# Responsive Design

## Mobile-First: Write It Right

Start with base styles for mobile, use `min-width` queries to layer complexity. Desktop-first (`max-width`) means mobile loads unnecessary styles first.

```css
/* Mobile-first (correct) */
.container { padding: 1rem; }

@media (min-width: 768px) {
  .container { padding: 2rem; }
}

/* Desktop-first (wrong way) */
.container { padding: 2rem; }

@media (max-width: 768px) {
  .container { padding: 1rem; }
}
```

---

## Breakpoints: Content-Driven

Don't chase device sizes — let content tell you where to break. Start narrow, stretch until design breaks, add breakpoint there. Three breakpoints usually suffice: 640, 768, 1024px. Use `clamp()` for fluid values without breakpoints.

---

## Detect Input Method, Not Just Screen Size

Screen size doesn't tell you input method. A laptop with touchscreen, a tablet with keyboard — use pointer and hover queries:

```css
/* Fine pointer (mouse, trackpad) */
@media (pointer: fine) {
  .button { padding: 8px 16px; }
}

/* Coarse pointer (touch, stylus) */
@media (pointer: coarse) {
  .button { padding: 12px 20px; }
}

/* Device supports hover */
@media (hover: hover) {
  .card:hover { transform: translateY(-2px); }
}

/* Device doesn't support hover (touch) */
@media (hover: none) {
  .card { /* No hover state — use active instead */ }
}
```

**Critical**: Don't rely on hover for functionality. Touch users can't hover.

---

## Safe Areas: Handle the Notch

Modern phones have notches, rounded corners, and home indicators:

```css
body {
  padding-top: env(safe-area-inset-top);
  padding-bottom: env(safe-area-inset-bottom);
  padding-left: env(safe-area-inset-left);
  padding-right: env(safe-area-inset-right);
}

/* With fallback */
.footer {
  padding-bottom: max(1rem, env(safe-area-inset-bottom));
}
```

Enable viewport-fit in your meta tag:
```html
<meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
```

---

## Responsive Images

### srcset with Width Descriptors

```html
<img
  src="hero-800.jpg"
  srcset="
    hero-400.jpg 400w,
    hero-800.jpg 800w,
    hero-1200.jpg 1200w
  "
  sizes="(max-width: 768px) 100vw, 50vw"
  alt="Hero image"
>
```

- `srcset` lists available images with their actual widths
- `sizes` tells the browser how wide the image will display
- Browser picks the best file based on viewport width AND device pixel ratio

### Picture Element for Art Direction

When you need different crops/compositions (not just resolutions):

```html
<picture>
  <source media="(min-width: 768px)" srcset="wide.jpg">
  <source media="(max-width: 767px)" srcset="tall.jpg">
  <img src="fallback.jpg" alt="...">
</picture>
```

---

## Layout Adaptation Patterns

**Navigation** — three stages:
- Mobile: hamburger + drawer
- Tablet: horizontal compact
- Desktop: full with labels

**Tables** — transform to cards on mobile:
```css
@media (max-width: 640px) {
  table, thead, tbody, tr, td {
    display: block;
  }
  
  td::before {
    content: attr(data-label) ": ";
    font-weight: bold;
  }
  
  thead { display: none; }
}
```

**Progressive disclosure** — use `<details>/<summary>` for content that can collapse on mobile.

---

## Testing: Don't Trust DevTools Alone

DevTools device emulation is useful for layout but misses:
- Actual touch interactions
- Real CPU/memory constraints
- Network latency patterns
- Font rendering differences
- Browser chrome/keyboard appearances

**Test on at least**: One real iPhone, one real Android, a tablet if relevant. Cheap Android phones reveal performance issues you'll never see on simulators.

---

**Avoid**: Desktop-first design. Device detection instead of feature detection. Separate mobile/desktop codebases. Ignoring tablet and landscape. Assuming all mobile devices are powerful.