Mobile-First Designer

Overview

This skill provides expert guidance on creating mobile-first, responsive designs that prioritize usability, performance, and accessibility across all devices. It enforces industry standards for touch interactions, responsive layouts, and progressive web app capabilities.

Core Philosophy: Design for mobile constraints first, then progressively enhance for larger screens. This ensures optimal experiences on the devices most users rely on.

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Critical Mobile-First Standards

Touch Target Requirements

Minimum Size: 44x44 pixels (Apple HIG standard)

• All interactive elements (buttons, links, inputs) must meet this minimum
• Provides comfortable tap area for various finger sizes
• Prevents accidental taps and user frustration

Spacing: Minimum 8px between interactive elements

• Prevents accidental activation of adjacent controls
• Improves accuracy and reduces user errors
• Critical for forms, navigation menus, and action buttons

Examples:

/* Good - Meets minimum touch target */
.button {
  min-height: 44px;
  min-width: 44px;
  padding: 12px 24px;
}

/* Good - Adequate spacing */
.action-buttons {
  display: flex;
  gap: 8px; /* Minimum spacing */
}

/* Bad - Too small for touch */
.small-link {
  font-size: 10px;
  padding: 2px;
}

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Mobile-First CSS Architecture

Start with Base Mobile Styles

Write CSS for mobile devices first, then use min-width media queries to enhance for larger screens:

/* Base styles for mobile (320px+) */
body {
  font-size: 16px; /* Never go below 16px to prevent zoom on iOS */
  line-height: 1.5;
  padding: 16px;
}

.container {
  width: 100%;
  max-width: 100%;
}

/* Tablet enhancement (768px+) */
@media (min-width: 768px) {
  body {
    font-size: 18px;
    padding: 24px;
  }
  
  .container {
    max-width: 720px;
    margin: 0 auto;
  }
}

/* Desktop enhancement (1024px+) */
@media (min-width: 1024px) {
  body {
    font-size: 20px;
  }
  
  .container {
    max-width: 960px;
  }
}

Key Breakpoints (2025 Standards)

• 320px: Small phones (base)
• 480px: Large phones (landscape)
• 768px: Tablets (portrait)
• 1024px: Tablets (landscape) / Small desktops
• 1280px: Desktops
• 1920px: Large screens

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Typography Standards

Font Sizing

Mobile Requirements:

• Body text: Minimum 16px (prevents iOS auto-zoom on input focus)
• Headings: Scale proportionally (1.25-2x body size)
• Line height: 1.5 for body text, 1.2-1.3 for headings
• Line length: Maximum 65 characters per line for readability

/* Mobile typography */
body {
  font-size: 16px;
  line-height: 1.5;
}

h1 { font-size: 32px; line-height: 1.2; }
h2 { font-size: 28px; line-height: 1.25; }
h3 { font-size: 24px; line-height: 1.3; }

.content {
  max-width: 65ch; /* Optimal readability */
}

/* Scale up for larger screens */
@media (min-width: 768px) {
  body { font-size: 18px; }
  h1 { font-size: 40px; }
}

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Responsive Layout Patterns

Fluid Grid System

Use flexible, percentage-based layouts that adapt naturally:

/* Flexbox approach */
.grid {
  display: flex;
  flex-wrap: wrap;
  gap: 16px;
}

.grid-item {
  flex: 1 1 100%; /* Mobile: full width */
  min-width: 0; /* Prevents overflow */
}

@media (min-width: 768px) {
  .grid-item {
    flex: 1 1 calc(50% - 8px); /* Tablet: 2 columns */
  }
}

@media (min-width: 1024px) {
  .grid-item {
    flex: 1 1 calc(33.333% - 12px); /* Desktop: 3 columns */
  }
}

Responsive Images

Serve appropriately sized images for different devices:

<!-- Responsive image with srcset -->
<img 
  src="image-320w.jpg" 
  srcset="image-320w.jpg 320w,
          image-640w.jpg 640w,
          image-1280w.jpg 1280w"
  sizes="(max-width: 480px) 100vw,
         (max-width: 768px) 50vw,
         25vw"
  alt="Description"
>

<!-- Modern image formats -->
<picture>
  <source srcset="image.webp" type="image/webp">
  <source srcset="image.avif" type="image/avif">
  <img src="image.jpg" alt="Fallback">
</picture>

---

Navigation Patterns

Mobile-Optimized Navigation

Hamburger Menu (for complex navigation):

/* Mobile: Show hamburger */
.mobile-menu {
  display: none;
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100vh;
  background: white;
  z-index: 1000;
}

.hamburger {
  display: block;
  width: 44px;
  height: 44px;
  padding: 8px;
}

.mobile-menu.active {
  display: block;
}

/* Desktop: Show full navigation */
@media (min-width: 768px) {
  .hamburger {
    display: none;
  }
  
  .desktop-nav {
    display: flex;
  }
}

Fixed Header (stays accessible during scroll):

header {
  position: sticky;
  top: 0;
  z-index: 100;
  background: white;
  box-shadow: 0 2px 4px rgba(0,0,0,0.1);
}

---

Performance Optimization

Core Web Vitals Targets (2025)

Largest Contentful Paint (LCP): < 2.5 seconds

• Load critical content first
• Optimize images and fonts
• Minimize render-blocking resources

First Input Delay (FID): < 100 milliseconds

• Reduce JavaScript execution time
• Break up long tasks
• Use web workers for heavy operations

Cumulative Layout Shift (CLS): < 0.1

• Reserve space for images (use aspect-ratio or explicit dimensions)
• Avoid inserting content above existing content
• Use CSS transforms for animations

Performance Techniques

/* Lazy loading for images */
<img src="image.jpg" loading="lazy" alt="Description">

/* Aspect ratio to prevent layout shift */
.image-container {
  aspect-ratio: 16 / 9;
}

/* Optimize animations */
.animated-element {
  will-change: transform; /* Hint to browser */
  transform: translateX(0); /* Use transform instead of left/margin */
}

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Progressive Web App (PWA) Enhancements

Essential PWA Requirements

1. HTTPS Required

• PWAs must be served over secure connections
• Use SSL certificates from Let's Encrypt, Cloudflare, etc.

2. Web App Manifest Create manifest.json in your root directory:

{
  "name": "Your App Name",
  "short_name": "App",
  "start_url": "/",
  "display": "standalone",
  "background_color": "#ffffff",
  "theme_color": "#007bff",
  "icons": [
    {
      "src": "/icons/icon-192.png",
      "sizes": "192x192",
      "type": "image/png"
    },
    {
      "src": "/icons/icon-512.png",
      "sizes": "512x512",
      "type": "image/png"
    }
  ]
}

3. Service Worker for Offline Support Basic service worker structure:

// sw.js
const CACHE_NAME = 'v1';
const urlsToCache = [
  '/',
  '/styles/main.css',
  '/scripts/main.js'
];

self.addEventListener('install', event => {
  event.waitUntil(
    caches.open(CACHE_NAME)
      .then(cache => cache.addAll(urlsToCache))
  );
});

self.addEventListener('fetch', event => {
  event.respondWith(
    caches.match(event.request)
      .then(response => response || fetch(event.request))
  );
});

4. Add to Home Screen Prompt

// Detect install prompt
let deferredPrompt;

window.addEventListener('beforeinstallprompt', (e) => {
  e.preventDefault();
  deferredPrompt = e;
  // Show custom install button
  showInstallButton();
});

---

Accessibility Standards

WCAG 2.1 AA Compliance

Color Contrast:

• Normal text: Minimum 4.5:1 contrast ratio
• Large text (18px+): Minimum 3:1 contrast ratio
• Test with tools like WebAIM Contrast Checker

Keyboard Navigation:

• All interactive elements must be keyboard accessible
• Visible focus indicators required
• Logical tab order maintained

/* Clear focus indicators */
button:focus,
a:focus,
input:focus {
  outline: 2px solid #007bff;
  outline-offset: 2px;
}

/* Skip to content link */
.skip-link {
  position: absolute;
  top: -40px;
  left: 0;
  background: #000;
  color: white;
  padding: 8px;
  text-decoration: none;
  z-index: 100;
}

.skip-link:focus {
  top: 0;
}

ARIA Labels:

<!-- Screen reader support -->
<button aria-label="Close menu">
  <span aria-hidden="true">&times;</span>
</button>

<nav aria-label="Main navigation">
  <!-- Navigation items -->
</nav>

<img src="chart.png" alt="Sales increased 25% in Q4 2024">

---

Touch Interaction Guidelines

Avoid Hover-Dependent Features

Touch devices don't support hover states. Design with tap/press interactions:

Bad:

/* Don't rely on hover for essential info */
.tooltip {
  display: none;
}
.element:hover .tooltip {
  display: block;
}

Good:

/* Use tap/press states */
.element:active .tooltip,
.element:focus .tooltip {
  display: block;
}

Touch Gestures

Support common mobile gestures:

• Swipe: Navigation, carousels, dismissing items
• Pull to refresh: Content updates
• Pinch to zoom: Images, maps (if appropriate)
• Long press: Context menus, additional options

// Touch event handling
let touchStartX = 0;
let touchEndX = 0;

element.addEventListener('touchstart', e => {
  touchStartX = e.changedTouches[0].screenX;
});

element.addEventListener('touchend', e => {
  touchEndX = e.changedTouches[0].screenX;
  handleSwipe();
});

function handleSwipe() {
  if (touchEndX < touchStartX - 50) {
    // Swipe left
  }
  if (touchEndX > touchStartX + 50) {
    // Swipe right
  }
}

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Review Checklist

When reviewing mobile designs, verify:

Layout & Spacing

• Touch targets minimum 44x44px
• 8px minimum spacing between interactive elements
• No horizontal scroll at any breakpoint
• Content fits viewport without zooming
• Adequate padding/margins on all sides (16px minimum)

Typography

• Body text minimum 16px
• Line height 1.5 for body text
• Line length maximum 65 characters
• Text scales appropriately across breakpoints

Performance

• Images optimized and responsive
• LCP < 2.5 seconds
• FID < 100 milliseconds
• CLS < 0.1
• No render-blocking resources

Navigation

• Easy to access and use on mobile
• Touch-friendly menu items
• Clear visual hierarchy
• Hamburger menu for complex navigation
• Fixed header if needed

Accessibility

• Color contrast meets WCAG AA (4.5:1)
• All interactive elements keyboard accessible
• Visible focus indicators
• ARIA labels where needed
• Alt text for images

Touch Interactions

• No hover-dependent features
• Touch gestures supported where appropriate
• Visual feedback for taps/presses
• Swipe-friendly carousels/galleries

PWA Considerations

• HTTPS enabled
• Web app manifest present
• Service worker for offline support
• Installable on home screen
• Works offline (if applicable)

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Common Mobile Design Mistakes

❌ Avoid These Patterns

1. Tiny Touch Targets

/* BAD - Too small */
.link {
  font-size: 12px;
  padding: 2px 4px;
}

2. Desktop-First Thinking

/* BAD - Requires override for mobile */
.element {
  width: 1200px; /* Doesn't fit mobile */
}

@media (max-width: 768px) {
  .element {
    width: 100%; /* Unnecessary override */
  }
}

3. Ignoring Touch Interactions

/* BAD - Hover only */
.dropdown:hover .menu {
  display: block;
}

4. Fixed Widths

/* BAD */
.container {
  width: 960px;
}

/* GOOD */
.container {
  max-width: 960px;
  width: 100%;
  padding: 0 16px;
}

5. No Viewport Meta Tag

<!-- Always include this -->
<meta name="viewport" content="width=device-width, initial-scale=1">

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Testing Recommendations

Test on Real Devices

Minimum Test Matrix:

• iOS (iPhone 13+, iPad)
• Android (Pixel, Samsung Galaxy)
• Various screen sizes (small phone to tablet)
• Different network conditions (3G, 4G, 5G, offline)

Testing Tools

Browser DevTools:

• Chrome DevTools: Device simulation, network throttling
• Firefox Responsive Design Mode
• Safari Web Inspector (for iOS testing)

Performance Testing:

• Google PageSpeed Insights
• Lighthouse (in Chrome DevTools)
• WebPageTest
• GTmetrix

Accessibility Testing:

• WAVE (Web Accessibility Evaluation Tool)
• axe DevTools
• Color contrast analyzers

Cross-Browser Testing:

• BrowserStack
• LambdaTest
• Sauce Labs

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Implementation Workflow

Step-by-Step Process

1. Start with Mobile Wireframes

• Design for 320-375px width first
• Focus on essential content and actions
• Establish visual hierarchy

2. Build Mobile-First CSS

• Write base styles for mobile
• Add progressive enhancements with min-width media queries
• Test at each breakpoint

3. Optimize Performance

• Compress and optimize images
• Minify CSS/JS
• Implement lazy loading
• Test Core Web Vitals

4. Add Touch Interactions

• Ensure all buttons meet 44px minimum
• Add appropriate spacing
• Test on actual touch devices

5. Implement PWA Features (if applicable)

• Add manifest.json
• Create service worker
• Test offline functionality
• Verify installability

6. Accessibility Audit

• Check color contrast
• Test keyboard navigation
• Add ARIA labels
• Verify screen reader compatibility

7. Cross-Device Testing

• Test on multiple devices
• Verify responsive breakpoints
• Check performance metrics
• Gather user feedback

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Resources & Standards

Official Guidelines

• Apple Human Interface Guidelines: 44pt minimum touch targets
• Google Material Design: Touch target guidance
• W3C Mobile Web Best Practices: Accessibility and usability
• WCAG 2.1: Accessibility standards

Key Metrics (2025)

• Mobile traffic: 60%+ of global web traffic
• Expected load time: < 3 seconds (53% abandon otherwise)
• Touch target size: 44x44px minimum
• Font size: 16px minimum for body text
• Spacing: 8px minimum between interactive elements

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Summary

Mobile-first design is not optional in 2025—it's essential. By following these guidelines, you'll create experiences that:

✅ Work beautifully on all devices ✅ Load fast and perform smoothly ✅ Are accessible to all users ✅ Convert better and engage more effectively ✅ Rank higher in search results (mobile-first indexing)

Remember: Design for mobile constraints first, then progressively enhance for larger screens. This ensures your design works where it matters most—on the devices most users rely on daily.