position: relative; z-index: 10; -> Option A

4. Quick Check:

   z-index works only with positioned elements [OK]

Hint: z-index needs position other than static [OK]

Common Mistakes:

• Using z-index without setting position
• Assuming display affects stacking
• Using margin or padding with z-index expecting effect

3. Given the CSS below, which element will appear on top?

div.a { position: relative; z-index: 5; }
div.b { position: relative; z-index: 10; }

medium

A. div.a will be on top

B. div.b will be on top

C. Both will appear side by side

D. Neither will overlap

Solution

1. Step 1: Compare z-index values

   div.a has z-index 5, div.b has z-index 10. Higher z-index means on top.

2. Step 2: Confirm both are positioned

   Both have position: relative, so z-index applies.

3. Final Answer:

   div.b will be on top -> Option B

4. Quick Check:

   Higher z-index = top element [OK]

Hint: Higher z-index number means element is on top [OK]

Common Mistakes:

• Ignoring position property
• Thinking lower z-index is on top
• Assuming elements don't overlap

4. Why does the z-index property not work on this element?

.box { z-index: 100; }

medium

A. Because z-index requires a background color

B. Because z-index only works on inline elements

C. Because the element has no position set or is static

D. Because z-index only works on elements with margin

Solution

1. Step 1: Check element positioning

   By default, elements have position: static, which ignores z-index.

2. Step 2: Understand z-index requirements

   z-index only works if position is relative, absolute, fixed, or sticky.

3. Final Answer:

   Because the element has no position set or is static -> Option C

4. Quick Check:

   z-index needs non-static position [OK]

Hint: Set position to relative or absolute for z-index to work [OK]

Common Mistakes:

• Assuming z-index works without position
• Thinking z-index depends on background color
• Believing margin affects stacking order

5. You have three overlapping elements with these styles:

.one { position: relative; z-index: 1; }
.two { position: absolute; z-index: 3; }
.three { position: relative; z-index: 2; }

Which order will they stack from bottom to top?

hard

A. .one, .two, .three

B. .two, .three, .one

C. .three, .one, .two

D. .one, .three, .two

Solution

1. Step 1: List elements with their z-index

   .one = 1, .three = 2, .two = 3.

2. Step 2: Order by ascending z-index

   Lower z-index is below higher z-index, so stacking is .one (bottom), .three (middle), .two (top).

3. Final Answer:

   .one, .three, .two -> Option D

4. Quick Check:

   Stack order = ascending z-index [OK]

Hint: Stack from lowest to highest z-index [OK]

Common Mistakes:

• Ignoring absolute vs relative position effect
• Mixing up stacking order direction
• Assuming position type changes z-index order