Mobile devices account for 63% of all web traffic worldwide (Statcounter, February 2026). Sites failing mobile usability benchmarks experience 38% higher bounce rates and 24% lower conversion rates. Every 100ms of added mobile load time costs 1.1% conversion (Deloitte, 2025).
Core Principles
Thumb Zones
67% of mobile interactions involve one-handed use (Hoober, 2024). The comfortable reach zone covers the lower-center 40% of the screen. Apps that moved primary nav from top to bottom saw 20-30% increase in engagement. This connects to core UX fundamentals about reducing interaction cost.
Touch Targets
WCAG 2.2 recommends 44x44 CSS pixels minimum. 31% of mobile sites have critical touch target violations on their primary conversion path. Audit with our touch target tester. Spacing: at least 8px between adjacent interactive elements.
Progressive Disclosure
Primary actions always visible, secondary one tap away, tertiary in menus. This applies Hick's Law — reducing choices reduces decision time.
Navigation Patterns
Bottom tab bar: Standard for 3-5 sections. Use icons with labels (icons alone reduce discoverability 50%). Contextual navigation: Adapts to current page per interaction flow principles. Hamburger menu: Valid for secondary navigation only. Products that buried search in hamburger saw 70% drop in usage.
Form Optimization
27% of mobile cart abandonments from form issues (Baymard, 2025). Use correct input types (type="email", type="tel", inputmode="numeric") — reduces input time 20-30%. Minimize fields (each adds 4-7% abandonment). Single-column layouts. Inline validation (+14% completion). Support autofill with correct autocomplete attributes.
Performance Budgets
Core Web Vitals thresholds: LCP under 2.5s, INP under 200ms, CLS under 0.1. Target under 1MB total transfer. Use WebP/AVIF images, responsive srcset, lazy-load below-fold. Developers should evaluate rendering strategy against these targets.
Accessibility on Mobile
Color contrast: 4.5:1 minimum — test with our contrast checker. Text sizing: Minimum 16px, never disable user scaling. Screen readers: Verify heading structure with our heading hierarchy checker. Gestures: Every gesture action must have a simple tap alternative (WCAG 2.2 criterion 2.5.7).
Testing Mobile UX
Test on: current iPhone, flagship Android, mid-range Android ($150-200). Test contexts: bright sunlight, one-handed standing, slow network. Heurilens analyzes mobile interfaces against usability heuristics. Compare with Maze for different approaches.
Common Failures
Intrusive interstitials (96% dismissal, SEO penalty). Horizontal scrolling. Tiny close buttons on modals (need 44x44px minimum). Desktop-oriented content. Ignoring system preferences (dark mode, reduced motion, dynamic type).
Building Mobile-First
Start designs mobile-first. Track mobile metrics separately. Heurilens evaluates against these principles — touch targets, heading structure, contrast. Combined with the UX Laws library, you have a complete framework.
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