Agent-skills 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).

install

source · Clone the upstream repo

git clone https://github.com/tech-leads-club/agent-skills

Claude Code · Install into ~/.claude/skills/

T=$(mktemp -d) && git clone --depth=1 https://github.com/tech-leads-club/agent-skills "$T" && mkdir -p ~/.claude/skills && cp -r "$T/packages/skills-catalog/skills/(quality)/web-best-practices" ~/.claude/skills/tech-leads-club-agent-skills-best-practices && rm -rf "$T"

manifest: packages/skills-catalog/skills/(quality)/web-best-practices/SKILL.md

source content

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:

<!-- ❌ Mixed content -->
<img src="http://example.com/image.jpg" />
<script src="http://cdn.example.com/script.js"></script>

<!-- ✅ HTTPS only -->
<img src="https://example.com/image.jpg" />
<script src="https://cdn.example.com/script.js"></script>

<!-- ✅ Protocol-relative (will use page's protocol) -->
<img src="//example.com/image.jpg" />

HSTS Header:

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

Content Security Policy (CSP)

<!-- 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 -->

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:

<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

# Check for vulnerabilities
npm audit
yarn audit

# Auto-fix when possible
npm audit fix

# Check specific package
npm ls lodash

Keep dependencies updated:

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

Known vulnerable patterns to avoid:

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

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

Input sanitization

// ❌ 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

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

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

Browser compatibility

Doctype declaration

<!-- ❌ Missing or invalid doctype -->
<html>
  <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">

  <!-- ✅ HTML5 doctype -->
  <!DOCTYPE html>
  <html lang="en"></html>
</html>

Character encoding

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

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

Viewport meta tag

<!-- ❌ 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>

Feature detection

// ❌ 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)

<!-- 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>

Deprecated APIs

Avoid these

// ❌ 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

// ❌ 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

// ❌ 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)

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

// 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

// ❌ 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

// ❌ 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

// ❌ 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

// ❌ 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

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

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

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

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

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

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

Semantic HTML

<!-- ❌ 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>

Image aspect ratios

<!-- ❌ 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;" />

Permissions & privacy

Request permissions properly

// ❌ 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

<!-- 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')" />

Audit checklist

Security (critical)

Compatibility

Code quality

UX

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

Tools

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

Agent-skills best-practices

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