Using variables in CSS - Performance & Optimization

Performance: Using variables

LOW IMPACT

Using CSS variables affects the style calculation and paint stages by enabling reuse of values without repeating code.

Defining and reusing colors across multiple elements

CSS

:root { --main-color: #3498db; }
.button { color: var(--main-color); }
.header { background-color: var(--main-color); }
.footer { border-color: var(--main-color); }

Using a variable centralizes the value, reducing repetition and style recalculation when the variable changes.

📈 Performance GainSingle style recalculation for all uses of the variable, smaller CSS size

Defining and reusing colors across multiple elements

CSS

:root { }
.button { color: #3498db; }
.header { background-color: #3498db; }
.footer { border-color: #3498db; }

Repeating the same color value multiple times increases CSS size and style recalculation when changes occur.

📉 Performance CostIncreases CSS size slightly and triggers multiple style recalculations if color changes

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Repeating color values	None	Multiple style recalculations if changed	Moderate paint cost if styles change	[X] Bad
Using CSS variables	None	Single style recalculation on variable change	Lower paint cost due to fewer recalculations	[OK] Good

Rendering Pipeline

CSS variables are resolved during style calculation. When a variable changes, only affected elements recalculate styles, reducing layout and paint work.

→Style Calculation

→Paint

⚠️ BottleneckStyle Calculation when variables change frequently

Optimization Tips

1Define CSS variables at the root level for broad reuse.

2Avoid frequent dynamic changes to CSS variables to reduce style recalculations.

3Use variables to reduce CSS size and improve maintainability with minimal performance cost.

Performance Quiz - 3 Questions

How do CSS variables improve performance compared to repeating the same value multiple times?

A They increase the number of DOM nodes

B They block rendering until all variables load

C They reduce style recalculations by centralizing values

D They cause more paint events

DevTools: Performance

How to check: Record a performance profile while changing CSS variables dynamically or switching themes. Observe style recalculation and paint events.

What to look for: Look for fewer style recalculations and paint events when using variables compared to repeated values.

Practice

1. What is the main purpose of CSS variables?

easy

A. To store reusable values like colors and sizes

B. To create new HTML elements

C. To write JavaScript code inside CSS

D. To add comments in CSS files

Solution

1. Step 1: Understand what CSS variables do

   CSS variables hold values that can be reused throughout the stylesheet, such as colors or font sizes.

2. Step 2: Compare options with this purpose

   Only To store reusable values like colors and sizes describes storing reusable values; others describe unrelated tasks.

3. Final Answer:

   To store reusable values like colors and sizes -> Option A

4. Quick Check:

   CSS variables = reusable values [OK]

Hint: CSS variables store values you reuse often [OK]

Common Mistakes:

• Thinking CSS variables create HTML elements
• Confusing CSS variables with JavaScript
• Believing CSS variables add comments

2. Which is the correct way to define a CSS variable for a primary color globally?

easy

A. :root { --primary-color: #3498db; }

B. body { primary-color: #3498db; }

C. :root { primary-color = #3498db; }

D. html { --primary-color #3498db; }

Solution

1. Step 1: Recall CSS variable syntax

   Variables are defined with two dashes and a colon inside a selector, usually :root for global scope.

2. Step 2: Check each option's syntax

   :root { --primary-color: #3498db; } uses correct syntax: :root { --name: value; }. Others have missing dashes, wrong selectors, or wrong assignment.

3. Final Answer:

   :root { --primary-color: #3498db; } -> Option A

4. Quick Check:

   Define variables with --name: value; inside :root [OK]

Hint: Use :root and --var-name: value; to define variables [OK]

Common Mistakes:

• Missing double dashes before variable name
• Using = instead of : for assignment
• Defining variables outside :root for global use

3. Given the CSS below, what color will the paragraph text be?

:root { --main-color: #ff0000; } p { color: var(--main-color); }

medium

A. Blue

B. Green

C. Black

D. Red

Solution

1. Step 1: Identify the variable value

   The variable --main-color is set to #ff0000, which is red.

2. Step 2: Check how the variable is used

   The paragraph's color is set using var(--main-color), so it uses the red color.

3. Final Answer:

   Red -> Option D

4. Quick Check:

   var(--main-color) = #ff0000 (red) [OK]

Hint: Match variable value hex to color name [OK]

Common Mistakes:

• Ignoring the variable and using default color
• Confusing hex codes with other colors
• Not using var() to apply variable

4. What is wrong with this CSS code?

:root { --font-size 16px; } h1 { font-size: var(--font-size); }

medium

A. Variable name should not start with dashes

B. Missing colon after --font-size in variable definition

C. Using var() incorrectly in h1 font-size

D. font-size property cannot use variables

Solution

1. Step 1: Check variable definition syntax

   The variable definition is missing a colon after --font-size; it should be --font-size: 16px;

2. Step 2: Verify usage of var()

   The usage var(--font-size) is correct in h1 font-size property.

3. Final Answer:

   Missing colon after --font-size in variable definition -> Option B

4. Quick Check:

   Variable definitions need colon after name [OK]

Hint: Variable definitions need colon after name [OK]

Common Mistakes:

• Forgetting colon in variable definition
• Thinking var() usage is wrong
• Believing variable names can't start with --

5. You want to create a theme with two colors using CSS variables: primary as blue (#0000ff) and secondary as gray (#888888). How do you apply these variables to style a button's background and border color?

hard

A. :root { --primary: blue; --secondary: gray; } button { background-color: --primary; border: 2px solid --secondary; }

B. :root { primary: #0000ff; secondary: #888888; } button { background-color: primary; border: 2px solid secondary; }

C. :root { --primary: #0000ff; --secondary: #888888; } button { background-color: var(--primary); border: 2px solid var(--secondary); }

D. button { --primary: #0000ff; --secondary: #888888; background-color: var(primary); border: 2px solid var(secondary); }

Solution

1. Step 1: Define variables globally with correct syntax

   Variables must be defined inside :root with double dashes and colon, e.g., --primary: #0000ff;

2. Step 2: Use variables with var() in button styles

   Apply variables using var(--primary) for background and var(--secondary) for border color.

3. Final Answer:

   :root { --primary: #0000ff; --secondary: #888888; } button { background-color: var(--primary); border: 2px solid var(--secondary); } -> Option C

4. Quick Check:

   Define with --name: value; use with var(--name) [OK]

Hint: Define variables in :root and use var(--name) to apply [OK]

Common Mistakes:

• Omitting -- in variable names
• Not using var() when applying variables
• Defining variables inside button instead of :root