| 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". |
| license | MIT |
| metadata | {"author":"web-quality-skills","version":"2.0"} |
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
<HTML>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<!DOCTYPE html>
<html lang="en">
Character encoding
<html>
<head>
<title>Page</title>
<meta charset="UTF-8">
</head>
<html>
<head>
<meta charset="UTF-8">
<title>Page</title>
</head>
Viewport meta tag
<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
if (navigator.userAgent.includes('Chrome')) {
}
if ('IntersectionObserver' in window) {
} else {
}
@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('<script src="..."></script>');
const script = document.createElement('script');
script.src = '...';
document.head.appendChild(script);
const xhr = new XMLHttpRequest();
xhr.open('GET', url, false);
const response = await fetch(url);
<html manifest="cache.manifest">
// ✅ Service Workers
if ('serviceWorker' in navigator) {
navigator.serviceWorker.register('/sw.js');
}
Event listener passive
element.addEventListener('touchstart', handler);
element.addEventListener('wheel', handler);
element.addEventListener('touchstart', handler, { passive: true });
element.addEventListener('wheel', handler, { passive: true });
element.addEventListener('touchstart', handler, { passive: false });
Console & errors
No console errors
console.log('Debug info');
throw new Error('Unhandled');
try {
riskyOperation();
} catch (error) {
errorTracker.captureException(error);
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;
}
}
<ErrorBoundary>
<App />
</ErrorBoundary>
Global error handler
window.addEventListener('error', (event) => {
errorTracker.captureException(event.error);
});
window.addEventListener('unhandledrejection', (event) => {
errorTracker.captureException(event.reason);
});
Source maps
Production configuration
module.exports = {
devtool: 'source-map',
};
module.exports = {
devtool: 'hidden-source-map',
};
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
<script src="heavy-library.js"></script>
<script defer src="heavy-library.js"></script>
@import url('other-styles.css');
<link rel="stylesheet" href="styles.css">
<link rel="stylesheet" href="other-styles.css">
Efficient event handlers
items.forEach(item => {
item.addEventListener('click', handleClick);
});
container.addEventListener('click', (e) => {
if (e.target.matches('.item')) {
handleClick(e);
}
});
Memory management
const handler = () => { };
window.addEventListener('resize', handler);
const handler = () => { };
window.addEventListener('resize', handler);
window.removeEventListener('resize', handler);
const controller = new AbortController();
window.addEventListener('resize', handler, { signal: controller.signal });
controller.abort();
Code quality
Valid HTML
<div id="header">
<div id="header">
<ul>
<div>Item</div>
</ul>
<a href="/"><button>Click</button></a>
<header id="site-header">
</header>
<ul>
<li>Item</li>
</ul>
<a href="/" class="button">Click</a>
Semantic 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
<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
navigator.geolocation.getCurrentPosition(success, error);
findNearbyButton.addEventListener('click', async () => {
if (await showPermissionExplanation()) {
navigator.geolocation.getCurrentPosition(success, error);
}
});
Permissions policy
<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)
Compatibility
Code quality
UX
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
