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HTML Performance Optimization

Optimize HTML for faster page loads and better user experience. Learn techniques for reducing page weight, improving render speed, and maximizing web performance.

Why Performance Matters

53%

of users abandon sites that take > 3 seconds to load

1 second

delay = 7% reduction in conversions

100ms

faster = 1% increase in revenue (Amazon)

Top 3

Google ranking factor for search

Critical Rendering Path

Understanding how browsers render HTML is key to optimization:

  1. Parse HTML → Build DOM tree
  2. Parse CSS → Build CSSOM tree
  3. Combine DOM + CSSOM → Render tree
  4. Layout → Calculate positions
  5. Paint → Draw pixels

Goal: Minimize blocking resources and reduce time to first paint.

Optimize HTML Document Structure

✅ Optimized HTML Template

HTML
<!DOCTYPE html>
<html lang="en">
<head>
    <!-- Essential meta tags first --&gt;
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    
    <!-- Preconnect to external domains --&gt;
    <link rel="preconnect" href="https://fonts.googleapis.com">
    <link rel="preconnect" href="https://cdn.example.com">
    
    <!-- Preload critical resources --&gt;
    <link rel="preload" href="/fonts/main.woff2" as="font" type="font/woff2" crossorigin>
    <link rel="preload" href="/images/hero.jpg" as="image">
    
    <!-- Critical CSS inline --&gt;
    <style>
        /* Above-the-fold CSS here */
        body { margin: 0; font-family: sans-serif; }
        .hero { height: 100vh; background: #333; }
    </style>
    
    <title>Page Title</title>
    
    <!-- Defer non-critical CSS --&gt;
    <link rel="stylesheet" href="/css/main.css" media="print" onload="this.media='all'">
    <noscript><link rel="stylesheet" href="/css/main.css"></noscript>
</head>
<body>
    <!-- Content here --&gt;
    
    <!-- Scripts at end with defer/async --&gt;
    <script src="/js/main.js" defer></script>
    <script src="/js/analytics.js" async></script>
</body>
</html>

Optimize Images

Lazy Loading

HTML
<!-- Native lazy loading (modern browsers) --&gt;
<img src="image.jpg" 
     alt="Description" 
     loading="lazy"
     width="800" 
     height="600">

<!-- Load hero image immediately --&gt;
<img src="hero.jpg" 
     alt="Hero" 
     loading="eager"
     fetchpriority="high">

<!-- Lazy load all below-the-fold images --&gt;
<img src="photo1.jpg" alt="Photo 1" loading="lazy">
<img src="photo2.jpg" alt="Photo 2" loading="lazy">
<img src="photo3.jpg" alt="Photo 3" loading="lazy">

Responsive Images (srcset)

HTML
<!-- Serve appropriate image size --&gt;
<img src="image-400w.jpg"
     srcset="image-400w.jpg 400w,
             image-800w.jpg 800w,
             image-1200w.jpg 1200w"
     sizes="(max-width: 600px) 100vw,
            (max-width: 900px) 50vw,
            800px"
     alt="Responsive image"
     loading="lazy">

Modern Image Formats

HTML
<!-- Use WebP with fallback --&gt;
<picture>
    <source type="image/webp" srcset="image.webp">
    <source type="image/jpeg" srcset="image.jpg">
    <img src="image.jpg" alt="Image" loading="lazy">
</picture>

<!-- Or AVIF (even better compression) --&gt;
<picture>
    <source type="image/avif" srcset="image.avif">
    <source type="image/webp" srcset="image.webp">
    <img src="image.jpg" alt="Image" loading="lazy">
</picture>

Image Dimensions

HTML
<!-- Always specify width and height (prevents layout shift) --&gt;
<img src="image.jpg" 
     alt="Description"
     width="800" 
     height="600"
     loading="lazy">

<!-- Use aspect-ratio in CSS --&gt;
<style>
img {
    aspect-ratio: 16 / 9;
    width: 100%;
    height: auto;
}
</style>

Optimize JavaScript Loading

defer vs async vs blocking

HTML
<!-- ❌ Blocking (blocks HTML parsing) --&gt;
<script src="script.js"></script>

<!-- ✅ async: Download in parallel, execute ASAP --&gt;
<script src="analytics.js" async></script>
<!-- Good for: Independent scripts (analytics, ads) --&gt;

<!-- ✅ defer: Download in parallel, execute after HTML parsed --&gt;
<script src="app.js" defer></script>
<!-- Good for: Scripts that need DOM, maintain order --&gt;

<!-- ✅ Multiple deferred scripts execute in order --&gt;
<script src="library.js" defer></script>
<script src="app.js" defer></script>  <!-- Runs after library.js --&gt;

<!-- ✅ Put scripts at end of body --&gt;
<body>
    <!-- Content --&gt;
    <script src="script.js"></script>
</body>

Inline Critical JavaScript

HTML
<!-- Inline small, critical scripts --&gt;
<script>
    // Feature detection
    if ('serviceWorker' in navigator) {
        navigator.serviceWorker.register('/sw.js');
    }
    
    // Critical functionality
    const isDarkMode = localStorage.getItem('theme') === 'dark';
    if (isDarkMode) {
        document.documentElement.classList.add('dark');
    }
</script>

<!-- Load non-critical scripts later --&gt;
<script src="non-critical.js" defer></script>

Optimize CSS Loading

Critical CSS Inline

HTML
<head>
    <!-- Inline critical above-the-fold CSS --&gt;
    <style>
        /* Only styles needed for initial render */
        body {
            margin: 0;
            font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
        }
        .hero {
            min-height: 100vh;
            background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
        }
    </style>
    
    <!-- Defer non-critical CSS --&gt;
    <link rel="stylesheet" href="main.css" media="print" onload="this.media='all'">
    <noscript><link rel="stylesheet" href="main.css"></noscript>
</head>

Avoid CSS @import

HTML
<!-- ❌ CSS @import blocks rendering --&gt;
<style>
    @import url('other.css');
</style>
HTML
<!-- ✅ Use link tags instead --&gt;
<link rel="stylesheet" href="main.css">
<link rel="stylesheet" href="other.css">

Resource Hints

Preconnect

HTML
<!-- Establish early connections to important origins --&gt;
<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://cdn.example.com">
<link rel="preconnect" href="https://api.example.com">

DNS Prefetch

HTML
<!-- Resolve DNS ahead of time --&gt;
<link rel="dns-prefetch" href="https://analytics.google.com">
<link rel="dns-prefetch" href="https://www.googletagmanager.com">

Preload

HTML
<!-- Preload critical resources --&gt;
<link rel="preload" href="/fonts/main.woff2" as="font" type="font/woff2" crossorigin>
<link rel="preload" href="/hero-image.jpg" as="image">
<link rel="preload" href="/critical.css" as="style">
<link rel="preload" href="/app.js" as="script">

Prefetch

HTML
<!-- Prefetch resources for next navigation --&gt;
<link rel="prefetch" href="/next-page.html">
<link rel="prefetch" href="/images/next-page-hero.jpg">

Minimize HTML Size

Remove Unnecessary Code

HTML
<!-- ❌ Bloated HTML --&gt;
<div class="container">
    <div class="wrapper">
        <div class="inner">
            <div class="content">
                <p>Text</p>
            </div>
        </div>
    </div>
</div>

<!-- Excessive comments --&gt;
<!-- This is a div --&gt;
<!-- This contains content --&gt;
<div>Content</div>
HTML
<!-- ✅ Minimal HTML --&gt;
<div class="container">
    <p>Text</p>
</div>

<!-- Useful comments only --&gt;
<!-- Product Grid Section --&gt;
<div class="product-grid">...</div>

Minify HTML

HTML
<!-- Before minification (2.5KB) --&gt;
<!DOCTYPE html>
<html>
    <head>
        <title>Page</title>
    </head>
    <body>
        <div class="container">
            <p>Content</p>
        </div>
    </body>
</html>

<!-- After minification (1.8KB) --&gt;
<!DOCTYPE html><html><head><title>Page</title></head><body><div class="container"><p>Content</p></div></body></html>

<!-- Use build tools:
   - html-minifier
   - terser
   - Webpack/Vite plugins --&gt;

Reduce Render-Blocking Resources

HTML
<head>
    <!-- ✅ Critical resources only --&gt;
    <style>
        /* Critical CSS inline */
    </style>
    
    <!-- ❌ Don't block rendering with non-critical CSS --&gt;
    <!-- <link rel="stylesheet" href="large.css"> --&gt;
    
    <!-- ✅ Defer non-critical CSS --&gt;
    <link rel="stylesheet" href="large.css" media="print" onload="this.media='all'">
    
    <!-- ❌ Don't load sync scripts in head --&gt;
    <!-- <script src="app.js"></script> --&gt;
    
    <!-- ✅ Defer or async scripts --&gt;
    <script src="app.js" defer></script>
</head>

Optimize Web Fonts

HTML
<head>
    <!-- Preconnect to font provider --&gt;
    <link rel="preconnect" href="https://fonts.googleapis.com">
    <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
    
    <!-- Load fonts with font-display --&gt;
    <link href="https://fonts.googleapis.com/css2?family=Roboto:wght@400;700&display=swap" rel="stylesheet">
    
    <!-- Or self-host fonts --&gt;
    <style>
        @font-face {
            font-family: 'Roboto';
            src: url('/fonts/roboto.woff2') format('woff2');
            font-display: swap;  /* Show fallback font first */
            font-weight: 400;
        }
    </style>
</head>

Reduce Layout Shift (CLS)

Reserve Space for Images

HTML
<!-- Always specify dimensions --&gt;
<img src="image.jpg" 
     alt="Description"
     width="800" 
     height="600">

<!-- Or use CSS aspect-ratio --&gt;
<style>
.image-container {
    aspect-ratio: 16 / 9;
}
</style>

Reserve Space for Ads/Embeds

HTML
<!-- Reserve space for ad --&gt;
<div class="ad-container" style="min-height: 250px;">
    <!-- Ad loads here --&gt;
</div>

<!-- Reserve space for iframe --&gt;
<div style="aspect-ratio: 16 / 9;">
    <iframe src="video.html" style="width: 100%; height: 100%;"></iframe>
</div>

Performance Measurement Tools

Google Lighthouse

  • Open Chrome DevTools (F12)
  • Go to Lighthouse tab
  • Run audit
  • Get performance score (0-100)

Core Web Vitals

  • LCP (Largest Contentful Paint): < 2.5s
  • FID (First Input Delay): < 100ms
  • CLS (Cumulative Layout Shift): < 0.1

Other Tools

  • WebPageTest: Detailed performance analysis
  • PageSpeed Insights: Google's performance tool
  • Chrome DevTools Performance: Record and analyze

Performance Checklist

HTML

  • ☑ Minify HTML in production
  • ☑ Remove unnecessary divs/wrappers
  • ☑ Remove unused code
  • ☑ Use semantic HTML

CSS

  • ☑ Inline critical CSS
  • ☑ Defer non-critical CSS
  • ☑ Minify CSS
  • ☑ Remove unused CSS
  • ☑ Avoid @import

JavaScript

  • ☑ Use defer/async
  • ☑ Put scripts at end of body
  • ☑ Minify JavaScript
  • ☑ Code splitting
  • ☑ Remove unused JavaScript

Images

  • ☑ Lazy load below-the-fold images
  • ☑ Use srcset for responsive images
  • ☑ Use WebP/AVIF formats
  • ☑ Compress images
  • ☑ Specify width/height
  • ☑ Use appropriate image sizes

Fonts

  • ☑ Use font-display: swap
  • ☑ Preload critical fonts
  • ☑ Use WOFF2 format
  • ☑ Limit font variations

Resources

  • ☑ Preconnect to external domains
  • ☑ Preload critical resources
  • ☑ Use CDN for static assets
  • ☑ Enable compression (gzip/brotli)
  • ☑ Use caching headers

What's Next?

Learn about modern HTML development with components and progressive web apps.

Next: Modern HTML Components →