web-best-practices

by tech-leads-club

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SKILL.md About

---
name: best-practices
description: Apply modern web development best practices for security, compatibility, and code quality. Use when asked to "apply best practices", "security audit", "modernize code", "code quality review", or "check for vulnerabilities". Do NOT use for accessibility (use web-accessibility), SEO (use seo), performance (use core-web-vitals), or comprehensive multi-area audits (use web-quality-audit).
license: MIT
metadata:
  author: web-quality-skills
  version: '1.0'
---

# Best practices

Modern web development standards based on Lighthouse best practices audits. Covers security, browser compatibility, and code quality patterns.

## Security

### HTTPS everywhere

**Enforce HTTPS:**

```html

<img src="http://example.com/image.jpg" />
<script src="http://cdn.example.com/script.js"></script>

<img src="https://example.com/image.jpg" />
<script src="https://cdn.example.com/script.js"></script>

<img src="//example.com/image.jpg" />
```

**HSTS Header:**

```
Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
```

### Content Security Policy (CSP)

```html

<meta
  http-equiv="Content-Security-Policy"
  content="default-src 'self'; 
               script-src 'self' https://trusted-cdn.com; 
               style-src 'self' 'unsafe-inline';
               img-src 'self' data: https:;
               connect-src 'self' https://api.example.com;"
/>

```

**CSP Header (recommended):**

```
Content-Security-Policy:
  default-src 'self';
  script-src 'self' 'nonce-abc123' https://trusted.com;
  style-src 'self' 'nonce-abc123';
  img-src 'self' data: https:;
  connect-src 'self' https://api.example.com;
  frame-ancestors 'self';
  base-uri 'self';
  form-action 'self';
```

**Using nonces for inline scripts:**

```html
<script nonce="abc123">
  // This inline script is allowed
</script>
```

### Security headers

```
# Prevent clickjacking
X-Frame-Options: DENY

# Prevent MIME type sniffing
X-Content-Type-Options: nosniff

# Enable XSS filter (legacy browsers)
X-XSS-Protection: 1; mode=block

# Control referrer information
Referrer-Policy: strict-origin-when-cross-origin

# Permissions policy (formerly Feature-Policy)
Permissions-Policy: geolocation=(), microphone=(), camera=()
```

### No vulnerable libraries

```bash
# Check for vulnerabilities
npm audit
yarn audit

# Auto-fix when possible
npm audit fix

# Check specific package
npm ls lodash
```

**Keep dependencies updated:**

```json
// package.json
{
  "scripts": {
    "audit": "npm audit --audit-level=moderate",
    "update": "npm update && npm audit fix"
  }
}
```

**Known vulnerable patterns to avoid:**

```javascript
// ❌ Prototype pollution vulnerable patterns
Object.assign(target, userInput)
_.merge(target, userInput)

// ✅ Safer alternatives
const safeData = JSON.parse(JSON.stringify(userInput))
```

### Input sanitization

```javascript
// ❌ XSS vulnerable
element.innerHTML = userInput
document.write(userInput)

// ✅ Safe text content
element.textContent = userInput

// ✅ If HTML needed, sanitize
import DOMPurify from 'dompurify'
element.innerHTML = DOMPurify.sanitize(userInput)
```

### Secure cookies

```javascript
// ❌ Insecure cookie
document.cookie = "session=abc123";

// ✅ Secure cookie (server-side)
Set-Cookie: session=abc123; Secure; HttpOnly; SameSite=Strict; Path=/
```

---

## Browser compatibility

### Doctype declaration

```html

<html>
  <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">

<!DOCTYPE html>
  <html lang="en"></html>
</html>
```

### Character encoding

```html

<html>
  <head>
    <title>Page</title>
    <meta charset="UTF-8" />
  </head>

<html>
    <head>
      <meta charset="UTF-8" />
      <title>Page</title>
    </head>
  </html>
</html>
```

### Viewport meta tag

```html

<head>
  <title>Page</title>
</head>

<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1" />
  <title>Page</title>
</head>
```

### Feature detection

```javascript
// ❌ Browser detection (brittle)
if (navigator.userAgent.includes('Chrome')) {
  // Chrome-specific code
}

// ✅ Feature detection
if ('IntersectionObserver' in window) {
  // Use IntersectionObserver
} else {
  // Fallback
}

// ✅ Using @supports in CSS
@supports (display: grid) {
  .container {
    display: grid;
  }
}

@supports not (display: grid) {
  .container {
    display: flex;
  }
}
```

### Polyfills (when needed)

```html

<script>
  if (!('fetch' in window)) {
    document.write('<script src="/polyfills/fetch.js"><\/script>')
  }
</script>

<script src="https://polyfill.io/v3/polyfill.min.js?features=fetch,IntersectionObserver"></script>
```

---

## Deprecated APIs

### Avoid these

```javascript
// ❌ document.write (blocks parsing)
document.write('<script src="..."></script>');

// ✅ Dynamic script loading
const script = document.createElement('script');
script.src = '...';
document.head.appendChild(script);

// ❌ Synchronous XHR (blocks main thread)
const xhr = new XMLHttpRequest();
xhr.open('GET', url, false); // false = synchronous

// ✅ Async fetch
const response = await fetch(url);

// ❌ Application Cache (deprecated)
<html manifest="cache.manifest">

// ✅ Service Workers
if ('serviceWorker' in navigator) {
  navigator.serviceWorker.register('/sw.js');
}
```

### Event listener passive

```javascript
// ❌ Non-passive touch/wheel (may block scrolling)
element.addEventListener('touchstart', handler)
element.addEventListener('wheel', handler)

// ✅ Passive listeners (allows smooth scrolling)
element.addEventListener('touchstart', handler, { passive: true })
element.addEventListener('wheel', handler, { passive: true })

// ✅ If you need preventDefault, be explicit
element.addEventListener('touchstart', handler, { passive: false })
```

---

## Console & errors

### No console errors

```javascript
// ❌ Errors in production
console.log('Debug info') // Remove in production
throw new Error('Unhandled') // Catch all errors

// ✅ Proper error handling
try {
  riskyOperation()
} catch (error) {
  // Log to error tracking service
  errorTracker.captureException(error)
  // Show user-friendly message
  showErrorMessage('Something went wrong. Please try again.')
}
```

### Error boundaries (React)

```jsx
class ErrorBoundary extends React.Component {
  state = { hasError: false }

static getDerivedStateFromError(error) {
    return { hasError: true }
  }

componentDidCatch(error, info) {
    errorTracker.captureException(error, { extra: info })
  }

render() {
    if (this.state.hasError) {
      return <FallbackUI />
    }
    return this.props.children
  }
}

// Usage
;<ErrorBoundary>
  <App />
</ErrorBoundary>
```

### Global error handler

```javascript
// Catch unhandled errors
window.addEventListener('error', (event) => {
  errorTracker.captureException(event.error)
})

// Catch unhandled promise rejections
window.addEventListener('unhandledrejection', (event) => {
  errorTracker.captureException(event.reason)
})
```

---

## Source maps

### Production configuration

```javascript
// ❌ Source maps exposed in production
// webpack.config.js
module.exports = {
  devtool: 'source-map', // Exposes source code
}

// ✅ Hidden source maps (uploaded to error tracker)
module.exports = {
  devtool: 'hidden-source-map',
}

// ✅ Or no source maps in production
module.exports = {
  devtool: process.env.NODE_ENV === 'production' ? false : 'source-map',
}
```

---

## Performance best practices

### Avoid blocking patterns

```javascript
// ❌ Blocking script
<script src="heavy-library.js"></script>

// ✅ Deferred script
<script defer src="heavy-library.js"></script>

// ❌ Blocking CSS import
@import url('other-styles.css');

// ✅ Link tags (parallel loading)
<link rel="stylesheet" href="styles.css">
<link rel="stylesheet" href="other-styles.css">
```

### Efficient event handlers

```javascript
// ❌ Handler on every element
items.forEach((item) => {
  item.addEventListener('click', handleClick)
})

// ✅ Event delegation
container.addEventListener('click', (e) => {
  if (e.target.matches('.item')) {
    handleClick(e)
  }
})
```

### Memory management

```javascript
// ❌ Memory leak (never removed)
const handler = () => {
  /* ... */
}
window.addEventListener('resize', handler)

// ✅ Cleanup when done
const handler = () => {
  /* ... */
}
window.addEventListener('resize', handler)

// Later, when component unmounts:
window.removeEventListener('resize', handler)

// ✅ Using AbortController
const controller = new AbortController()
window.addEventListener('resize', handler, { signal: controller.signal })

// Cleanup:
controller.abort()
```

---

## Code quality

### Valid HTML

```html

<div id="header">
  <div id="header">

<a href="/"><button>Click</button></a>

<header id="site-header"></header>

<a href="/" class="button">Click</a>
  </div>
</div>
```

### Semantic HTML

```html

<div class="header">
  <div class="nav">
    <div class="nav-item">Home</div>
  </div>
</div>
<div class="main">
  <div class="article">
    <div class="title">Headline</div>
  </div>
</div>

<header>
  <nav>
    <a href="/">Home</a>
  </nav>
</header>
<main>
  <article>
    <h1>Headline</h1>
  </article>
</main>
```

### Image aspect ratios

```html

<img src="photo.jpg" width="300" height="100" />

<img src="photo.jpg" width="300" height="225" />

<img src="photo.jpg" style="width: 300px; height: 200px; object-fit: cover;" />
```

---

## Permissions & privacy

### Request permissions properly

```javascript
// ❌ Request on page load (bad UX, often denied)
navigator.geolocation.getCurrentPosition(success, error)

// ✅ Request in context, after user action
findNearbyButton.addEventListener('click', async () => {
  // Explain why you need it
  if (await showPermissionExplanation()) {
    navigator.geolocation.getCurrentPosition(success, error)
  }
})
```

### Permissions policy

```html

<meta http-equiv="Permissions-Policy" content="geolocation=(), camera=(), microphone=()" />

<meta http-equiv="Permissions-Policy" content="geolocation=(self 'https://maps.example.com')" />
```

---

## Audit checklist

### Security (critical)

- [ ] HTTPS enabled, no mixed content
- [ ] No vulnerable dependencies (`npm audit`)
- [ ] CSP headers configured
- [ ] Security headers present
- [ ] No exposed source maps

### Compatibility

- [ ] Valid HTML5 doctype
- [ ] Charset declared first in head
- [ ] Viewport meta tag present
- [ ] No deprecated APIs used
- [ ] Passive event listeners for scroll/touch

### Code quality

- [ ] No console errors
- [ ] Valid HTML (no duplicate IDs)
- [ ] Semantic HTML elements used
- [ ] Proper error handling
- [ ] Memory cleanup in components

### UX

- [ ] No intrusive interstitials
- [ ] Permission requests in context
- [ ] Clear error messages
- [ ] Appropriate image aspect ratios

## Tools

| Tool                                               | Purpose                    |
| -------------------------------------------------- | -------------------------- |
| `npm audit`                                        | Dependency vulnerabilities |
| [SecurityHeaders.com](https://securityheaders.com) | Header analysis            |
| [W3C Validator](https://validator.w3.org)          | HTML validation            |
| Lighthouse                                         | Best practices audit       |
| [Observatory](https://observatory.mozilla.org)     | Security scan              |

## References

- [MDN Web Security](https://developer.mozilla.org/en-US/docs/Web/Security)
- [OWASP Top 10](https://owasp.org/www-project-top-ten/)
- [Web Quality Audit](../web-quality-audit/SKILL.md)