Mobile-first approach in CSS - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why use a mobile-first approach in CSS?

Which of the following best explains the main reason developers use a mobile-first approach when writing CSS?

A It uses JavaScript to detect screen size and load styles dynamically.

B It ensures styles for small screens load first, improving performance on mobile devices.

C It applies desktop styles first, then overrides them for mobile devices.

D It disables styles on mobile devices to save bandwidth.

💡 Hint

Think about which devices usually have slower connections and smaller screens.

Mobile-first means writing CSS for small screens first. This helps pages load faster on phones and tablets because the browser applies simple styles first. Then, larger screen styles are added with media queries.

📝 Syntax

intermediate

Identify the correct mobile-first media query

Which CSS media query correctly follows the mobile-first approach to apply styles for screens wider than 600px?

A @media (max-width: 600px) { body { background: blue; } }

B @media screen and (min-device-width: 600px) { body { background: blue; } }

C @media (min-width: 600px) { body { background: blue; } }

D @media screen and (max-device-width: 600px) { body { background: blue; } }

💡 Hint

Mobile-first uses min-width to add styles for bigger screens.

Mobile-first CSS writes base styles for small screens. Then it uses @media (min-width: 600px) to add or override styles for screens 600px wide or larger.

❓ rendering

advanced

What background color will the page show on a 500px wide screen?

Given this CSS, what background color will the page have when viewed on a screen 500px wide?

CSS

body {
  background-color: white;
}

@media (min-width: 600px) {
  body {
    background-color: green;
  }
}

@media (min-width: 400px) {
  body {
    background-color: yellow;
  }
}

A Yellow

B White

C Green

D No background color

💡 Hint

Check which media queries apply at 500px width.

Base background is white. The @media (min-width: 400px) applies at 500px, changing background to yellow. The @media (min-width: 600px) does not apply because 500px is less than 600px.

❓ selector

advanced

Which CSS selector targets only mobile devices in a mobile-first approach?

You want to style a nav element only on screens smaller than 480px. Which CSS snippet correctly does this using a mobile-first approach?

A nav { font-size: 1.2rem; } @media (min-width: 480px) { nav { font-size: 1rem; } }

B @media (min-width: 480px) { nav { font-size: 1.2rem; } }

C @media (max-width: 479px) { nav { font-size: 1.2rem; } }

D nav { font-size: 1rem; } @media (max-width: 479px) { nav { font-size: 1.2rem; } }

💡 Hint

Mobile-first means base styles for small screens, then bigger screen overrides.

Option A sets the base font size for mobile (small screens). Then it uses @media (min-width: 480px) to reduce font size for bigger screens. This matches mobile-first principles.

❓ accessibility

expert

How does mobile-first CSS improve accessibility?

Which statement best explains how a mobile-first CSS approach can help users with disabilities?

A It hides content on mobile devices to reduce cognitive load.

B It disables animations on mobile devices to reduce distractions.

C It automatically adds ARIA labels to all elements for screen readers.

D It ensures simpler layouts and larger touch targets on small screens, aiding users with motor impairments.

💡 Hint

Think about how mobile design affects ease of use for everyone.

Mobile-first design encourages simple layouts and bigger buttons for small screens. This helps users with motor difficulties by making controls easier to tap and content easier to read.

Practice

1. What does the mobile-first approach in CSS mean?

easy

A. Write all styles without using media queries

B. Start styling for small screens first, then add styles for larger screens

C. Start styling for desktop screens first, then add styles for smaller screens

D. Only style for mobile devices and ignore desktops

Solution

1. Step 1: Understand mobile-first approach

   Mobile-first means you write CSS targeting small screens first, like phones.

2. Step 2: Add styles for bigger screens

   Then you use media queries with min-width to add or change styles for tablets and desktops.

3. Final Answer:

   Start styling for small screens first, then add styles for larger screens -> Option B

4. Quick Check:

   Mobile-first = Start small, add bigger [OK]

Hint: Think: style phones first, then bigger screens [OK]

Common Mistakes:

• Confusing mobile-first with desktop-first
• Using max-width instead of min-width for bigger screens
• Ignoring media queries completely

2. Which CSS syntax correctly applies styles for screens wider than 600px in a mobile-first approach?

easy

A. @media (max-width: 600px) { ... }

B. @media (width: 600px) { ... }

C. @media screen and (min-width: 600px) { ... }

D. @media screen and (max-width: 600px) { ... }

Solution

1. Step 1: Identify mobile-first media query

   Mobile-first uses min-width to add styles for bigger screens.

2. Step 2: Choose correct syntax with media type

   Using @media screen and (min-width: 600px) targets screens wider than 600px correctly.

3. Final Answer:

   @media screen and (min-width: 600px) { ... } -> Option C

4. Quick Check:

   Mobile-first uses min-width with media type [OK]

Hint: Use min-width for bigger screens in mobile-first [OK]

Common Mistakes:

• Using max-width instead of min-width for bigger screens
• Using 'width' instead of 'min-width'
• Confusing min-width and max-width

3. Given this CSS, what background color will a 700px wide screen see?

body { background: white; }
@media (min-width: 600px) { body { background: blue; } }
@media (min-width: 800px) { body { background: green; } }

medium

A. Blue

B. White

C. Green

D. No background color

Solution

1. Step 1: Check base style

   Base background is white for all screen sizes.

2. Step 2: Check media queries for 700px width

   700px is >= 600px but < 800px, so blue applies, green does not.

3. Final Answer:

   Blue -> Option A

4. Quick Check:

   700px triggers min-width 600px but not 800px [OK]

Hint: Check which min-width media query matches screen size [OK]

Common Mistakes:

• Choosing green because 700 > 600 but ignoring 800px limit
• Choosing white ignoring media queries
• Confusing min-width with max-width

4. What is wrong with this mobile-first CSS snippet?

body { font-size: 14px; }
@media (max-width: 600px) { body { font-size: 16px; } }

medium

A. It uses max-width instead of min-width for bigger screens

B. No error, this is correct mobile-first CSS

C. The base font size should be larger than media query size

D. Media query should use min-width for mobile-first approach

Solution

1. Step 1: Identify media query type

   The snippet uses max-width: 600px, which targets small screens.

2. Step 2: Check mobile-first rule

   Mobile-first uses base styles for small screens and min-width for bigger screens, so this is reversed.

3. Final Answer:

   Media query should use min-width for mobile-first approach -> Option D

4. Quick Check:

   Mobile-first uses min-width, not max-width [OK]

Hint: Mobile-first uses min-width, max-width is desktop-first [OK]

Common Mistakes:

• Thinking max-width is correct for mobile-first
• Ignoring base styles importance
• Assuming no error in reversed media query

5. You want a button to have 100% width on phones and 200px width on screens wider than 768px. Which CSS correctly uses mobile-first approach?

hard

A. button { width: 100%; } @media (min-width: 768px) { button { width: 200px; } }

B. button { width: 200px; } @media (max-width: 768px) { button { width: 100%; } }

C. button { width: 100%; } @media (max-width: 768px) { button { width: 200px; } }

D. button { width: 200px; } @media (min-width: 768px) { button { width: 100%; } }

Solution

1. Step 1: Set base style for small screens

   Mobile-first means base style is for phones, so width 100% is base.

2. Step 2: Use min-width media query for bigger screens

   Use @media (min-width: 768px) to set width 200px for larger screens.

3. Final Answer:

   button { width: 100%; } @media (min-width: 768px) { button { width: 200px; } } -> Option A

4. Quick Check:

   Base small, min-width bigger = mobile-first [OK]

Hint: Base styles for mobile, min-width for bigger screens [OK]

Common Mistakes:

• Using max-width for bigger screens
• Setting desktop styles as base
• Reversing widths in media queries