gabrielmoreira 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 |