BE
best-practices
Applies modern web development standards to improve security, compatibility, and quality.
Install
mkdir -p .claude/skills/best-practices && curl -L -o skill.zip "https://agentskills.codes/api/skills/download/6797" && unzip -o skill.zip -d .claude/skills/best-practices && rm skill.zipInstalls to .claude/skills/best-practices
Activation
This is the description your AI agent reads to decide when to run this skill — the better it matches your request, the more reliably it fires.
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".223 chars✓ has a “when” trigger
Beginner
Key capabilities
- →Verifies HTTPS enforcement and mixed content
- →Generates secure CSP header configurations
- →Sets modern security headers like X-Content-Type-Options
- →Applies nonces to inline scripts
- →Configures referrer-policy for data control
How it works
Matches existing code patterns against a database of security standards and provides replacements for insecure implementation practices.
Inputs & outputs
You give it
Code snippet or project configuration
You get back
Hardened security header snippets and updated code blocks
When to use best-practices
- →Modernize code for security
- →Perform a code quality audit
- →Fix security vulnerabilities
About this skill
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">
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>
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>
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
---
*Content truncated.*
When not to use it
- →Legacy sites using strictly non-HTTPS dependencies
- →Development environments requiring permissive unsafe-inline
Prerequisites
Access to server-side header configuration
Limitations
- →Does not handle site-wide architectural security flaws
- →Requires manual configuration of server-level headers
- →Might break older external assets lacking HTTPS support
How it compares
Rather than generic feedback, it provides specific, production-ready security header implementation snippets.
Compared to similar skills
best-practices side by side with the closest alternatives in the catalog.
| Skill | Installs | Updated | Safety | Difficulty |
|---|---|---|---|---|
| best-practices (this skill) | 3 | 6mo | Caution | Beginner |
| github-code-review | 13 | 2mo | Review | Advanced |
| reviewing-nextjs-16-patterns | 11 | 8mo | Review | Intermediate |
| agent-code-analyzer | 3 | 6mo | Review | Intermediate |
Try saying
Example prompts that trigger this skill in your AI assistant.
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