best-practices

Modern web development standards based on Lighthouse best practices audits. Covers security, browser compatibility, and code quality patterns.

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Install skill "best-practices" with this command: npx skills add davila7/claude-code-templates/davila7-claude-code-templates-best-practices

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">&#x3C;/script>

<!-- ✅ HTTPS only --> <img src="https://example.com/image.jpg"> <script src="https://cdn.example.com/script.js">&#x3C;/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">&#x3C;/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 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

  • Security headers present

  • No exposed source maps

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

Lighthouse Best practices audit

Observatory Security scan

References

  • MDN Web Security

  • OWASP Top 10

  • Web Quality Audit

Source Transparency

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