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".
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Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/addyosmani/web-quality-skills --skill best-practices- 01
Install
Run the command in your project. The CLI asks which agents to install for.
- 02
Describe the task
Ask your agent for something this skill covers. It loads best-practices automatically when relevant.
- 03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.
Security audits
Independent scans of the skill's instructions and bundled files.
SKILL.md
The instructions your agent reads when the skill is triggered.
Best practices
Modern web development standards based on Lighthouse best practices audits. Covers security, browser compatibility, and code quality patterns.
Evidence-led audit workflow
When a rendered page is available:
- Run a live Lighthouse Best Practices audit when that capability is available; with Chrome DevTools MCP, use
lighthouse_audit. Use navigation mode for a normal page load or snapshot mode when the current state must be preserved. - Inspect the listed console and network failures and fetch individual details only when they support a finding.
- Supplement runtime evidence with dependency, header, configuration, and source inspection; Lighthouse is not a complete security assessment.
- Fix the implicated code, re-run the same audit, and keep security findings separate from style preferences.
If live tools are unavailable, use the Lighthouse CLI plus focused dependency and header checks. Never report a high Lighthouse score as proof that the application is secure.
Security
Read the security reference when security is in scope or a live audit surfaces a related failure. It covers HTTPS/HSTS, CSP and Trusted Types, Subresource Integrity, headers, dependencies, sanitization, and cookies.
At minimum:
- Use HTTPS without mixed content. Add HSTS only after confirming every relevant subdomain supports HTTPS.
- Treat a strict CSP as defense in depth. Prefer nonces or hashes and test with report-only before enforcement.
- Sanitize untrusted HTML and protect DOM XSS sinks. Prefer text APIs when markup is not required.
- Pin and review third-party code. Use SRI where the delivery model supports it and keep dependencies patched.
- Verify response headers at runtime. Source configuration alone does not prove what the deployed page sends.
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)
Prefer bundling polyfills at build time (Babel/SWC + core-js, or @vitejs/plugin-legacy) targeted by your supported-browsers list. This eliminates the runtime check entirely and avoids shipping polyfill bytes to modern browsers.
If you must load a polyfill at runtime, append a script element — never use document.write (it blocks the parser and is broken in async/deferred contexts):
<script>
if (!('fetch' in window)) {
const s = document.createElement('script');
s.src = '/polyfills/fetch.js';
s.defer = true;
document.head.appendChild(s);
}
</script>
Never load polyfills from a third-party CDN you don't control. The polyfill.io service was compromised in mid-2024 in a supply-chain attack and used to serve malware to ~100k sites. Self-host, or use a vetted mirror (e.g. Cloudflare's cdnjs polyfill build) — and pin the version with Subresource Integrity.
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',
};
Strip sourcesContent from production maps when uploading to your error tracker. By default, bundlers embed the full original source inside the .map file — anyone who obtains the map (including via a misconfigured upload step) gets your unminified code. Configure your bundler to omit sourcesContent, or use a Sentry/Bugsnag CLI flag that does so when uploading.
For Vite, prefer sourcemap: 'hidden' over 'true' so the //# sourceMappingURL= comment isn't emitted into the bundle.
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
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)
- HTTPS enabled, no mixed content
- No vulnerable dependencies (
npm audit) - CSP headers configured (with
frame-ancestors,base-uri,form-action) -
require-trusted-types-for 'script'enforced (or report-only during rollout) - Third-party
<script>/<link rel="stylesheet">pinned with SRI hashes - Security headers present (HSTS, X-Content-Type-Options, Referrer-Policy)
- No exposed source maps (and
sourcesContentstripped from uploaded ones)
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 | Header analysis |
| W3C Validator | HTML validation |
Live Lighthouse audit (Chrome DevTools MCP: lighthouse_audit) | Rendered Best Practices checks for agents |
| Lighthouse CLI | Best Practices audit fallback |
| Observatory | Security scan |
References
- MDN Web Security
- OWASP Top 10
- Web Quality Audit
FAQ
What does the best-practices skill do?
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".
How do I install best-practices?
Run npx skills add https://github.com/addyosmani/web-quality-skills --skill best-practices in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/best-practices/ (personal) or .claude/skills/best-practices/ (project).
Which AI agents can use best-practices?
Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.
Is best-practices safe to install?
Security partners have reviewed it: Gen Agent Trust Hub passed; Socket passed; Snyk flagged items to review; Runlayer passed; ZeroLeaks passed. As with any skill, read the SKILL.md and bundled scripts before installing.
Who maintains best-practices?
It is published by addyosmani in addyosmani/web-quality-skills. It has 29,029 recorded installs.
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