<!-- Base HTML (works without CSS/JS) -->
<form action="/submit" method="POST">
  <label for="email">Email:</label>
  <input type="email" id="email" name="email" required>
  <button type="submit">Submit</button>
</form>

/* Enhanced styling */
form {
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

// Enhanced interactivity
form.addEventListener('submit', async (e) => {
  e.preventDefault();
  await fetch('/api/submit', { /* ... */ });
});

/* Mobile-first base styles */
.container {
  padding: 1rem;
}

/* Tablet and up */
@media (min-width: 768px) {
  .container {
    padding: 2rem;
    max-width: 1200px;
    margin: 0 auto;
  }
}

/* Desktop */
@media (min-width: 1024px) {
  .container {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 2rem;
  }
}

<nav aria-label="Main navigation">
  <ul role="menubar">
    <li role="none">
      <a href="/" role="menuitem">Home</a>
    </li>
    <li role="none">
      <button
        role="menuitem"
        aria-expanded="false"
        aria-haspopup="true"
      >
        Products
      </button>
    </li>
  </ul>
</nav>

<!-- Lazy load images -->
<img
  src="placeholder.jpg"
  data-src="high-res.jpg"
  loading="lazy"
  alt="Description"
>

<!-- Preload critical resources -->
<link rel="preload" href="critical.css" as="style">
<link rel="preconnect" href="https://api.example.com">

<!-- Async/defer non-critical scripts -->
<script src="analytics.js" async></script>
<script src="app.js" defer></script>

// Measure Core Web Vitals
const observer = new PerformanceObserver((list) => {
  for (const entry of list.getEntries()) {
    console.log('Performance metric:', {
      name: entry.name,
      value: entry.value,
      rating: entry.rating
    });
  }
});

observer.observe({ entryTypes: ['largest-contentful-paint', 'first-input', 'layout-shift'] });

class AccessibleTabs {
  constructor(element) {
    this.tablist = element.querySelector('[role="tablist"]');
    this.tabs = Array.from(this.tablist.querySelectorAll('[role="tab"]'));
    this.panels = Array.from(element.querySelectorAll('[role="tabpanel"]'));
    
    this.tabs.forEach((tab, index) => {
      tab.addEventListener('click', () => this.selectTab(index));
      tab.addEventListener('keydown', (e) => this.handleKeydown(e, index));
    });
  }
  
  selectTab(index) {
    // Deselect all tabs
    this.tabs.forEach(tab => {
      tab.setAttribute('aria-selected', 'false');
      tab.setAttribute('tabindex', '-1');
    });
    this.panels.forEach(panel => panel.hidden = true);
    
    // Select target tab
    this.tabs[index].setAttribute('aria-selected', 'true');
    this.tabs[index].setAttribute('tabindex', '0');
    this.tabs[index].focus();
    this.panels[index].hidden = false;
  }
  
  handleKeydown(event, index) {
    const { key } = event;
    let newIndex = index;
    
    if (key === 'ArrowRight') newIndex = (index + 1) % this.tabs.length;
    if (key === 'ArrowLeft') newIndex = (index - 1 + this.tabs.length) % this.tabs.length;
    if (key === 'Home') newIndex = 0;
    if (key === 'End') newIndex = this.tabs.length - 1;
    
    if (newIndex !== index) {
      event.preventDefault();
      this.selectTab(newIndex);
    }
  }
}

/* Container queries (modern browsers) */
@container (min-width: 400px) {
  .card {
    display: grid;
    grid-template-columns: 1fr 2fr;
  }
}

/* CSS Grid with subgrid */
.grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
  gap: 2rem;
}

.grid-item {
  display: grid;
  grid-template-rows: subgrid;
  grid-row: span 3;
}

/* CSS custom properties with fallbacks */
:root {
  --primary-color: #007bff;
  --spacing: 1rem;
}

.element {
  color: var(--primary-color, blue);
  padding: var(--spacing, 16px);
}

// Express.js example
app.use((req, res, next) => {
  // Content Security Policy
  res.setHeader('Content-Security-Policy', 
    "default-src 'self'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'");
  
  // Strict Transport Security
  res.setHeader('Strict-Transport-Security', 'max-age=31536000; includeSubDomains; preload');
  
  // XSS Protection
  res.setHeader('X-Content-Type-Options', 'nosniff');
  res.setHeader('X-Frame-Options', 'DENY');
  res.setHeader('X-XSS-Protection', '1; mode=block');
  
  // Referrer Policy
  res.setHeader('Referrer-Policy', 'strict-origin-when-cross-origin');
  
  next();
});