Bird
Raised Fist0
SASSmarkup~7 mins

Critical CSS extraction strategies in SASS

Choose your learning style10 modes available
LearnWhyDeepRenderTryChallengeProjectRecallPerf

Start learning this pattern below

Jump into concepts and practice - no test required

or
Recommended
Test this pattern10 questions across easy, medium, and hard to know if this pattern is strong
Introduction

Critical CSS helps your webpage load faster by showing important styles first. It makes the page look good quickly while other styles load in the background.

When you want your website's main content to appear quickly on screen.
If your page has many styles but only a few are needed right away.
To improve user experience on slow internet connections.
When optimizing for search engines that value fast loading pages.
If you want to reduce the chance of users leaving before the page loads.
Syntax
SASS
// Example of critical CSS in SASS
// Define critical styles separately
$critical-bg: #fff;
$critical-color: #333;

.critical {
  background-color: $critical-bg;
  color: $critical-color;
  font-weight: bold;
}

// Other styles load later
.other {
  color: #666;
  padding: 1rem;
}

Critical CSS is usually small and focused on above-the-fold content.

Use variables to keep critical styles consistent and easy to update.

Examples
This styles the main header quickly for fast display.
SASS
// Simple critical CSS block
.critical-header {
  font-size: 2rem;
  color: navy;
  margin: 0;
}
Adjusts critical styles for small screens to keep text readable.
SASS
// Critical CSS with media query
@media (max-width: 600px) {
  .critical-header {
    font-size: 1.5rem;
  }
}
Mixins help reuse critical style rules easily.
SASS
// Using SASS mixin for critical styles
@mixin critical-text {
  font-weight: 700;
  color: darkred;
}

.critical-alert {
  @include critical-text;
  background: #ffe0e0;
}
Sample Program

This example shows critical CSS in the <style> tag inside <head>. It styles the page background and header quickly. The rest of the styles come from an external file loaded later.

SASS
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1" />
  <title>Critical CSS Example</title>
  <style>
    /* Critical CSS extracted and inlined */
    body {
      margin: 0;
      font-family: Arial, sans-serif;
      background-color: #fff;
      color: #222;
    }
    header {
      background-color: #007acc;
      color: white;
      padding: 1rem;
      font-size: 1.5rem;
      font-weight: bold;
    }
  </style>
  <link rel="stylesheet" href="styles.css" />
</head>
<body>
  <header>Welcome to My Site</header>
  <main>
    <p>This content loads styled after the critical CSS.</p>
  </main>
</body>
</html>
OutputSuccess
Important Notes

Keep critical CSS as small as possible to speed up loading.

Use tools or build steps to automatically extract critical CSS from your full styles.

Test on different devices to ensure critical styles cover all important content.

Summary

Critical CSS shows important styles first for faster page display.

Extract critical CSS by focusing on above-the-fold content and using SASS features like variables and mixins.

Inline critical CSS in the HTML head and load other styles separately for best performance.

Practice

(1/5)
1. What is the main goal of Critical CSS extraction in web development?
easy
A. To load only the CSS needed for above-the-fold content first
B. To combine all CSS files into one large file
C. To remove all CSS comments for smaller file size
D. To delay loading CSS until user interaction

Solution

  1. Step 1: Understand the purpose of critical CSS

    Critical CSS focuses on styles needed to display the visible part of the page immediately.

  2. Step 2: Identify the correct goal

    Loading only above-the-fold CSS first speeds up page rendering and improves user experience.

  3. Final Answer:

    To load only the CSS needed for above-the-fold content first -> Option A

  4. Quick Check:

    Critical CSS = Above-the-fold styles first [OK]
Hint: Critical CSS means styles for visible content load first [OK]
Common Mistakes:
  • Thinking critical CSS means all CSS combined
  • Believing critical CSS removes comments only
  • Assuming CSS loads only after user clicks
2. Which SASS feature helps reuse styles when extracting critical CSS?
easy
A. Mixins
B. Variables
C. Nesting
D. Functions

Solution

  1. Step 1: Review SASS features for reuse

    Mixins allow you to define reusable blocks of styles that can be included where needed.

  2. Step 2: Match feature to critical CSS extraction

    Using mixins helps keep critical CSS organized and reusable across components.

  3. Final Answer:

    Mixins -> Option A

  4. Quick Check:

    Reusable style blocks = Mixins [OK]
Hint: Mixins reuse style blocks easily in SASS [OK]
Common Mistakes:
  • Confusing variables with reusable style blocks
  • Thinking nesting creates reusable styles
  • Assuming functions output CSS directly
3. Given this SASS snippet for critical CSS extraction:
@mixin critical-button {
  background-color: blue;
  color: white;
  padding: 1rem;
}

.critical {
  @include critical-button;
  font-weight: bold;
}
What CSS will be generated for the .critical class?
medium
A. Error: mixin not found
B. .critical { background-color: blue; color: white; padding: 1rem; }
C. .critical { background-color: blue; color: white; padding: 1rem; font-weight: bold; }
D. .critical { font-weight: bold; }

Solution

  1. Step 1: Understand mixin inclusion

    The @include critical-button inserts all styles from the mixin into .critical.

  2. Step 2: Combine mixin styles with additional styles

    The .critical class gets background-color, color, padding from mixin plus font-weight: bold.

  3. Final Answer:

    .critical { background-color: blue; color: white; padding: 1rem; font-weight: bold; } -> Option C

  4. Quick Check:

    Mixin styles + extra styles = full CSS block [OK]
Hint: Mixins insert all their styles where included [OK]
Common Mistakes:
  • Ignoring styles outside the mixin
  • Thinking mixins create separate CSS blocks
  • Assuming syntax error due to mixin
4. Identify the error in this SASS code for critical CSS extraction:
@mixin critical-text {
  font-size 1.2rem;
  color: black;
}

.text {
  @include critical-text;
}
medium
A. Cannot include mixins inside classes
B. Missing colon after font-size property
C. Mixin name is invalid
D. Color property value is incorrect

Solution

  1. Step 1: Check property syntax inside mixin

    The line font-size 1.2rem; is missing a colon between property and value.

  2. Step 2: Confirm correct SASS syntax

    All CSS properties require a colon, so this causes a syntax error.

  3. Final Answer:

    Missing colon after font-size property -> Option B

  4. Quick Check:

    Property syntax needs colon : [OK]
Hint: CSS properties always need a colon between name and value [OK]
Common Mistakes:
  • Assuming mixin names must be special
  • Thinking mixins can't be included in classes
  • Believing color value 'black' is invalid
5. You want to extract critical CSS for a webpage header using SASS. Which strategy best improves page load speed?
hard
A. Load all styles in one CSS file at the end of the body
B. Place critical CSS in an external file loaded with normally
C. Use SASS variables only without mixins for critical styles
D. Inline critical header styles in the HTML <head> and load other styles asynchronously

Solution

  1. Step 1: Understand critical CSS loading best practice

    Inlining critical CSS in the HTML <head> ensures styles are applied immediately during page load.

  2. Step 2: Compare other options

    Loading all styles at the end delays rendering; variables alone don't extract CSS; normal external loading delays critical styles.

  3. Final Answer:

    Inline critical header styles in the HTML <head> and load other styles asynchronously -> Option D

  4. Quick Check:

    Inline critical CSS + async load = faster page [OK]
Hint: Inline critical CSS in head for fastest render [OK]
Common Mistakes:
  • Loading all CSS at the end delays visible content
  • Using variables alone doesn't extract critical CSS
  • Loading critical CSS as normal external file delays render