best-practices

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).

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Repository: tech-leads-club/agent-skills
Last source activity: June 11, 2026 at 19:07
Detected SKILL.md language: English
Stars: 5,022
Forks: 459

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SKILL.md

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

<!-- Basic CSP via meta tag -->
<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;"
/>

<!-- Better: HTTP header -->

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';

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

# 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=()

# Check for vulnerabilities
npm audit
yarn audit

# Auto-fix when possible
npm audit fix

# Check specific package
npm ls lodash

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

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

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

// ❌ 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)

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

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

<!-- ❌ Missing or invalid doctype -->
<html lang="en">
  <head>
    <title>Page</title>
  </head>
  <body></body>
</html>

<!-- ✅ HTML5 doctype -->
<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Page</title>
  </head>
  <body></body>
</html>

<!-- ❌ Missing or late charset -->
<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Page</title>
    <meta charset="UTF-8" />
  </head>
  <body></body>
</html>

<!-- ✅ Charset as first element in head -->
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>Page</title>
  </head>
  <body></body>
</html>

<!-- ❌ Missing viewport -->
<head>
  <title>Page</title>
</head>

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

// ❌ 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;
  }
}

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

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

// ❌ 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');
}

// ❌ 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 })

// ❌ 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.')
}

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>

// 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 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',
}

// ❌ 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">

// ❌ 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 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()

<!-- ❌ Invalid HTML -->
<div id="header">
  <div id="header">
    <!-- Duplicate ID -->
  </div>

  <ul>
    <div>Item</div>
    <!-- Invalid child -->
  </ul>

  <a href="/"><button>Click</button></a>
  <!-- Invalid nesting -->
</div>

<!-- ✅ Valid HTML -->
<header id="site-header"></header>

<ul>
  <li>Item</li>
</ul>

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

<!-- ❌ Non-semantic -->
<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>

<!-- ✅ Semantic HTML5 -->
<header>
  <nav>
    <a href="/">Home</a>
  </nav>
</header>
<main>
  <article>
    <h1>Headline</h1>
  </article>
</main>

<!-- ❌ Distorted images -->
<img src="photo.jpg" width="300" height="100" />
<!-- If actual ratio is 4:3, this squishes the image -->

<!-- ✅ Preserve aspect ratio -->
<img src="photo.jpg" width="300" height="225" />
<!-- Actual 4:3 dimensions -->

<!-- ✅ CSS object-fit for flexibility -->
<img src="photo.jpg" style="width: 300px; height: 200px; object-fit: cover;" />

// ❌ 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)
  }
})

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

<!-- Or allow for specific origins -->
<meta http-equiv="Permissions-Policy" content="geolocation=(self 'https://maps.example.com')" />

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"}

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"}

Tool	Purpose
`npm audit`	Dependency vulnerabilities
SecurityHeaders.com	Header analysis
W3C Validator	HTML validation
Lighthouse	Best practices audit
Observatory	Security scan

best-practices

Source facts

Install options

Review the source files

More from this repository

More from this repository

Best practices

Security

HTTPS everywhere

Best practices

Security

HTTPS everywhere

Content Security Policy (CSP)

Security headers

No vulnerable libraries

Input sanitization

Secure cookies

Browser compatibility

Doctype declaration

Character encoding

Viewport meta tag

Feature detection

Polyfills (when needed)

Deprecated APIs

Avoid these

Event listener passive

Console & errors

No console errors

Error boundaries (React)

Global error handler

Source maps

Production configuration

Performance best practices

Avoid blocking patterns

Efficient event handlers

Memory management

Code quality

Valid HTML

Semantic HTML

Image aspect ratios

Permissions & privacy

Request permissions properly

Permissions policy

Audit checklist

Security (critical)

Compatibility

Code quality

UX

Tools

References