CSS Performance ⚡

Optimize your CSS for faster loading, smoother interactions, and better user experience.

Why CSS Performance Matters?

CSS performance directly impacts user experience, conversion rates, and SEO. Optimized CSS leads to faster page loads, smoother animations, and better Core Web Vitals scores.

🚀 Faster Loading

Reduce render-blocking CSS

🎯 Better UX

Smooth interactions and animations

📈 Higher Conversions

Improved engagement and retention

Critical Rendering Path Optimization

Key Concepts:

DOM Construction: HTML parsing and DOM tree creation
CSSOM Construction: CSS parsing and CSSOM tree creation
Render Tree: Combination of DOM and CSSOM
Layout: Calculating element positions and sizes
Paint: Filling pixels with colors and textures

Critical Rendering Path Implementation

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>CSS Performance - Critical Rendering Path Optimization</title>
  <style>
    * {
      margin: 0;
      padding: 0;
      box-sizing: border-box;
    }
    
    body {
      font-family: 'Inter', sans-serif;
      line-height: 1.6;
      background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
      min-height: 100vh;
      padding: 2rem;
      color: #2c3e50;
    }
    
    .container {
      max-width: 1200px;
      margin: 0 auto;
    }
    
    .header {
      text-align: center;
      margin-bottom: 3rem;
      color: white;
    }
    
    .header h1 {
      font-size: 3rem;
      margin-bottom: 1rem;
      text-shadow: 2px 2px 4px rgba(0,0,0,0.3);
    }
    
    .concept-grid {
      display: grid;
      grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
      gap: 2rem;
      margin-bottom: 3rem;
    }
    
    .concept-card {
      background: white;
      border-radius: 15px;
      padding: 2rem;
      box-shadow: 0 10px 30px rgba(0,0,0,0.1);
    }
    
    .step-visual {
      background: #2c3e50;
      color: white;
      padding: 2rem;
      border-radius: 10px;
      margin: 2rem 0;
      font-family: 'Fira Code', monospace;
    }
    
    .step {
      margin: 1rem 0;
      padding: 1rem;
      border-left: 4px solid #3498db;
      background: #34495e;
    }
    
    .step-critical { border-color: #e74c3c; }
    .step-important { border-color: #f39c12; }
    .step-optimized { border-color: #2ecc71; }
    
    .comparison-grid {
      display: grid;
      grid-template-columns: 1fr 1fr;
      gap: 2rem;
      margin: 2rem 0;
    }
    
    .code-panel {
      background: white;
      border-radius: 10px;
      padding: 1.5rem;
      box-shadow: 0 5px 15px rgba(0,0,0,0.1);
    }
    
    .code-panel h3 {
      color: #2c3e50;
      margin-bottom: 1rem;
      padding-bottom: 0.5rem;
      border-bottom: 2px solid #3498db;
    }
    
    .good-code, .bad-code {
      background: #2c3e50;
      color: #ecf0f1;
      padding: 1rem;
      border-radius: 5px;
      font-family: 'Fira Code', monospace;
      font-size: 0.9rem;
      line-height: 1.4;
      overflow-x: auto;
    }
    
    .good-code { border-left: 4px solid #2ecc71; }
    .bad-code { border-left: 4px solid #e74c3c; }
    
    .comment { color: #7f8c8d; }
    .selector { color: #9b59b6; }
    .property { color: #3498db; }
    .value { color: #2ecc71; }
    
    .demo-area {
      background: white;
      border-radius: 15px;
      padding: 2rem;
      margin: 2rem 0;
      box-shadow: 0 10px 30px rgba(0,0,0,0.1);
    }
    
    .performance-metric {
      background: #f8f9fa;
      padding: 1rem;
      border-radius: 8px;
      margin: 1rem 0;
      border-left: 4px solid #3498db;
    }
    
    .metric-value {
      font-size: 1.5rem;
      font-weight: bold;
      color: #2c3e50;
    }
    
    .optimization-tip {
      background: #d4edda;
      border-left: 4px solid #28a745;
      padding: 1rem;
      margin: 1rem 0;
      border-radius: 5px;
    }
    
    @media (max-width: 768px) {
      .comparison-grid {
        grid-template-columns: 1fr;
      }
      
      .header h1 {
        font-size: 2rem;
      }
    }
  </style>
</head>
<body>
  <div class="container">
    <div class="header">
      <h1>⚡ CSS Performance</h1>
      <p>Optimize Critical Rendering Path and Improve User Experience</p>
    </div>
    
    <div class="concept-grid">
      <div class="concept-card">
        <h3>🎯 Critical Rendering Path</h3>
        <p>The sequence of steps browsers take to render content on screen.</p>
        <div class="performance-metric">
          <div class="metric-value">1. DOM Construction</div>
          <p>HTML → DOM Tree</p>
        </div>
        <div class="performance-metric">
          <div class="metric-value">2. CSSOM Construction</div>
          <p>CSS → CSSOM Tree</p>
        </div>
        <div class="performance-metric">
          <div class="metric-value">3. Render Tree</div>
          <p>DOM + CSSOM = Render Tree</p>
        </div>
      </div>
      
      <div class="concept-card">
        <h3>🚀 Performance Goals</h3>
        <ul>
          <li>First Contentful Paint &lt; 1s</li>
          <li>Largest Contentful Paint &lt; 2.5s</li>
          <li>Cumulative Layout Shift &lt; 0.1</li>
          <li>First Input Delay &lt; 100ms</li>
        </ul>
      </div>
    </div>
    
    <div class="step-visual">
      <h3>🔄 Browser Rendering Process</h3>
      <div class="step step-critical">
        <strong>1. DOM Construction (Critical)</strong><br>
        HTML parsing → DOM Tree creation
      </div>
      <div class="step step-critical">
        <strong>2. CSSOM Construction (Render-Blocking)</strong><br>
        CSS parsing → CSSOM Tree creation
      </div>
      <div class="step step-important">
        <strong>3. Render Tree</strong><br>
        DOM + CSSOM = Visible elements
      </div>
      <div class="step step-optimized">
        <strong>4. Layout</strong><br>
        Calculate element positions and sizes
      </div>
      <div class="step step-optimized">
        <strong>5. Paint</strong><br>
        Fill pixels with colors and textures
      </div>
    </div>
    
    <div class="comparison-grid">
      <div class="code-panel">
        <h3>✅ Optimized CSS Loading</h3>
        <div class="good-code">
          <span class="comment">&lt;!-- Critical CSS inlined --&gt;</span><br>
          <span class="selector">&lt;style&gt;</span><br>
          &nbsp;&nbsp;<span class="selector">.critical</span> {<br>
          &nbsp;&nbsp;&nbsp;&nbsp;<span class="property">display</span>: <span class="value">block</span>;<br>
          &nbsp;&nbsp;&nbsp;&nbsp;<span class="property">visibility</span>: <span class="value">visible</span>;<br>
          &nbsp;&nbsp;}<br>
          <span class="selector">&lt;/style&gt;</span><br><br>
          
          <span class="comment">&lt;!-- Non-critical CSS deferred --&gt;</span><br>
          <span class="selector">&lt;link</span> <span class="property">rel</span>=<span class="value">"preload"</span> <span class="property">href</span>=<span class="value">"styles.css"</span> <span class="property">as</span>=<span class="value">"style"</span> <span class="selector">/&gt;</span><br>
          <span class="selector">&lt;link</span> <span class="property">rel</span>=<span class="value">"stylesheet"</span> <span class="property">href</span>=<span class="value">"styles.css"</span> <span class="property">media</span>=<span class="value">"print"</span> <span class="property">onload</span>=<span class="value">"this.media='all'"</span> <span class="selector">/&gt;</span>
        </div>
      </div>
      
      <div class="code-panel">
        <h3>❌ Unoptimized CSS Loading</h3>
        <div class="bad-code">
          <span class="comment">&lt;!-- Multiple render-blocking CSS --&gt;</span><br>
          <span class="selector">&lt;link</span> <span class="property">rel</span>=<span class="value">"stylesheet"</span> <span class="property">href</span>=<span class="value">"reset.css"</span> <span class="selector">/&gt;</span><br>
          <span class="selector">&lt;link</span> <span class="property">rel</span>=<span class="value">"stylesheet"</span> <span class="property">href</span>=<span class="value">"framework.css"</span> <span class="selector">/&gt;</span><br>
          <span class="selector">&lt;link</span> <span class="property">rel</span>=<span class="value">"stylesheet"</span> <span class="property">href</span>=<span class="value">"components.css"</span> <span class="selector">/&gt;</span><br>
          <span class="selector">&lt;link</span> <span class="property">rel</span>=<span class="value">"stylesheet"</span> <span class="property">href</span>=<span class="value">"theme.css"</span> <span class="selector">/&gt;</span><br><br>
          
          <span class="comment">&lt;!-- CSS after content --&gt;</span><br>
          <span class="selector">&lt;div&gt;</span>Content loads before styles<span class="selector">&lt;/div&gt;</span><br>
          <span class="selector">&lt;link</span> <span class="property">rel</span>=<span class="value">"stylesheet"</span> <span class="property">href</span>=<span class="value">"styles.css"</span> <span class="selector">/&gt;</span>
        </div>
      </div>
    </div>
    
    <div class="demo-area">
      <h3>📊 Performance Impact Comparison</h3>
      
      <div class="comparison-grid">
        <div class="concept-card">
          <h4>Optimized Loading</h4>
          <div class="performance-metric">
            <div class="metric-value">~800ms</div>
            <p>First Contentful Paint</p>
          </div>
          <div class="performance-metric">
            <div class="metric-value">~1.2s</div>
            <p>Largest Contentful Paint</p>
          </div>
          <div class="performance-metric">
            <div class="metric-value">0.05</div>
            <p>Cumulative Layout Shift</p>
          </div>
        </div>
        
        <div class="concept-card">
          <h4>Unoptimized Loading</h4>
          <div class="performance-metric">
            <div class="metric-value">~2.5s</div>
            <p>First Contentful Paint</p>
          </div>
          <div class="performance-metric">
            <div class="metric-value">~4.1s</div>
            <p>Largest Contentful Paint</p>
          </div>
          <div class="performance-metric">
            <div class="metric-value">0.3</div>
            <p>Cumulative Layout Shift</p>
          </div>
        </div>
      </div>
    </div>
    
    <div class="optimization-tip">
      <strong>💡 Pro Tip:</strong> Use CSS containment (contain: layout paint style) to isolate rendering 
      of components and prevent unnecessary reflows and repaints.
    </div>
    
    <div class="concept-card">
      <h3>🔧 Critical CSS Extraction</h3>
      <p>Identify and inline CSS needed for above-the-fold content:</p>
      <div class="good-code">
        <span class="comment">/* Critical CSS (above-the-fold) */</span><br>
        <span class="selector">.header</span>, <span class="selector">.hero</span>, <span class="selector">.navigation</span> {<br>
        &nbsp;&nbsp;<span class="property">display</span>: <span class="value">block</span>;<br>
        &nbsp;&nbsp;<span class="property">visibility</span>: <span class="value">visible</span>;<br>
        &nbsp;&nbsp;<span class="property">opacity</span>: <span class="value">1</span>;<br>
        }<br><br>
        
        <span class="comment">/* Non-critical CSS (below-the-fold) */</span><br>
        <span class="selector">.footer</span>, <span class="selector">.sidebar</span>, <span class="selector">.related-posts</span> {<br>
        &nbsp;&nbsp;<span class="property">display</span>: <span class="value">none</span>;<br>
        &nbsp;&nbsp;<span class="property">animation</span>: <span class="value">fadeIn 0.5s ease-in</span>;<br>
        }
      </div>
    </div>
  </div>
</body>
</html>

Selector Efficiency & Optimization

Efficient Selectors

Fast, single-class selectors that browsers can match quickly.

.button-primary

#main-nav

input[type="text"]

Inefficient Selectors

Complex selectors that require extensive matching.

div nav ul li a

.container *

[class*="icon-"]

Selector Performance Implementation

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>CSS Performance - Selector Efficiency & Optimization</title>
  <style>
    * {
      margin: 0;
      padding: 0;
      box-sizing: border-box;
    }
    
    body {
      font-family: 'Inter', sans-serif;
      line-height: 1.6;
      background: linear-gradient(135deg, #4facfe 0%, #00f2fe 100%);
      min-height: 100vh;
      padding: 2rem;
      color: #2c3e50;
    }
    
    .container {
      max-width: 1200px;
      margin: 0 auto;
    }
    
    .header {
      text-align: center;
      margin-bottom: 3rem;
      color: white;
    }
    
    .header h1 {
      font-size: 3rem;
      margin-bottom: 1rem;
      text-shadow: 2px 2px 4px rgba(0,0,0,0.3);
    }
    
    .performance-grid {
      display: grid;
      grid-template-columns: repeat(auto-fit, minmax(500px, 1fr));
      gap: 2rem;
      margin-bottom: 3rem;
    }
    
    .performance-card {
      background: white;
      border-radius: 15px;
      padding: 2rem;
      box-shadow: 0 10px 30px rgba(0,0,0,0.1);
    }
    
    .selector-comparison {
      display: grid;
      grid-template-columns: 1fr 1fr;
      gap: 1rem;
      margin: 1rem 0;
    }
    
    .efficient, .inefficient {
      background: #2c3e50;
      color: #ecf0f1;
      padding: 1rem;
      border-radius: 5px;
      font-family: 'Fira Code', monospace;
      font-size: 0.8rem;
      line-height: 1.4;
    }
    
    .efficient { border-left: 4px solid #2ecc71; }
    .inefficient { border-left: 4px solid #e74c3c; }
    
    .comment { color: #7f8c8d; }
    .selector { color: #9b59b6; }
    .property { color: #3498db; }
    .value { color: #2ecc71; }
    
    .specificity-scale {
      background: #34495e;
      color: white;
      padding: 1.5rem;
      border-radius: 8px;
      margin: 1rem 0;
    }
    
    .specificity-level {
      margin: 0.5rem 0;
      padding: 0.5rem;
      border-radius: 4px;
    }
    
    .level-low { background: #27ae60; }
    .level-medium { background: #f39c12; }
    .level-high { background: #e74c3c; }
    .level-very-high { background: #8e44ad; }
    
    .performance-metrics {
      display: grid;
      grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
      gap: 1rem;
      margin: 2rem 0;
    }
    
    .metric-card {
      background: #f8f9fa;
      padding: 1.5rem;
      border-radius: 8px;
      text-align: center;
    }
    
    .metric-value {
      font-size: 1.5rem;
      font-weight: bold;
      color: #2c3e50;
    }
    
    .browser-process {
      background: #2c3e50;
      color: white;
      padding: 1.5rem;
      border-radius: 8px;
      margin: 1rem 0;
      font-family: 'Fira Code', monospace;
    }
    
    .process-step {
      margin: 0.5rem 0;
      color: #3498db;
    }
    
    .demo-area {
      background: white;
      border-radius: 15px;
      padding: 2rem;
      margin: 2rem 0;
      box-shadow: 0 10px 30px rgba(0,0,0,0.1);
    }
    
    @media (max-width: 768px) {
      .performance-grid {
        grid-template-columns: 1fr;
      }
      
      .selector-comparison {
        grid-template-columns: 1fr;
      }
      
      .header h1 {
        font-size: 2rem;
      }
    }
  </style>
</head>
<body>
  <div class="container">
    <div class="header">
      <h1>🎯 Selector Performance</h1>
      <p>Write efficient CSS selectors for faster rendering</p>
    </div>
    
    <!-- Selector Efficiency -->
    <div class="performance-card">
      <h3>⚡ Selector Efficiency Guide</h3>
      <p>Browsers read selectors from right to left. Efficient selectors are fast to match.</p>
      
      <div class="selector-comparison">
        <div class="efficient">
          <span class="comment">/* Efficient Selectors */</span><br><br>
          <span class="selector">.button-primary</span> <span class="comment">// Fast - single class</span><br>
          <span class="selector">#main-nav</span> <span class="comment">// Fast - single ID</span><br>
          <span class="selector">input[type="text"]</span> <span class="comment">// Fast - attribute</span><br>
          <span class="selector">.card.featured</span> <span class="comment">// Good - chained classes</span><br>
          <span class="selector">.menu > .item</span> <span class="comment">// Good - direct child</span>
        </div>
        
        <div class="inefficient">
          <span class="comment">/* Inefficient Selectors */</span><br><br>
          <span class="selector">div nav ul li a</span> <span class="comment">// Slow - tag chain</span><br>
          <span class="selector">.container *</span> <span class="comment">// Very slow - universal</span><br>
          <span class="selector">[class*="icon-"]</span> <span class="comment">// Slow - partial attribute</span><br>
          <span class="selector">:nth-child(2n+1)</span> <span class="comment">// Slow - complex pseudo</span><br>
          <span class="selector">.sidebar .widget ~ p</span> <span class="comment">// Slow - general sibling</span>
        </div>
      </div>
    </div>
    
    <!-- Specificity Management -->
    <div class="performance-card">
      <h3>🎚️ CSS Specificity Scale</h3>
      
      <div class="specificity-scale">
        <div class="specificity-level level-low">
          <strong>Low Specificity (0,1,0)</strong><br>
          .class, [attribute], :pseudo-class
        </div>
        
        <div class="specificity-level level-medium">
          <strong>Medium Specificity (0,2,0 - 0,3,0)</strong><br>
          .class.class, .parent .child
        </div>
        
        <div class="specificity-level level-high">
          <strong>High Specificity (1,0,0)</strong><br>
          #id, style=""
        </div>
        
        <div class="specificity-level level-very-high">
          <strong>Very High Specificity (1,1,0+)</strong><br>
          #id .class, !important
        </div>
      </div>
      
      <div class="optimization-tip">
        <strong>💡 Best Practice:</strong> Keep specificity as low as possible. Use classes instead of IDs 
        and avoid !important for maintainable and performant CSS.
      </div>
    </div>
    
    <!-- Browser Matching Process -->
    <div class="performance-card">
      <h3>🔍 How Browsers Match Selectors</h3>
      
      <div class="browser-process">
        <div class="process-step">1. Start with rightmost selector</div>
        <div class="process-step">2. Find all matching elements</div>
        <div class="process-step">3. Move left through selector</div>
        <div class="process-step">4. Filter elements at each step</div>
        <div class="process-step">5. Apply styles to final matches</div>
      </div>
      
      <div class="selector-comparison">
        <div class="efficient">
          <span class="comment">/* Fast matching process */</span><br>
          <span class="selector">.featured .card</span><br><br>
          <span class="comment">1. Find all .card elements</span><br>
          <span class="comment">2. Check if parent has .featured</span><br>
          <span class="comment">→ 2 steps total</span>
        </div>
        
        <div class="inefficient">
          <span class="comment">/* Slow matching process */</span><br>
          <span class="selector">div section article .card</span><br><br>
          <span class="comment">1. Find all .card elements</span><br>
          <span class="comment">2. Check parent is article</span><br>
          <span class="comment">3. Check parent is section</span><br>
          <span class="comment">4. Check parent is div</span><br>
          <span class="comment">→ 4 steps total</span>
        </div>
      </div>
    </div>
    
    <!-- Performance Impact -->
    <div class="demo-area">
      <h3>📊 Selector Performance Impact</h3>
      
      <div class="performance-metrics">
        <div class="metric-card">
          <div class="metric-value">0.1ms</div>
          <p>Single Class Selector</p>
        </div>
        
        <div class="metric-card">
          <div class="metric-value">0.3ms</div>
          <p>Chained Classes</p>
        </div>
        
        <div class="metric-card">
          <div class="metric-value">1.2ms</div>
          <p>Descendant Selector</p>
        </div>
        
        <div class="metric-card">
          <div class="metric-value">2.5ms</div>
          <p>Complex Pseudo-class</p>
        </div>
        
        <div class="metric-card">
          <div class="metric-value">5.8ms</div>
          <p>Universal Selector</p>
        </div>
      </div>
      
      <div class="optimization-tip">
        <strong>🚀 Performance Tip:</strong> On a page with 1000 elements, switching from complex selectors 
        to simple class selectors can save 5-10ms in style calculation time.
      </div>
    </div>
    
    <!-- Real-world Examples -->
    <div class="performance-card">
      <h3>🏗️ Real-world Selector Optimization</h3>
      
      <div class="selector-comparison">
        <div class="efficient">
          <span class="comment">/* Optimized - BEM methodology */</span><br>
          <span class="selector">.card</span> { ... }<br>
          <span class="selector">.card__header</span> { ... }<br>
          <span class="selector">.card__title</span> { ... }<br>
          <span class="selector">.card--featured</span> { ... }<br>
          <span class="selector">.card--featured .card__title</span> { ... }
        </div>
        
        <div class="inefficient">
          <span class="comment">/* Unoptimized - Overly specific */</span><br>
          <span class="selector">div.container section.main .card .header h3.title</span> { ... }<br>
          <span class="selector">body.home .content .sidebar .widget.featured p.text</span> { ... }<br>
          <span class="selector">ul.menu > li.item > a.link[href="#home"]</span> { ... }
        </div>
      </div>
    </div>
  </div>
</body>
</html>

Layout & Painting Optimization

CSS Property Performance Costs

Different CSS properties have different performance costs. Some trigger layout recalculations, some trigger repaints, and some can be handled entirely by the GPU.

// Cheap: transform, opacity, filter

// Medium: color, background-color

// Expensive: width, height, margin

Layout & Painting Performance Implementation

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>CSS Performance - Layout & Painting Optimization</title>
  <style>
    * {
      margin: 0;
      padding: 0;
      box-sizing: border-box;
    }
    
    body {
      font-family: 'Inter', sans-serif;
      line-height: 1.6;
      background: linear-gradient(135deg, #ff9a9e 0%, #fecfef 100%);
      min-height: 100vh;
      padding: 2rem;
      color: #2c3e50;
    }
    
    .container {
      max-width: 1200px;
      margin: 0 auto;
    }
    
    .header {
      text-align: center;
      margin-bottom: 3rem;
    }
    
    .header h1 {
      font-size: 3rem;
      margin-bottom: 1rem;
      color: #2c3e50;
    }
    
    .performance-grid {
      display: grid;
      grid-template-columns: repeat(auto-fit, minmax(350px, 1fr));
      gap: 2rem;
      margin-bottom: 3rem;
    }
    
    .performance-card {
      background: white;
      border-radius: 15px;
      padding: 2rem;
      box-shadow: 0 10px 30px rgba(0,0,0,0.1);
    }
    
    .property-comparison {
      display: grid;
      grid-template-columns: 1fr 1fr;
      gap: 1rem;
      margin: 1rem 0;
    }
    
    .cheap, .expensive {
      background: #2c3e50;
      color: #ecf0f1;
      padding: 1rem;
      border-radius: 5px;
      font-family: 'Fira Code', monospace;
      font-size: 0.8rem;
    }
    
    .cheap { border-left: 4px solid #2ecc71; }
    .expensive { border-left: 4px solid #e74c3c; }
    
    .comment { color: #7f8c8d; }
    .property { color: #3498db; }
    .value { color: #2ecc71; }
    
    .performance-cost {
      background: #34495e;
      color: white;
      padding: 1.5rem;
      border-radius: 8px;
      margin: 1rem 0;
    }
    
    .cost-level {
      margin: 0.5rem 0;
      padding: 0.5rem;
      border-radius: 4px;
    }
    
    .cost-low { background: #27ae60; }
    .cost-medium { background: #f39c12; }
    .cost-high { background: #e74c3c; }
    
    .demo-visual {
      background: #f8f9fa;
      padding: 2rem;
      border-radius: 10px;
      margin: 2rem 0;
      border: 2px dashed #bdc3c7;
    }
    
    .demo-element {
      padding: 1rem;
      margin: 0.5rem;
      background: #3498db;
      color: white;
      border-radius: 4px;
      transition: all 0.3s ease;
    }
    
    .demo-element:hover {
      transform: translateX(20px);
      background: #e74c3c;
    }
    
    .optimization-tip {
      background: #d4edda;
      border-left: 4px solid #28a745;
      padding: 1rem;
      margin: 1rem 0;
      border-radius: 5px;
    }
    
    .warning {
      background: #f8d7da;
      border-left: 4px solid #dc3545;
      padding: 1rem;
      margin: 1rem 0;
      border-radius: 5px;
    }
    
    .browser-timeline {
      background: #2c3e50;
      color: white;
      padding: 1.5rem;
      border-radius: 8px;
      margin: 1rem 0;
      font-family: 'Fira Code', monospace;
    }
    
    .timeline-event {
      margin: 0.5rem 0;
      color: #3498db;
    }
    
    @media (max-width: 768px) {
      .performance-grid {
        grid-template-columns: 1fr;
      }
      
      .property-comparison {
        grid-template-columns: 1fr;
      }
      
      .header h1 {
        font-size: 2rem;
      }
    }
  </style>
</head>
<body>
  <div class="container">
    <div class="header">
      <h1>🎨 Layout & Painting Performance</h1>
      <p>Minimize reflows and repaints for smooth user experiences</p>
    </div>
    
    <!-- CSS Property Costs -->
    <div class="performance-card">
      <h3>💰 CSS Property Performance Costs</h3>
      <p>Some CSS properties are more expensive than others:</p>
      
      <div class="property-comparison">
        <div class="cheap">
          <span class="comment">/* Cheap Properties */</span><br><br>
          <span class="property">transform</span><br>
          <span class="property">opacity</span><br>
          <span class="property">filter</span><br>
          <span class="property">will-change</span><br>
          <span class="property">backface-visibility</span>
        </div>
        
        <div class="expensive">
          <span class="comment">/* Expensive Properties */</span><br><br>
          <span class="property">width</span> / <span class="property">height</span><br>
          <span class="property">margin</span> / <span class="property">padding</span><br>
          <span class="property">top</span> / <span class="property">left</span><br>
          <span class="property">font-size</span><br>
          <span class="property">border-radius</span>
        </div>
      </div>
    </div>
    
    <!-- Performance Cost Scale -->
    <div class="performance-card">
      <h3>📊 Property Performance Impact</h3>
      
      <div class="performance-cost">
        <div class="cost-level cost-low">
          <strong>Low Cost (Composite Only)</strong><br>
          transform, opacity, filter - Uses GPU, no layout or paint
        </div>
        
        <div class="cost-level cost-medium">
          <strong>Medium Cost (Paint Only)</strong><br>
          color, background-color, border-color - Repaints but no layout
        </div>
        
        <div class="cost-level cost-high">
          <strong>High Cost (Layout + Paint)</strong><br>
          width, height, margin, padding - Triggers reflow and repaint
        </div>
      </div>
    </div>
    
    <!-- Browser Rendering Pipeline -->
    <div class="performance-card">
      <h3>🔄 Browser Rendering Pipeline</h3>
      
      <div class="browser-timeline">
        <div class="timeline-event">1. JavaScript → Style → Layout → Paint → Composite</div>
        <div class="timeline-event">2. Changing layout properties → Reflow needed</div>
        <div class="timeline-event">3. Changing paint properties → Repaint needed</div>
        <div class="timeline-event">4. Changing composite properties → No reflow/repaint</div>
      </div>
      
      <div class="optimization-tip">
        <strong>🚀 Performance Tip:</strong> Use transform and opacity for animations whenever possible. 
        These properties can be handled by the GPU without triggering layout or paint.
      </div>
    </div>
    
    <!-- Demo Area -->
    <div class="demo-visual">
      <h3>🎭 Performance Demo</h3>
      <p>Hover over these elements to see different types of changes:</p>
      
      <div class="demo-element" style="transform: translateX(0);">
        Cheap: Uses transform (GPU accelerated)
      </div>
      
      <div class="demo-element" style="margin-left: 0;">
        Expensive: Uses margin (triggers layout)
      </div>
      
      <div class="demo-element" style="background-color: #3498db;">
        Medium: Changes background-color (repaint only)
      </div>
    </div>
    
    <!-- Optimization Strategies -->
    <div class="performance-card">
      <h3>🔧 Layout & Painting Optimization</h3>
      
      <div class="property-comparison">
        <div class="cheap">
          <span class="comment">/* Optimized Approaches */</span><br><br>
          <span class="property">transform</span>: <span class="value">translateX(100px)</span>;<br>
          <span class="comment">// Instead of left: 100px</span><br><br>
          
          <span class="property">will-change</span>: <span class="value">transform</span>;<br>
          <span class="comment">// Hint for browser optimization</span><br><br>
          
          <span class="property">contain</span>: <span class="value">layout paint</span>;<br>
          <span class="comment">// Isolate rendering</span>
        </div>
        
        <div class="expensive">
          <span class="comment">/* Unoptimized Approaches */</span><br><br>
          <span class="property">left</span>: <span class="value">100px</span>;<br>
          <span class="comment">// Triggers layout recalc</span><br><br>
          
          <span class="property">width</span>: <span class="value">calc(100% - 20px)</span>;<br>
          <span class="comment">// Expensive calculation</span><br><br>
          
          <span class="property">margin</span>: <span class="value">10px</span>;<br>
          <span class="comment">// Affects surrounding elements</span>
        </div>
      </div>
    </div>
    
    <!-- Real-world Impact -->
    <div class="performance-card">
      <h3>📈 Real Performance Impact</h3>
      
      <div class="performance-metrics">
        <div class="metric-card">
          <div class="metric-value">0.5ms</div>
          <p>Transform Animation</p>
        </div>
        
        <div class="metric-card">
          <div class="metric-value">3.2ms</div>
          <p>Layout Property Change</p>
        </div>
        
        <div class="metric-card">
          <div class="metric-value">8.7ms</div>
          <p>Multiple Layout Changes</p>
        </div>
      </div>
      
      <div class="warning">
        <strong>⚠️ Warning:</strong> Frequent layout thrashing (multiple sequential layout changes) 
        can cause jank and poor user experience. Batch your DOM changes when possible.
      </div>
    </div>
    
    <!-- Best Practices -->
    <div class="performance-card">
      <h3>✅ Best Practices Summary</h3>
      <ul>
        <li><strong>Use transform and opacity for animations</strong></li>
        <li><strong>Avoid forced synchronous layouts</strong></li>
        <li><strong>Use CSS Containment for complex components</strong></li>
        <li><strong>Batch DOM read/write operations</strong></li>
        <li><strong>Use will-change sparingly for known changes</strong></li>
        <li><strong>Prefer flexbox/grid over floats for layout</strong></li>
        <li><strong>Debounce scroll and resize handlers</strong></li>
      </ul>
    </div>
  </div>
</body>
</html>

Performance Tools & Measurement

🔍 Chrome DevTools

Performance panel, coverage tool, rendering tools

📊 Lighthouse

Performance scoring and optimization opportunities

⚡ WebPageTest

Multi-location testing and filmstrip analysis

Tools & Measurement Implementation

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>CSS Performance - Tools, Measurement & Best Practices</title>
  <style>
    * {
      margin: 0;
      padding: 0;
      box-sizing: border-box;
    }
    
    body {
      font-family: 'Inter', sans-serif;
      line-height: 1.6;
      background: linear-gradient(135deg, #a8edea 0%, #fed6e3 100%);
      min-height: 100vh;
      padding: 2rem;
      color: #2c3e50;
    }
    
    .container {
      max-width: 1200px;
      margin: 0 auto;
    }
    
    .header {
      text-align: center;
      margin-bottom: 3rem;
    }
    
    .header h1 {
      font-size: 3rem;
      margin-bottom: 1rem;
      color: #2c3e50;
    }
    
    .tools-grid {
      display: grid;
      grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
      gap: 2rem;
      margin-bottom: 3rem;
    }
    
    .tool-card {
      background: white;
      border-radius: 15px;
      padding: 2rem;
      box-shadow: 0 10px 30px rgba(0,0,0,0.1);
    }
    
    .tool-icon {
      font-size: 2.5rem;
      margin-bottom: 1rem;
    }
    
    .metrics-grid {
      display: grid;
      grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
      gap: 1rem;
      margin: 2rem 0;
    }
    
    .metric-card {
      background: #f8f9fa;
      padding: 1.5rem;
      border-radius: 10px;
      text-align: center;
    }
    
    .metric-value {
      font-size: 1.5rem;
      font-weight: bold;
      color: #2c3e50;
    }
    
    .good { color: #27ae60; }
    .average { color: #f39c12; }
    .poor { color: #e74c3c; }
    
    .code-example {
      background: #2c3e50;
      color: #ecf0f1;
      padding: 1rem;
      border-radius: 5px;
      font-family: 'Fira Code', monospace;
      font-size: 0.8rem;
      margin: 1rem 0;
    }
    
    .checklist {
      background: #e8f4fd;
      padding: 1.5rem;
      border-radius: 10px;
      margin: 2rem 0;
      border-left: 4px solid #3498db;
    }
    
    .checklist-item {
      margin: 0.5rem 0;
      display: flex;
      align-items: center;
    }
    
    .checklist-item::before {
      content: "✅";
      margin-right: 0.5rem;
    }
    
    .performance-tip {
      background: #fff3cd;
      border-left: 4px solid #ffc107;
      padding: 1rem;
      margin: 1rem 0;
      border-radius: 5px;
    }
    
    .browser-tool {
      background: #34495e;
      color: white;
      padding: 1rem;
      border-radius: 8px;
      margin: 1rem 0;
      font-family: 'Fira Code', monospace;
    }
    
    .workflow-steps {
      display: grid;
      grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
      gap: 1rem;
      margin: 2rem 0;
    }
    
    .step {
      background: white;
      padding: 1.5rem;
      border-radius: 10px;
      text-align: center;
      box-shadow: 0 2px 10px rgba(0,0,0,0.1);
    }
    
    .step-number {
      background: #3498db;
      color: white;
      width: 40px;
      height: 40px;
      border-radius: 50%;
      display: flex;
      align-items: center;
      justify-content: center;
      margin: 0 auto 1rem;
      font-weight: bold;
    }
    
    @media (max-width: 768px) {
      .tools-grid {
        grid-template-columns: 1fr;
      }
      
      .workflow-steps {
        grid-template-columns: 1fr;
      }
      
      .header h1 {
        font-size: 2rem;
      }
    }
  </style>
</head>
<body>
  <div class="container">
    <div class="header">
      <h1>🛠️ Performance Tools & Measurement</h1>
      <p>Measure, analyze, and optimize CSS performance</p>
    </div>
    
    <!-- Performance Tools -->
    <div class="tools-grid">
      <div class="tool-card">
        <div class="tool-icon">🔍</div>
        <h3>Chrome DevTools</h3>
        <ul>
          <li>Performance Panel</li>
          <li>Lighthouse Audits</li>
          <li>Coverage Tool</li>
          <li>Rendering Tools</li>
        </ul>
      </div>
      
      <div class="tool-card">
        <div class="tool-icon">📊</div>
        <h3>Lighthouse</h3>
        <ul>
          <li>Performance Scoring</li>
          <li>Opportunities</li>
          <li>Best Practices</li>
          <li>SEO & Accessibility</li>
        </ul>
      </div>
      
      <div class="tool-card">
        <div class="tool-icon">⚡</div>
        <h3>WebPageTest</h3>
        <ul>
          <li>Multi-location Testing</li>
          <li>Filmstrip View</li>
          <li>Waterfall Charts</li>
          <li>Core Web Vitals</li>
        </ul>
      </div>
      
      <div class="tool-card">
        <div class="tool-icon">🎯</div>
        <h3>CSS Analysis</h3>
        <ul>
          <li>PurgeCSS</li>
          <li>UnCSS</li>
          <li>CSS Stats</li>
          <li>Specificity Graph</li>
        </ul>
      </div>
    </div>
    
    <!-- Performance Metrics -->
    <div class="tool-card">
      <h3>📈 Core Web Vitals</h3>
      
      <div class="metrics-grid">
        <div class="metric-card">
          <div class="metric-value good">&lt; 2.5s</div>
          <p>Largest Contentful Paint (LCP)</p>
          <small>Loading Performance</small>
        </div>
        
        <div class="metric-card">
          <div class="metric-value good">&lt; 100ms</div>
          <p>First Input Delay (FID)</p>
          <small>Interactivity</small>
        </div>
        
        <div class="metric-card">
          <div class="metric-value good">&lt; 0.1</div>
          <p>Cumulative Layout Shift (CLS)</p>
          <small>Visual Stability</small>
        </div>
        
        <div class="metric-card">
          <div class="metric-value good">&lt; 1s</div>
          <p>First Contentful Paint (FCP)</p>
          <small>Perceived Load</small>
        </div>
      </div>
    </div>
    
    <!-- Measurement Techniques -->
    <div class="tool-card">
      <h3>📏 Performance Measurement</h3>
      
      <div class="workflow-steps">
        <div class="step">
          <div class="step-number">1</div>
          <h4>Audit</h4>
          <p>Run Lighthouse analysis</p>
        </div>
        
        <div class="step">
          <div class="step-number">2</div>
          <h4>Identify</h4>
          <p>Find performance bottlenecks</p>
        </div>
        
        <div class="step">
          <div class="step-number">3</div>
          <h4>Optimize</h4>
          <p>Implement improvements</p>
        </div>
        
        <div class="step">
          <div class="step-number">4</div>
          <h4>Measure</h4>
          <p>Verify performance gains</p>
        </div>
      </div>
    </div>
    
    <!-- DevTools Examples -->
    <div class="tool-card">
      <h3>🔧 Chrome DevTools in Action</h3>
      
      <div class="browser-tool">
        <span class="comment">// Performance Panel Usage</span><br>
        1. Open DevTools → Performance tab<br>
        2. Click Record ⚫<br>
        3. Interact with page<br>
        4. Stop recording<br>
        5. Analyze results<br>
      </div>
      
      <div class="browser-tool">
        <span class="comment">// Coverage Tool</span><br>
        1. DevTools → More Tools → Coverage<br>
        2. Reload page<br>
        3. See unused CSS/JS<br>
        4. Identify critical vs non-critical<br>
      </div>
    </div>
    
    <!-- Optimization Checklist -->
    <div class="checklist">
      <h3>✅ CSS Performance Checklist</h3>
      
      <div class="checklist-item">Remove unused CSS with PurgeCSS</div>
      <div class="checklist-item">Minify and compress CSS files</div>
      <div class="checklist-item">Use efficient CSS selectors</div>
      <div class="checklist-item">Optimize critical rendering path</div>
      <div class="checklist-item">Use transform/opacity for animations</div>
      <div class="checklist-item">Implement lazy loading for non-critical CSS</div>
      <div class="checklist-item">Use CSS containment for complex components</div>
      <div class="checklist-item">Reduce CSS specificity conflicts</div>
      <div class="checklist-item">Optimize images and fonts</div>
      <div class="checklist-item">Use modern layout methods (Flexbox/Grid)</div>
    </div>
    
    <!-- Code Examples -->
    <div class="tool-card">
      <h3>💻 Performance Monitoring Code</h3>
      
      <div class="code-example">
        <span class="comment">// Monitor Largest Contentful Paint</span><br>
        <span class="selector">new PerformanceObserver</span>((entryList) => {<br>
        &nbsp;&nbsp;<span class="selector">for</span> (const entry of entryList.getEntries()) {<br>
        &nbsp;&nbsp;&nbsp;&nbsp;console.log('LCP:', entry.startTime);<br>
        &nbsp;&nbsp;}<br>
        }).observe({type: 'largest-contentful-paint', buffered: true});<br><br>
        
        <span class="comment">// Monitor Layout Shifts</span><br>
        <span class="selector">new PerformanceObserver</span>((entryList) => {<br>
        &nbsp;&nbsp;<span class="selector">for</span> (const entry of entryList.getEntries()) {<br>
        &nbsp;&nbsp;&nbsp;&nbsp;console.log('Layout Shift:', entry.value);<br>
        &nbsp;&nbsp;}<br>
        }).observe({type: 'layout-shift', buffered: true});
      </div>
    </div>
    
    <!-- Best Practices Summary -->
    <div class="tool-card">
      <h3>🎯 Continuous Performance Culture</h3>
      
      <div class="performance-tip">
        <strong>💡 Pro Tip:</strong> Integrate performance monitoring into your development workflow. 
        Use tools like Lighthouse CI to catch regressions before they reach production.
      </div>
      
      <ul>
        <li><strong>Set performance budgets</strong> for CSS size and Core Web Vitals</li>
        <li><strong>Monitor regularly</strong> with automated tools</li>
        <li><strong>Educate team members</strong> on performance best practices</li>
        <li><strong>Optimize progressively</strong> - focus on biggest wins first</li>
        <li><strong>Measure real user metrics</strong> alongside lab data</li>
      </ul>
    </div>
  </div>
</body>
</html>

Practical Performance Applications

🚀 Immediate Wins

Remove unused CSS with PurgeCSS
Minify and compress CSS files
Optimize critical rendering path
Use efficient CSS selectors
Implement lazy loading for non-critical CSS

🔧 Advanced Optimizations

CSS containment for complex components
GPU-accelerated animations
Reduced CSS specificity conflicts
Modern layout methods (Flexbox/Grid)
Performance monitoring and budgets

💡 Performance Culture Tips

Development:

Set performance budgets
Use performance monitoring tools
Educate team members
Optimize progressively

Measurement:

Monitor Core Web Vitals
Use real user monitoring
Test across devices and networks
Set up performance alerts

Ready to Optimize Your CSS? ⚡

Start implementing CSS performance optimizations to create faster, smoother, and more engaging user experiences. Measure your progress and watch your Core Web Vitals scores improve.

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