Z-index basics in CSS - Mini Project: Build & Apply

Z-index basics

📖 Scenario: You are creating a simple webpage with two overlapping colored boxes. You want to control which box appears on top using the z-index property.

🎯 Goal: Build a CSS snippet that sets up two overlapping boxes and uses z-index to control their stacking order so that the red box appears above the blue box.

📋 What You'll Learn

Create two boxes with class names .box1 and .box2

Set the position property to relative for both boxes

Set the background colors: .box1 to blue and .box2 to red

Use z-index to make sure the red box (.box2) appears above the blue box (.box1)

Set the size of each box to 100px by 100px and position them so they overlap

💡 Why This Matters

🌍 Real World

Web designers often need to control which elements appear on top when they overlap, such as menus, modals, or images.

💼 Career

Understanding z-index is essential for front-end developers and UI designers to create visually correct and user-friendly web pages.

1

Create two boxes with basic styles

Create CSS rules for .box1 and .box2 that set their width and height to 100px and background colors to blue for .box1 and red for .box2.

CSS

/* Your code here to create .box1 and .box2 with size and background colors */

Hint

Use width, height, and background-color properties for each class.

2

Add positioning to the boxes

Add position: relative; to both .box1 and .box2 CSS rules to enable stacking control.

CSS

.box1 {
  width: 100px;
  height: 100px;
  background-color: blue;
  /* Add position relative here */
}

.box2 {
  width: 100px;
  height: 100px;
  background-color: red;
  /* Add position relative here */
}

Hint

Set position: relative; inside both class rules.

3

Position the boxes to overlap

Add top: 20px; and left: 20px; to .box2 so it overlaps .box1 which stays at the default position.

CSS

.box1 {
  width: 100px;
  height: 100px;
  background-color: blue;
  position: relative;
}

.box2 {
  width: 100px;
  height: 100px;
  background-color: red;
  position: relative;
  /* Add top and left to overlap .box1 */
}

Hint

Use top and left properties to move the red box.

4

Use z-index to bring red box on top

Add z-index: 1; to .box2 to make sure the red box appears above the blue box.

CSS

.box1 {
  width: 100px;
  height: 100px;
  background-color: blue;
  position: relative;
}

.box2 {
  width: 100px;
  height: 100px;
  background-color: red;
  position: relative;
  top: 20px;
  left: 20px;
  /* Add z-index to bring .box2 on top */
}

Hint

Set z-index: 1; inside the .box2 rule.

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