Mobile Optimization Checklist

Run Lighthouse in Chrome DevTools with the Mobile preset and Slow 4G throttling against the top 10 landing pages by GA4 sessions. Save the JSON report — you'll compare LCP, INP, and CLS to PageSpeed Insights field data later in the run.

Open Search Console → Core Web Vitals → Mobile. Lab data from Lighthouse and field data from CrUX often disagree; the Search Console mobile report is what affects rankings. Note any URL groups flagged Poor or Needs Improvement.

Spot-check the homepage, top 3 conversion pages, and top 3 organic landing pages. Google retired the standalone Mobile-Friendly Test in late 2023 — use Search Console's URL Inspection tool and the rendered screenshot instead.

Hero images are typically the LCP element on mobile. Serve WebP or AVIF with srcset breakpoints at 360, 768, and 1200 px. Add fetchpriority="high" on the LCP image and lazy-load everything below the fold.

Body text under 16px triggers iOS Safari auto-zoom on form focus and fails WCAG 2.1 readability guidance. Confirm line height around 1.5 and contrast ratio at least 4.5:1 against the background.

Test on a 375x667 viewport (iPhone SE) — the smallest mainstream screen. The primary CTA should be visible without scrolling. Sticky headers eating 60+ pixels are a common culprit.

Open and close the menu on Safari iOS, Chrome Android, and Samsung Internet. Watch for focus traps, scroll-lock issues, and the iOS bottom toolbar overlapping menu items. Confirm aria-expanded toggles correctly for screen readers.

Set inputmode="search" so iOS shows the search keyboard. Confirm autocomplete suggestions don't push the submit button below the fold. Test that filter drawers don't trap scroll on the underlying page.

Use type="email", type="tel", inputmode="numeric" for postal codes, and autocomplete tokens (given-name, family-name, email, tel, postal-code). Wrong input types are the single biggest mobile form-completion killer — users abandon when the wrong keyboard appears.

Largest Contentful Paint over 2.5s on mobile is the most common Core Web Vitals failure. Preload the LCP image, defer non-critical JavaScript, and inline critical CSS. Confirm the fix in CrUX field data — lab improvements don't always show in field data for 28 days.

Interaction to Next Paint replaced FID in March 2024. Long tasks from third-party scripts (chat widgets, A/B testing tools, marketing tags) are the typical culprits. Profile with the Performance panel and defer non-essential tags via consent-state gating in GTM.

Cumulative Layout Shift over 0.1 mostly comes from images without explicit width/height, late-loading web fonts, and ad slots without reserved space. Set aspect-ratio CSS on media containers and use font-display: optional or swap with size-adjust descriptors.

Open GA4 DebugView with the device on the same network. Confirm page_view, scroll, click, form_submit, and any custom conversion events fire on mobile. A common gotcha: enhanced measurement form_submit fires on email-blur rather than actual submit, inflating reported conversions.

Build a GA4 exploration with Device Category as a dimension. A mobile conversion rate under 40% of desktop on the same campaigns usually indicates a mobile UX or tracking gap, not user intent. Segment by traffic source — paid social skews heavily mobile and distorts blended rates.