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Critical CSS Generator - Extract Above-the-Fold Styles

Critical CSS Generator

Extract inline styles and identify external stylesheets from this page

What is Critical CSS?

Critical CSS is the minimum set of CSS needed to render the above-the-fold content (the part of the webpage visible without scrolling). By inlining critical CSS and deferring non-critical styles, you can significantly improve page load performance and Core Web Vitals scores.

Why Critical CSS Matters
  • Faster First Paint: Reduces time to first meaningful content render
  • Better Core Web Vitals: Improves Largest Contentful Paint (LCP) score
  • Eliminates Render Blocking: Prevents CSS from blocking page rendering
  • Improved Perceived Performance: Users see content faster
  • Better Mobile Experience: Especially important on slower connections
How to Implement Critical CSS
1. Extract Critical CSS

Use this tool or other tools like:

2. Inline Critical CSS
<head>
  <style>
    /* Critical CSS inlined here */
    body { margin: 0; font-family: sans-serif; }
    .header { background: #333; color: white; padding: 20px; }
    /* Only include styles needed for above-the-fold content */
  </style>

  <!-- Defer non-critical CSS -->
  <link rel="preload" href="/styles/main.css" as="style" onload="this.onload=null;this.rel='stylesheet'">
  <noscript><link rel="stylesheet" href="/styles/main.css"></noscript>
</head>
3. Using JavaScript to Load CSS Asynchronously
<script>
  // Load CSS asynchronously
  function loadCSS(href) {
    var link = document.createElement('link');
    link.rel = 'stylesheet';
    link.href = href;
    document.head.appendChild(link);
  }

  // Load non-critical CSS after page load
  window.addEventListener('load', function() {
    loadCSS('/styles/non-critical.css');
  });
</script>
Best Practices
  • Keep it small: Critical CSS should be under 14KB (compressed)
  • Test on different viewports: Critical CSS differs for mobile vs desktop
  • Automate extraction: Use build tools to extract critical CSS automatically
  • Update regularly: Regenerate when you make layout changes
  • Don't duplicate: Remove critical CSS from main stylesheet to avoid duplication
  • Test thoroughly: Ensure deferred styles load correctly
Alternative Approaches
1. HTTP/2 Server Push

Push critical CSS to the browser before it's requested:

# Server configuration
Link: </styles/critical.css>; rel=preload; as=style
2. Modern CSS Loading
<!-- Modern browsers -->
<link rel="stylesheet" href="/styles/main.css" media="print" onload="this.media='all'">

<!-- Fallback for browsers without JS -->
<noscript>
  <link rel="stylesheet" href="/styles/main.css">
</noscript>
3. CSS-in-JS Solutions

Frameworks like styled-components or Emotion automatically handle critical CSS extraction in React applications.

Measuring Impact

Use these tools to measure the impact of critical CSS optimization:

  • Google Lighthouse: Check performance score and LCP
  • WebPageTest: View filmstrip and start render time
  • Chrome DevTools: Performance tab and Coverage tool
  • PageSpeed Insights: Real-world Core Web Vitals data
Common Pitfalls
  • Too much critical CSS: Keep it focused on above-the-fold content only
  • Flash of unstyled content (FOUC): Test that deferred styles load properly
  • Duplicate CSS: Remove critical styles from main stylesheet
  • Outdated critical CSS: Regenerate after design changes
  • Not testing mobile: Mobile critical CSS is often different from desktop
Example: Before and After
Before (Render Blocking): 
<head>
  <link rel="stylesheet" href="/css/main.css">
  <link rel="stylesheet" href="/css/components.css">
</head>

Page waits for all CSS to download before rendering

After (Optimized): 
<head>
  <style>
    /* Critical CSS inlined */
  </style>
  <link rel="preload" href="/css/main.css" as="style" onload="this.rel='stylesheet'">
</head>

Page renders immediately with critical styles

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