Website Development Rules

Performance

Images are the #1 cause of slow sites — use WebP/AVIF, lazy-load below-the-fold, and set explicit width/height to prevent layout shift
Render-blocking CSS delays first paint — inline critical CSS in <head>, defer the rest
Third-party scripts (analytics, chat widgets) often add 500ms+ — load them with async or defer, audit regularly
Fonts cause invisible text flash (FOIT) — use font-display: swap and preload critical fonts
Measure with Lighthouse in incognito mode — extensions skew results

Start CSS with mobile styles, add complexity with min-width media queries — easier to scale up than strip down
Touch targets need 44x44px minimum — fingers are imprecise, small buttons frustrate users
Test on real devices, not just browser DevTools — throttling simulation misses real-world jank
Horizontal scroll is a critical bug — test every page at 320px width minimum
viewport meta tag is required: <meta name="viewport" content="width=device-width, initial-scale=1">

Every <img> needs alt text — empty alt="" for decorative images, descriptive text for meaningful ones
Color contrast ratio 4.5:1 minimum for body text — use WebAIM contrast checker
Form inputs must have associated <label> elements — placeholders alone are not accessible
Keyboard navigation must work — test every interactive element with Tab key
Screen readers announce heading hierarchy — use H1-H6 in logical order, never skip levels

One <h1> per page only — it's the page title, not a styling tool
Use semantic elements: <nav>, <main>, <article>, <aside>, <footer> — they communicate structure to browsers and assistive tech
<button> for actions, <a> for navigation — don't use divs with click handlers
External links should have rel="noopener" — prevents security vulnerability with target="_blank"
Validate HTML — broken markup causes unpredictable rendering across browsers

Avoid !important — it breaks cascade and makes debugging painful. Fix specificity instead
Use relative units (rem, em, %) over fixed px for text — respects user font size preferences
CSS custom properties (variables) reduce repetition — define colors and spacing once, use everywhere
Flexbox for 1D layouts, Grid for 2D — don't force one to do the other's job
Test without CSS loading — content should still be readable in plain HTML

Missing favicon causes 404 spam in server logs — always include one, even a simple PNG
Not setting <html lang="en"> breaks screen reader pronunciation
Hardcoded http:// links break on HTTPS sites — use protocol-relative // or always https://
Assuming JavaScript is available — core content should work without JS (progressive enhancement)
Forgetting print styles — add @media print for pages users might print (receipts, articles)

Test all forms actually submit — broken contact forms lose leads silently
Check 404 page exists and is helpful — default server 404 looks unprofessional
Verify social sharing previews with Open Graph tags — test in Facebook/Twitter debuggers
Submit sitemap to Google Search Console — speeds up indexing
Set up uptime monitoring — know when your site goes down before users tell you