Writing reusable CSS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ selector

intermediate

Which CSS selector targets all buttons inside a section with class card?

You want to style all <button> elements that are inside any <section> with class card. Which selector correctly does this?

A button.card section { color: blue; }

B .card > button { color: blue; }

C section.card button { color: blue; }

D section button.card { color: blue; }

💡 Hint

Think about how to select buttons that are inside a section with a specific class.

Option C uses the selector section.card button which means any button inside a section with class card. Option C selects buttons that are direct children of any element with class card, but .card could be any element, not necessarily a section. Options A and D have incorrect order or class placement.

❓ layout

intermediate

What is the effect of this CSS on a container with multiple items?

Consider this CSS code:

.container { display: flex; flex-wrap: wrap; gap: 1rem; }

What will you see when multiple child items are inside .container?

A Items will be arranged in a row and wrap to the next line if needed, with 1rem space between them.

B Items will stack vertically with 1rem space between them.

C Items will overlap each other with no spacing.

D Items will be arranged in a column and never wrap.

💡 Hint

Remember what flex-wrap: wrap and gap do in flexbox.

The display: flex makes children line up in a row by default. flex-wrap: wrap allows items to move to the next line if they don't fit. gap: 1rem adds space between items both horizontally and vertically.

❓ accessibility

advanced

Which CSS practice improves accessibility for keyboard users?

You want to make sure focus outlines are visible for keyboard users but not distracting for mouse users. Which CSS snippet achieves this best?

A :active { outline: none; }

B :focus { outline: none; }

C *:hover { outline: 2px solid #005fcc; }

D :focus-visible { outline: 2px solid #005fcc; }

💡 Hint

Focus outlines should appear only when using keyboard navigation.

The :focus-visible pseudo-class applies styles only when the element is focused via keyboard or similar input, not mouse. This keeps outlines visible for keyboard users but hides them for mouse users. Option D removes outlines completely, which harms accessibility. Options C and D do not address keyboard focus properly.

📝 Syntax

advanced

What error does this CSS code cause?

Look at this CSS snippet:

.btn { background-color: blue; color: white; border-radius 5px; }

What happens when a browser tries to read this?

CSS

.btn { background-color: blue; color: white; border-radius 5px; }

A The browser applies all styles except border-radius.

B The browser ignores the entire rule because of missing colon in border-radius.

C The browser throws a syntax error and stops rendering the page.

D The browser applies border-radius but ignores color.

💡 Hint

Check the syntax for property declarations.

The missing colon after border-radius makes that declaration invalid, so the browser ignores only that line but applies the rest. Browsers do not throw syntax errors for CSS but skip invalid declarations.

🧠 Conceptual

expert

How many unique CSS classes are used in this HTML snippet?

Given this HTML:

<div class="card primary">
  <button class="btn primary">Click</button>
  <button class="btn secondary">Cancel</button>
</div>

How many unique CSS class names are present in total?

CSS

<div class="card primary">
  <button class="btn primary">Click</button>
  <button class="btn secondary">Cancel</button>
</div>

A 2

B 4

C 5

D 3

💡 Hint

Count each distinct class name only once.

The classes used are: card, primary, btn, and secondary. That's 4 class names. But note primary is repeated on two elements, so unique classes are card, primary, btn, and secondary. So total unique classes are 4, not 3. The correct answer is 4.

Practice

1. What is the main benefit of using reusable CSS classes in web development?

easy

A. You can write styles once and use them many times.

B. It makes the website load slower.

C. It requires writing more code for each element.

D. It only works with inline styles.

Solution

1. Step 1: Understand the purpose of reusable CSS classes

   Reusable CSS classes allow you to write a style rule once and apply it to multiple elements, saving time and effort.

2. Step 2: Compare options to the main benefit

   Options B, C, and D describe drawbacks or incorrect uses, while You can write styles once and use them many times. correctly states the main benefit.

3. Final Answer:

   You can write styles once and use them many times. -> Option A

4. Quick Check:

   Reusable CSS = Write once, use many times [OK]

Hint: Reusable classes save time by reusing styles [OK]

Common Mistakes:

• Thinking reusable CSS slows down the site
• Confusing reusable classes with inline styles
• Believing reusable CSS requires more code

2. Which of the following is the correct syntax to define a reusable CSS class named button?

easy

A. button { color: blue; }

B. #button { color: blue; }

C. .button { color: blue; }

D. *button { color: blue; }

Solution

1. Step 1: Identify CSS selector for classes

   In CSS, classes are selected using a dot (.) before the class name.

2. Step 2: Match syntax to class selector

   .button { color: blue; } uses .button which is correct. #button { color: blue; } uses an ID selector (#), C uses an element selector, and D uses an invalid selector.

3. Final Answer:

   .button { color: blue; } -> Option C

4. Quick Check:

   Class selector starts with dot (.) [OK]

Hint: Class selectors start with a dot (.) in CSS [OK]

Common Mistakes:

• Using # instead of . for classes
• Omitting the dot before class name
• Confusing element selectors with class selectors

3. Given the CSS below, what color will the text inside the <p> tag have?

.red { color: red; } .bold { font-weight: bold; }

Hello World

medium

A. Default browser color

B. Bold only

C. Red only

D. Red and bold

Solution

1. Step 1: Analyze the applied classes

   The paragraph has two classes: red and bold. The red class sets text color to red, and bold sets font weight to bold.

2. Step 2: Combine effects of both classes

   Both styles apply together, so the text will be red and bold.

3. Final Answer:

   Red and bold -> Option D

4. Quick Check:

   Multiple classes combine styles [OK]

Hint: Multiple classes combine their styles on one element [OK]

Common Mistakes:

• Thinking only one class style applies
• Confusing color and font-weight properties
• Ignoring combined class effects

4. What is wrong with this CSS if the goal is to reuse the style for multiple buttons?

button { background-color: blue; } .button { background-color: red; }

Click me

medium

A. The class name should start with a #.

B. The element selector button and class .button conflict causing confusion.

C. Class selectors cannot be reused.

D. The CSS syntax is invalid.

Solution

1. Step 1: Understand selector types

   The selector button targets all <button> elements, while .button targets elements with class "button".

2. Step 2: Identify conflict in styles

   The paragraph has class "button" but is not a <button> element, so only .button applies. The similar names can confuse developers.

3. Final Answer:

   The element selector button and class .button conflict causing confusion. -> Option B

4. Quick Check:

   Element and class selectors with same name cause confusion [OK]

Hint: Avoid naming classes same as HTML elements [OK]

Common Mistakes:

• Using # instead of . for classes
• Assuming class selectors can't be reused
• Ignoring selector specificity and conflicts

5. You want to create a reusable CSS class for a card component that has a shadow, padding, and rounded corners. Which CSS snippet correctly combines these styles for reuse?

hard

A. .card { box-shadow: 0 4px 6px rgba(0,0,0,0.1); padding: 1rem; border-radius: 0.5rem; }

B. #card { box-shadow: 0 4px 6px rgba(0,0,0,0.1); padding: 1rem; border-radius: 0.5rem; }

C. card { box-shadow: 0 4px 6px rgba(0,0,0,0.1); padding: 1rem; border-radius: 0.5rem; }

D. .card { shadow: 0 4px 6px rgba(0,0,0,0.1); padding: 1rem; rounded: 0.5rem; }

Solution

1. Step 1: Use correct class selector syntax

   Classes use a dot (.) before the name, so .card is correct. #card { box-shadow: 0 4px 6px rgba(0,0,0,0.1); padding: 1rem; border-radius: 0.5rem; } uses an ID selector (#), C uses an invalid element selector, and D uses incorrect CSS properties.

2. Step 2: Verify CSS properties for shadow, padding, and rounded corners

   box-shadow, padding, and border-radius are correct CSS properties. .card { shadow: 0 4px 6px rgba(0,0,0,0.1); padding: 1rem; rounded: 0.5rem; } uses invalid properties shadow and rounded.

3. Final Answer:

   .card { box-shadow: 0 4px 6px rgba(0,0,0,0.1); padding: 1rem; border-radius: 0.5rem; } -> Option A

4. Quick Check:

   Correct class selector and CSS properties [OK]

Hint: Use dot for class and correct CSS property names [OK]

Common Mistakes:

• Using # instead of . for classes
• Using invalid CSS properties like shadow or rounded
• Omitting units or using wrong selectors