CSS custom properties vs SASS variables - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

How do CSS custom properties behave differently from SASS variables?

Which statement correctly describes a key difference between CSS custom properties and SASS variables?

A CSS custom properties are only available during SASS compilation, while SASS variables exist in the browser.

B SASS variables can be updated by JavaScript after the page loads, but CSS custom properties cannot.

C SASS variables support inheritance in CSS selectors, but CSS custom properties do not.

D CSS custom properties can be changed dynamically in the browser at runtime, while SASS variables are fixed at compile time.

💡 Hint

Think about when each variable type is processed and if they can be changed after the page loads.

CSS custom properties (variables) are part of the CSS language and exist in the browser. They can be updated dynamically using JavaScript or CSS at runtime. SASS variables are processed during the build step and do not exist in the browser, so they cannot be changed after compilation.

📝 Syntax

intermediate

Identify the valid CSS custom property syntax

Which option shows the correct way to define and use a CSS custom property?

A :root { $main-color: #3498db; } .btn { color: var($main-color); }

B :root { --main-color: #3498db; } .btn { color: $main-color; }

C :root { --main-color: #3498db; } .btn { color: var(--main-color); }

D :root { --main-color = #3498db; } .btn { color: var(--main-color); }

💡 Hint

CSS custom properties start with two dashes and use var() to access.

CSS custom properties are defined with two dashes (--) and accessed with the var() function. SASS variables use $ and are not valid in CSS.

❓ rendering

advanced

What color will the button text be?

Given this SASS and CSS code, what color will the button text appear in the browser?

SASS

$main-color: red;
:root {
  --main-color: blue;
}
.btn {
  color: $main-color;
  background-color: var(--main-color);
}

A Red text on blue background

B Blue text on blue background

C Red text on red background

D Blue text on red background

💡 Hint

Remember which variables are replaced during compilation and which are used by the browser.

The SASS variable $main-color is replaced with red during compilation, so color: $main-color; becomes color: red;. The CSS custom property --main-color is blue and used at runtime for background color.

❓ selector

advanced

Which selector correctly changes a CSS custom property for a specific element?

You want to change the CSS custom property --main-color only for buttons inside a <section>. Which CSS snippet does this correctly?

A section button { --main-color: green; }

B section button { color: var(--main-color: green); }

C section button { --main-color = green; }

D section button { $main-color: green; }

💡 Hint

CSS custom properties are set like normal CSS properties with two dashes.

To change a CSS custom property for specific elements, you set it as a normal CSS property with two dashes inside the selector. Option A does this correctly. Options A and B have invalid syntax. Option A uses SASS variable syntax, which is invalid in CSS.

❓ accessibility

expert

How can CSS custom properties improve accessibility?

Which approach best uses CSS custom properties to help users with visual impairments switch between color themes?

A Use SASS variables to define colors and recompile CSS for each theme.

B Define color variables with CSS custom properties and change their values on a parent element when toggling themes.

C Hardcode colors in CSS and use JavaScript to replace all color values inline when switching themes.

D Use inline styles on each element to set colors directly for each theme.

💡 Hint

Think about which method allows changing colors without recompiling or editing many elements.

CSS custom properties can be changed dynamically on a parent element, allowing easy theme switching without recompiling or editing many styles. This improves accessibility by letting users toggle themes instantly. SASS variables are static and require recompilation. Inline styles and JavaScript replacements are less efficient and harder to maintain.

Practice

1. Which of the following is true about CSS custom properties compared to SASS variables?

easy

A. SASS variables start with -- and are dynamic in the browser.

B. SASS variables start with -- and work in the browser at runtime.

C. CSS custom properties start with $ and are replaced before the browser sees CSS.

D. CSS custom properties start with -- and work in the browser at runtime.

Solution

1. Step 1: Identify CSS custom properties syntax

   CSS custom properties always start with -- and are live in the browser, meaning they can be changed dynamically.

2. Step 2: Identify SASS variables syntax and behavior

   SASS variables start with $ and are replaced during the build process, so they don't exist in the browser.

3. Final Answer:

   CSS custom properties start with -- and work in the browser at runtime. -> Option D

4. Quick Check:

   CSS variables = -- prefix and runtime [OK]

Hint: CSS vars use -- and work live; SASS vars use $ and compile away [OK]

Common Mistakes:

• Confusing $ and -- prefixes
• Thinking SASS variables work in the browser
• Assuming CSS variables are replaced before runtime

2. Which is the correct way to declare a SASS variable for a color?

easy

A. primary-color: #3498db;

B. --primary-color: #3498db;

C. $primary-color: #3498db;

D. $primary-color == #3498db;

Solution

1. Step 1: Recall SASS variable syntax

   SASS variables start with $ and use a colon : to assign values.

2. Step 2: Check each option

   $primary-color: #3498db; uses correct syntax. --primary-color: #3498db; uses the CSS custom property prefix. primary-color: #3498db; omits the $ prefix. $primary-color == #3498db; uses double equals which is invalid.

3. Final Answer:

   $primary-color: #3498db; -> Option C

4. Quick Check:

   SASS variable = $name: value; [OK]

Hint: SASS vars start with $ and use colon for assignment [OK]

Common Mistakes:

• Using -- instead of $ for SASS variables
• Using == instead of : for assignment
• Omitting $ prefix

3. Given the following code, what color will the text be in the browser?

:root { --main-color: red; }
.box { color: var(--main-color); }
$main-color: blue;
.box2 { color: $main-color; }

medium

A. Both .box and .box2 text will be red

B. .box text will be red; .box2 text will be blue

C. .box text will be blue; .box2 text will be red

D. Both .box and .box2 text will be blue

Solution

1. Step 1: Understand CSS custom property usage

   The --main-color is set to red in :root. The class .box uses var(--main-color), so it will be red at runtime.

2. Step 2: Understand SASS variable usage

   The SASS variable $main-color is blue and used in .box2. This is replaced during compilation, so .box2 color will be blue in the final CSS.

3. Final Answer:

   .box text will be red; .box2 text will be blue -> Option B

4. Quick Check:

   CSS var = red; SASS var = blue [OK]

Hint: CSS vars live in browser; SASS vars replaced before browser sees CSS [OK]

Common Mistakes:

• Thinking SASS variables work at runtime
• Confusing which variable affects which class
• Assuming CSS variables are replaced before runtime

4. What is wrong with this SASS code snippet?

$color: green;
.text { color: var($color); }

medium

A. The var() function is only for CSS custom properties, not SASS variables.

B. The variable name should start with -- instead of $.

C. SASS variables cannot be used inside var() function.

D. The color value green is invalid.

Solution

1. Step 1: Understand var() function usage

   The var() function is a CSS function used only for CSS custom properties (those starting with --).

2. Step 2: Understand SASS variable usage

   SASS variables start with $ and are used directly without var(). Using var($color) is invalid because var() expects a CSS custom property name.

3. Final Answer:

   The var() function is only for CSS custom properties, not SASS variables. -> Option A

4. Quick Check:

   var() = CSS vars only, not SASS [OK]

Hint: Use $var directly in SASS; var() only for CSS custom properties [OK]

Common Mistakes:

• Using var() with SASS variables
• Confusing $ and -- prefixes
• Thinking var() works for SASS variables

5. You want to create a theme where users can switch colors dynamically in the browser. Which approach is best and why?

$primary-color: #0055ff;
:root { --primary-color: #0055ff; }

hard

A. Use CSS custom property --primary-color because it can change dynamically in the browser.

B. Use SASS variable $primary-color because it compiles faster.

C. Use both SASS and CSS variables together for better performance.

D. Use inline styles instead of variables for dynamic changes.

Solution

1. Step 1: Understand dynamic changes in the browser

   CSS custom properties (starting with --) live in the browser and can be changed with JavaScript or media queries without recompiling CSS.

2. Step 2: Understand SASS variable limitations

   SASS variables are replaced during build time and cannot change after the CSS is loaded, so they are not suitable for dynamic theming.

3. Final Answer:

   Use CSS custom property --primary-color because it can change dynamically in the browser. -> Option A

4. Quick Check:

   Dynamic theme = CSS vars [OK]

Hint: Dynamic changes need CSS vars; SASS vars are static [OK]

Common Mistakes:

• Choosing SASS variables for runtime changes
• Thinking both together improve dynamic behavior
• Ignoring CSS custom properties' runtime flexibility