Web Development

Quick Reference

Critical Rules

1. DOCTYPE matters — Missing triggers quirks mode; layouts break unpredictably
2. CSS specificity beats cascade — .class overrides element selectors regardless of order
3. === not == — Type coercion causes "0" == false to be true
4. Async/await in loops — forEach doesn't await; use for...of or Promise.all
5. CORS is server-side — No client-side fix; configure Access-Control-Allow-Origin on the server
6. Responsive = viewport meta — Without , mobile renders desktop-width
7. Form without preventDefault — Page reloads; call e.preventDefault() in submit handler
8. Images need dimensions — Missing width/height causes layout shift (CLS penalty)
9. HTTPS or blocked — Mixed content (HTTP resources on HTTPS pages) gets blocked by browsers
10. Environment variables leak — NEXT_PUBLIC_* exposes to client; never prefix secrets

Common Requests

"Make it responsive" → Mobile-first CSS with media queries; test at 320px, 768px, 1024px

"Deploy to production" → See deploy.md for Vercel/Netlify/VPS patterns

"Fix CORS error" → Server must send headers; proxy through same-origin if you can't control server

"Improve performance" → Lighthouse audit; focus on LCP, CLS, FID; lazy-load below-fold images

"Add SEO" → Title/description per page, semantic HTML, OG tags, sitemap.xml

Framework Decision Tree

• Static content, fast builds → Astro or plain HTML
• Blog/docs with MDX → Astro or Next.js App Router
• Interactive app with auth → Next.js or Remix
• Full SSR/ISR control → Next.js
• Simple SPA, no SEO needed → Vite + React/Vue