Class selectors in CSS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a class selector in CSS?

A class selector targets HTML elements with a specific class attribute. It starts with a dot (.) followed by the class name.

beginner

How do you write a CSS rule to style all elements with the class highlight?

Use <code>.highlight { /* styles here */ }</code>. The dot means 'class', and <code>highlight</code> is the class name.

intermediate

Can an HTML element have multiple classes? How does CSS handle that?

Yes, an element can have many classes separated by spaces. CSS applies styles from all matching class selectors to that element.

beginner

What is the difference between a class selector and an ID selector?

A class selector starts with a dot (.) and can be used on many elements. An ID selector starts with a hash (#) and should be unique to one element.

intermediate

How do class selectors help with responsive design?

Class selectors let you group elements and apply styles that can change with screen size using media queries, making layouts adapt easily.

Which symbol starts a class selector in CSS?

A .

B #

C *

D $

Class selectors always start with a dot (.) followed by the class name.

How do you select all elements with class 'menu' in CSS?

A menu

B #menu

C .menu

D *menu

Use .menu to select elements with the class 'menu'.

If an element has class="btn primary", which selectors apply styles to it?

A .btn only

B #btn and #primary

C .primary only

D .btn and .primary

Both .btn and .primary selectors apply because the element has both classes.

Which selector should be unique on a page?

A ID selector

B Element selector

C Class selector

D Universal selector

ID selectors (#id) should be unique per page; classes can be reused.

What is the correct way to write a CSS rule for class 'card'?

A #card { }

B .card { }

C card { }

D *card { }

Use .card { } to target elements with class 'card'.

Explain how class selectors work in CSS and how you use them in HTML.

Think about how you mark elements in HTML and then style them in CSS.

Describe the difference between class selectors and ID selectors and when to use each.

Consider how many elements each selector can target.

Practice

1. What does a CSS class selector start with to select elements by their class?

easy

A. A dot (.) before the class name

B. A hash (#) before the class name

C. No symbol, just the class name

D. An asterisk (*) before the class name

Solution

1. Step 1: Understand CSS selector symbols

   Class selectors always start with a dot (.) to target elements with that class.

2. Step 2: Compare with other selectors

   ID selectors use #, element selectors use no symbol, and * selects all elements.

3. Final Answer:

   A dot (.) before the class name -> Option A

4. Quick Check:

   Class selector = dot (.) [OK]

Hint: Class selectors always begin with a dot (.) [OK]

Common Mistakes:

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

2. Which of the following is the correct CSS syntax to style all elements with class highlight?

easy

A. highlight { color: red; }

B. #highlight { color: red; }

C. .highlight { color: red; }

D. *highlight { color: red; }

Solution

1. Step 1: Identify correct class selector syntax

   The class selector uses a dot (.) followed by the class name, so .highlight is correct.

2. Step 2: Check other options

   #highlight targets an ID, highlight alone targets elements named 'highlight', and *highlight is invalid syntax.

3. Final Answer:

   .highlight { color: red; } -> Option C

4. Quick Check:

   Class selector syntax = dot + class name [OK]

Hint: Remember: dot + class name for class selectors [OK]

Common Mistakes:

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

3. Given this HTML and CSS, what color will the text inside the <p> tag be?

<p class="note">Hello!</p>
.note { color: blue; }
.alert { color: red; }

medium

A. Red

B. Blue

C. Black (default)

D. No color applied

Solution

1. Step 1: Match class in HTML with CSS selector

   The <p> tag has class "note", so the CSS rule .note { color: blue; } applies.

2. Step 2: Check other CSS rules

   The .alert class styles red but does not apply here because the element does not have that class.

3. Final Answer:

   Blue -> Option B

4. Quick Check:

   Class matches CSS selector = blue text [OK]

Hint: Match class attribute with dot selector name [OK]

Common Mistakes:

• Confusing class names and applying wrong styles
• Ignoring that only matching classes apply styles
• Assuming default color when class is present

4. What is wrong with this CSS if you want to style elements with class menu?

menu { font-size: 1.2rem; }

medium

A. Font size value is invalid

B. Missing curly braces

C. Class name should be uppercase

D. Missing dot before class name

Solution

1. Step 1: Check selector syntax

   The selector menu targets elements named <menu>, not class "menu". Class selectors need a dot before the name.

2. Step 2: Verify other parts

   Curly braces and font size are correct. Class names are case-sensitive but lowercase is valid.

3. Final Answer:

   Missing dot before class name -> Option D

4. Quick Check:

   Class selector needs dot (.) [OK]

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

Common Mistakes:

• Forgetting the dot before class name
• Confusing element selectors with class selectors
• Assuming uppercase class names are required

5. You want to style all buttons with class primary to have a blue background and white text, but only when hovered. Which CSS code correctly uses class selectors and pseudo-classes?

hard

A. .primary:hover { background-color: blue; color: white; }

B. #primary:hover { background-color: blue; color: white; }

C. button.primary { background-color: blue; color: white; }

D. .primary { background-color: blue; color: white; }

Solution

1. Step 1: Identify correct class selector with hover

   To style elements with class "primary" on hover, use .primary:hover.

2. Step 2: Check other options

   #primary targets an ID, not class. button.primary styles all buttons with class but not on hover. .primary styles all with class but no hover effect.

3. Final Answer:

   .primary:hover { background-color: blue; color: white; } -> Option A

4. Quick Check:

   Class selector + :hover pseudo-class = .primary:hover { background-color: blue; color: white; } [OK]

Hint: Use .class:hover for hover styles on class elements [OK]

Common Mistakes:

• Using # instead of . for class selectors
• Forgetting :hover for hover effect
• Styling without hover when hover is needed