CSS Modules in NextJS - Performance & Optimization

Performance: CSS Modules

MEDIUM IMPACT

CSS Modules affect how styles are loaded and scoped, impacting page load speed and style recalculation.

Applying styles scoped to components without global conflicts

NextJS

import styles from './Button.module.css';

function Button() {
  return <button className={styles.primary}>Click me</button>;
}

CSS Modules load only styles used by the component, reducing CSS size and avoiding conflicts.

📈 Performance GainSaves 30-70kb in CSS bundle, faster LCP due to smaller CSS

Applying styles scoped to components without global conflicts

NextJS

import './styles.css';

function Button() {
  return <button className="btn primary">Click me</button>;
}

Global CSS loads all styles on every page, increasing CSS size and causing potential style conflicts.

📉 Performance CostAdds 50-100kb to CSS bundle, blocks rendering until CSS loads

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Global CSS import	Low (styles applied globally)	Low (styles trigger once)	Medium (large CSS size)	[!OK]
CSS Modules import	Low (scoped styles)	Low (minimal style recalculation)	Low (smaller CSS size)	[OK] Good

Rendering Pipeline

CSS Modules generate scoped CSS classes that the browser applies during style calculation and layout, reducing style recalculation and repaint.

→Style Calculation

→Layout

→Paint

⚠️ BottleneckStyle Calculation

Core Web Vital Affected

LCP

CSS Modules affect how styles are loaded and scoped, impacting page load speed and style recalculation.

Optimization Tips

1Use CSS Modules to scope styles locally and reduce global CSS size.

2Smaller CSS bundles improve Largest Contentful Paint (LCP).

3Avoid loading large global CSS files that block rendering.

Performance Quiz - 3 Questions

How do CSS Modules improve page load performance compared to global CSS?

A By using inline styles instead of CSS files

B By loading only component-specific styles, reducing CSS bundle size

C By loading all styles globally to cache them faster

D By delaying CSS loading until user interaction

DevTools: Network

How to check: Open DevTools, go to Network tab, filter by CSS, and compare CSS file sizes loaded with global CSS vs CSS Modules.

What to look for: Look for smaller CSS file sizes and fewer style recalculations in Performance tab for CSS Modules.

Practice

1. What is the main benefit of using CSS Modules in a Next.js project?

easy

A. They enable global CSS styles to override component styles.

B. They automatically minify CSS files for faster loading.

C. They scope CSS styles locally to components, avoiding conflicts.

D. They allow CSS to be written inside JavaScript files only.

Solution

1. Step 1: Understand CSS Modules purpose

   CSS Modules scope CSS locally to components, so styles don't leak or conflict.

2. Step 2: Compare options with this purpose

   Only They scope CSS styles locally to components, avoiding conflicts. describes this key benefit. Others describe unrelated features.

3. Final Answer:

   They scope CSS styles locally to components, avoiding conflicts. -> Option C

4. Quick Check:

   CSS Modules = Local scoped styles [OK]

Hint: Remember: CSS Modules isolate styles per component [OK]

Common Mistakes:

• Thinking CSS Modules minify CSS automatically
• Confusing CSS Modules with CSS-in-JS
• Believing CSS Modules create global styles

2. Which of the following is the correct way to import a CSS Module file named Button.module.css in a Next.js component?

easy

A. import styles from './Button.css';

B. import styles from './Button.module.css';

C. import './Button.module.css';

D. import styles from './Button.cssmodule';

Solution

1. Step 1: Identify correct file extension for CSS Modules

   Next.js requires CSS Modules files to end with .module.css.

2. Step 2: Check import syntax

   Importing with import styles from './Button.module.css'; correctly imports the module as an object.

3. Final Answer:

   import styles from './Button.module.css'; -> Option B

4. Quick Check:

   Import CSS Modules with .module.css and assign to variable [OK]

Hint: Always use .module.css extension and import with 'from' [OK]

Common Mistakes:

• Using wrong file extension like .css or .cssmodule
• Importing without assigning to a variable
• Forgetting the .module part in filename

3. Given the CSS Module styles.module.css with content:

.title { color: red; }

And the React component:

import styles from './styles.module.css';

export default function Header() {
  return <h1 className={styles.title}>Hello</h1>
}

What color will the <h1> text be when rendered?

medium

A. Red

B. Black (default)

C. Blue

D. Green

Solution

1. Step 1: Check CSS Module style for .title

   The CSS sets .title color to red.

2. Step 2: Confirm component uses styles.title as className

   The component applies className={styles.title}, so the red color applies.

3. Final Answer:

   Red -> Option A

4. Quick Check:

   CSS Module class applied = red text [OK]

Hint: ClassName from styles object applies CSS Module styles [OK]

Common Mistakes:

• Assuming default black color without checking CSS
• Confusing CSS Modules with global CSS
• Not using styles.title but a string instead

4. What is wrong with this Next.js component using CSS Modules?

import styles from './Card.module.css';

export default function Card() {
  return <div className="styles.card">Content</div>
}

medium

A. The component must be a class component to use CSS Modules.

B. The CSS file should be imported with a different name.

C. The CSS Module file must be named Card.css, not Card.module.css.

D. The className should use curly braces, not quotes, to apply styles.card.

Solution

1. Step 1: Identify how className is assigned

   The code uses className="styles.card", which is a string literal, not the variable.

2. Step 2: Correct syntax for applying CSS Module class

   It should be className={styles.card} to use the imported styles object.

3. Final Answer:

   The className should use curly braces, not quotes, to apply styles.card. -> Option D

4. Quick Check:

   Use curly braces to apply CSS Module classes [OK]

Hint: Use {styles.className}, not "styles.className" for CSS Modules [OK]

Common Mistakes:

• Using quotes instead of curly braces for className
• Renaming CSS Module file incorrectly
• Thinking CSS Modules require class components

5. You want to style a Next.js component with two CSS Module classes: primary and large from Button.module.css. Which is the correct way to apply both classes to a <button> element?

hard

A. Click

B. Click

C. Click

D. Click

Solution

1. Step 1: Understand how to combine multiple CSS Module classes

   Use template literals or string concatenation to combine classes as strings.

2. Step 2: Evaluate each option

   Click uses template literals correctly to combine classes with a space.
   Click uses a string literal, so classes won't apply.
   Click concatenates without space, merging class names incorrectly.
   Click passes an array, which React does not accept as className.

3. Final Answer:

   <button className={`${styles.primary} ${styles.large}`}>Click</button> -> Option A

4. Quick Check:

   Combine CSS Module classes with template literals [OK]

Hint: Use backticks and ${} to combine multiple CSS Module classes [OK]

Common Mistakes:

• Using quotes instead of variables for className
• Concatenating without spaces between classes
• Passing arrays directly to className