Keyframe animations in CSS - Mini Project: Build & Apply

Create a Simple Keyframe Animation with CSS

📖 Scenario: You want to make a box on a webpage move smoothly from left to right and back again. This will catch the visitor's eye and make your page more lively.

🎯 Goal: Build a CSS animation using @keyframes that moves a square box horizontally across the screen and back continuously.

📋 What You'll Learn

Create a CSS @keyframes rule named slide that moves an element from left 0 to left 200px and back.

Create a CSS class called box that sets width, height, background color, and position relative.

Apply the slide animation to the box class with a duration of 4 seconds and infinite repetition.

Use semantic CSS properties and ensure the animation runs smoothly.

💡 Why This Matters

🌍 Real World

Keyframe animations are used on websites to create smooth movements and transitions that attract user attention and improve user experience.

💼 Career

Web developers often use CSS animations to enhance the look and feel of websites, making them more interactive and visually appealing.

1

Create the box style

Write CSS to create a class called box that sets width to 100px, height to 100px, background-color to blue, and position to relative.

CSS

/* Create the .box class with width, height, background-color, and position relative */
/* Your code here */

Hint

Use the CSS class selector .box and set the properties exactly as described.

2

Define the keyframes animation

Write a CSS @keyframes rule named slide that moves the element's left property from 0px at 0% to 200px at 50%, then back to 0px at 100%.

CSS

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

/* Define @keyframes slide to move left from 0px to 200px and back */
/* Your code here */

Hint

Use @keyframes slide and define the left property at 0%, 50%, and 100% as described.

3

Apply the animation to the box

Add to the .box class the CSS property animation with the value slide 4s infinite to run the slide animation continuously every 4 seconds.

CSS

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

@keyframes slide {
  0% { left: 0px; }
  50% { left: 200px; }
  100% { left: 0px; }
}

Hint

Use the animation property inside .box with the exact value slide 4s infinite.

4

Add HTML to show the animated box

Write the HTML code to create a <div> element with the class box so the animation is visible on the page.

CSS

.box {
  width: 100px;
  height: 100px;
  background-color: blue;
  position: relative;
  animation: slide 4s infinite;
}

@keyframes slide {
  0% { left: 0px; }
  50% { left: 200px; }
  100% { left: 0px; }
}

Hint

Use a <div> tag with the class attribute set to box.

Practice

1. What does the @keyframes rule do in CSS animations?

easy

A. Stops the animation immediately

B. Applies the animation to an HTML element

C. Sets the animation duration and delay

D. Defines the stages of an animation with style changes over time

Solution

1. Step 1: Understand the role of @keyframes

   The @keyframes rule defines how styles change at different points during the animation timeline.

2. Step 2: Differentiate from animation properties

   Properties like animation-duration or animation-name apply the animation, but @keyframes sets the actual style changes.

3. Final Answer:

   Defines the stages of an animation with style changes over time -> Option D

4. Quick Check:

   @keyframes defines animation steps [OK]

Hint: Remember: @keyframes sets animation steps [OK]

Common Mistakes:

• Confusing @keyframes with animation properties
• Thinking @keyframes applies animation to elements
• Mixing up animation duration with keyframe definitions

2. Which of the following is the correct syntax to define a simple keyframe animation named fade that changes opacity from 0 to 1?

easy

A. @keyframe fade { start { opacity: 0; } end { opacity: 1; } }

B. @keyframes fade { from { opacity: 0; } to { opacity: 1; } }

C. @animation fade { 0% { opacity: 0; } 100% { opacity: 1; } }

D. @keyframes fade { 0 { opacity: 0; } 1 { opacity: 1; } }

Solution

1. Step 1: Recall correct @keyframes syntax

   The correct rule is @keyframes followed by the animation name and curly braces containing percentage or keyword steps.

2. Step 2: Check the step keywords and values

   Valid steps are from and to or percentages like 0% and 100%. @keyframes fade { from { opacity: 0; } to { opacity: 1; } } uses from and to correctly.

3. Final Answer:

   @keyframes fade { from { opacity: 0; } to { opacity: 1; } } -> Option B

4. Quick Check:

   Correct @keyframes syntax uses from and to [OK]

Hint: Use @keyframes with from/to or 0%/100% [OK]

Common Mistakes:

• Using @animation instead of @keyframes
• Using invalid step names like start or end
• Omitting percentage signs or keywords in steps

3. Given the CSS below, what will happen to the <div> element when the page loads?

div {
  width: 100px;
  height: 100px;
  background-color: red;
  animation: slide 2s forwards;
}

@keyframes slide {
  0% { transform: translateX(0); }
  100% { transform: translateX(200px); }
}

medium

A. The div moves 200px to the right over 2 seconds and stays there

B. The div moves 200px to the right instantly with no animation

C. The div moves 200px to the left over 2 seconds and returns

D. The div does not move because animation is missing

Solution

1. Step 1: Analyze the animation properties

   The div has an animation named slide that lasts 2 seconds and uses forwards fill mode, meaning it keeps the final state.

2. Step 2: Understand the keyframe effect

   The slide animation moves the div from translateX(0) to translateX(200px), which moves it 200 pixels to the right.

3. Final Answer:

   The div moves 200px to the right over 2 seconds and stays there -> Option A

4. Quick Check:

   Animation moves right 200px and stays [OK]

Hint: Check animation duration and fill mode for final position [OK]

Common Mistakes:

• Confusing direction of translateX (right vs left)
• Ignoring the forwards fill mode effect
• Thinking animation happens instantly without duration

4. Identify the error in this CSS animation code:

@keyframes grow {
  0% { width: 100px; }
  100% { width: 200px }
}

.box {
  animation-name: grow;
  animation-duration: 3s;
  animation-iteration-count: infinite
}

medium

A. Missing semicolon after width: 200px in keyframes

B. Incorrect animation property name; should be animation

C. Animation duration must be in milliseconds, not seconds

D. Missing @keyframes keyword

Solution

1. Step 1: Check syntax inside @keyframes

   Each CSS declaration must end with a semicolon. The width: 200px line is missing a semicolon.

2. Step 2: Verify animation properties

   The properties animation-name, animation-duration, and animation-iteration-count are correct and properly used.

3. Final Answer:

   Missing semicolon after width: 200px in keyframes -> Option A

4. Quick Check:

   CSS declarations need semicolons [OK]

Hint: Always end CSS declarations with semicolons [OK]

Common Mistakes:

• Omitting semicolons inside keyframes
• Confusing animation property names
• Using wrong units for duration

5. You want to create a bouncing ball effect using keyframe animations. Which keyframe sequence best simulates a ball dropping and bouncing back up smoothly?

hard

A. @keyframes bounce {
0% { transform: translateX(0); }
50% { transform: translateX(100px); }
100% { transform: translateX(0); }
}

B. @keyframes bounce {
0% { transform: translateY(100px); }
50% { transform: translateY(0); }
100% { transform: translateY(100px); }
}

C. @keyframes bounce {
0%, 100% { transform: translateY(0); }
50% { transform: translateY(100px); }
}

D. @keyframes bounce {
0% { transform: translateY(0); }
50% { transform: translateY(-100px); }
100% { transform: translateY(0); }
}

Solution

1. Step 1: Understand bounce motion direction

   A bouncing ball moves down (positive Y) then back up (to zero). So translateY should go from 0 to positive value and back.

2. Step 2: Analyze each option's keyframes

   @keyframes bounce { 0%, 100% { transform: translateY(0); } 50% { transform: translateY(100px); } } moves from 0 to 100px down at 50% and back to 0 at 100%, simulating a bounce. @keyframes bounce { 0% { transform: translateY(100px); } 50% { transform: translateY(0); } 100% { transform: translateY(100px); } } starts at 100px down, which is unnatural. @keyframes bounce { 0% { transform: translateY(0); } 50% { transform: translateY(-100px); } 100% { transform: translateY(0); } } moves up (-100px), not down. @keyframes bounce { 0% { transform: translateX(0); } 50% { transform: translateX(100px); } 100% { transform: translateX(0); } } moves horizontally, not vertically.

3. Final Answer:

   @keyframes bounce { 0%, 100% { transform: translateY(0); } 50% { transform: translateY(100px); } } -> Option C

4. Quick Check:

   Bounce moves down then up vertically [OK]

Hint: Bounce means down (positive Y) then back up (zero) [OK]

Common Mistakes:

• Using negative translateY for bounce down
• Animating horizontal movement instead of vertical
• Starting animation at the bottom position