Stacking context in CSS - Mini Project: Build & Apply

Understanding CSS Stacking Context

📖 Scenario: You are creating a simple webpage with two overlapping colored boxes. You want to control which box appears on top using CSS stacking context rules.

🎯 Goal: Build a webpage with two overlapping <div> elements. Use CSS properties to create stacking contexts and control which box appears on top.

📋 What You'll Learn

Create two <div> elements with distinct background colors and sizes.

Position the boxes so they overlap partially.

Use CSS properties to create stacking contexts on the boxes.

Control the stacking order so the red box appears above the blue box.

💡 Why This Matters

🌍 Real World

Web developers often need to control how elements overlap on a page, such as menus, modals, or images. Understanding stacking context helps avoid visual bugs.

💼 Career

Knowing stacking context is essential for front-end developers to create visually correct and accessible user interfaces.

1

Create two overlapping boxes in HTML and CSS

Create two <div> elements with id attributes blue-box and red-box. Style them with CSS so that #blue-box has a blue background, width and height of 10rem, and is positioned relative at top: 2rem and left: 2rem. Style #red-box with a red background, width and height of 10rem, and position it relative at top: 0 and left: 5rem. This will make the boxes overlap partially.

CSS

Stacking Context Example

  /* Style #blue-box and #red-box with background colors, sizes, and relative positions */
  



  
  

Hint

Use position: relative; and top, left properties to move the boxes so they overlap.

2

Add z-index to create stacking context on the blue box

Add a CSS property z-index: 1; to the #blue-box style. This will create a stacking context for the blue box and set its stacking order.

CSS

Stacking Context Example

  #blue-box {
    /* Add z-index: 1 here */
  }



  
  

Hint

Adding z-index to a positioned element creates a stacking context.

3

Add z-index to the red box to appear on top

Add a CSS property z-index: 2; to the #red-box style. This will create a stacking context for the red box and make it appear above the blue box.

CSS

Stacking Context Example

  #red-box {
    /* Add z-index: 2 here */
  }



  
  

Hint

Use a higher z-index value on the red box to make it appear on top.

4

Add an aria-label for accessibility and finalize

Add an aria-label="Blue colored box" attribute to the <div> with id="blue-box" and an aria-label="Red colored box" attribute to the <div> with id="red-box". This improves accessibility by describing the boxes to screen readers.

CSS

Stacking Context Example

  /* CSS remains unchanged */



  
  

Hint

Use aria-label attributes on the <div> elements to describe them for screen readers.

Practice

1. What creates a new stacking context in CSS?

easy

A. An element with color property set

B. An element with position: relative and z-index set

C. Any element with display: block

D. An element with margin applied

Solution

1. Step 1: Understand stacking context creation

   A stacking context is created by elements with position other than static and a z-index value set.

2. Step 2: Analyze options

   Only An element with position: relative and z-index set mentions position: relative with z-index, which creates a stacking context. Other options do not create stacking contexts.

3. Final Answer:

   An element with position: relative and z-index set -> Option B

4. Quick Check:

   Stacking context = position + z-index [OK]

Hint: Look for position plus z-index to spot stacking contexts [OK]

Common Mistakes:

• Thinking any positioned element creates stacking context without z-index
• Confusing display or color properties with stacking context
• Assuming margin affects stacking order

2. Which CSS snippet correctly creates a stacking context?

easy

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

B. display: inline; z-index: 3;

C. position: relative; z-index: 5;

D. color: red; z-index: 1;

Solution

1. Step 1: Check position and z-index combination

   Only elements with position other than static and a z-index value create stacking contexts.

2. Step 2: Evaluate each option

   position: relative; z-index: 5; uses position: relative and z-index: 5, which correctly creates a stacking context. Others either have static position or irrelevant properties.

3. Final Answer:

   position: relative; z-index: 5; -> Option C

4. Quick Check:

   Position relative + z-index creates stacking context [OK]

Hint: Position must not be static to create stacking context [OK]

Common Mistakes:

• Using position static with z-index expecting stacking context
• Assuming display or color create stacking contexts
• Ignoring that inline elements don't create stacking contexts with z-index

3. Given this HTML and CSS, which element appears on top visually?

<div class='parent'>
  <div class='child1'>Child 1</div>
  <div class='child2'>Child 2</div>
</div>

.parent { position: relative; z-index: 1; }
.child1 { position: absolute; z-index: 2; }
.child2 { position: absolute; z-index: 1; }

medium

A. Parent appears on top of children

B. Child 2 appears on top

C. Both overlap equally, no stacking order

D. Child 1 appears on top

Solution

1. Step 1: Identify stacking contexts

   The parent has position: relative and z-index: 1, creating a stacking context. Children stack inside this context.

2. Step 2: Compare children's z-index inside stacking context

   Child 1 has z-index: 2, child 2 has z-index: 1. Higher z-index means it appears on top.

3. Final Answer:

   Child 1 appears on top -> Option D

4. Quick Check:

   Higher z-index inside stacking context = on top [OK]

Hint: Higher z-index inside same stacking context is on top [OK]

Common Mistakes:

• Ignoring stacking context created by parent
• Assuming parent overlays children
• Confusing absolute positioning with stacking order

4. Why does this CSS not create a stacking context as expected?

.box { position: absolute; }

What is missing?

medium

A. Missing z-index property

B. Position should be static

C. Need display: block

D. Color property must be set

Solution

1. Step 1: Understand stacking context requirements

   Positioned elements create stacking contexts only if z-index is set.

2. Step 2: Analyze given CSS

   The element has position: absolute but no z-index, so no stacking context is created.

3. Final Answer:

   Missing z-index property -> Option A

4. Quick Check:

   Position alone doesn't create stacking context without z-index [OK]

Hint: Position plus z-index needed for stacking context [OK]

Common Mistakes:

• Assuming position absolute alone creates stacking context
• Thinking display or color affect stacking context
• Confusing static position with stacking context creation

5. You have nested elements:

<div class='outer'>
  <div class='inner'>Content</div>
</div>

CSS:

.outer { position: relative; z-index: 1; }
.inner { position: relative; z-index: 10; }

Which statement is true about their stacking order?

hard

A. Inner's z-index 10 stacks only inside outer's stacking context

B. Inner's z-index 10 places it above all elements outside outer

C. Outer and inner share the same stacking context ignoring z-index

D. Inner's z-index is ignored because outer has lower z-index

Solution

1. Step 1: Identify stacking contexts

   The outer element creates a stacking context with position: relative and z-index: 1. The inner element creates a new stacking context inside outer with position: relative and z-index: 10.

2. Step 2: Understand stacking context isolation

   Inner's stacking order is only relative to siblings inside outer's stacking context. It cannot escape outer's stacking context to appear above elements outside it.

3. Final Answer:

   Inner's z-index 10 stacks only inside outer's stacking context -> Option A

4. Quick Check:

   Nested stacking contexts isolate z-index scope [OK]

Hint: Nested stacking contexts isolate z-index effects [OK]

Common Mistakes:

• Thinking inner z-index affects outside elements
• Assuming stacking contexts merge ignoring nesting
• Believing outer's lower z-index cancels inner's stacking