add frontend skills

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+# 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.
+
+## 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; }  /* Larger touch target */
+}
+
+/* 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. Use `env()`:
+
+```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: Get It Right
+
+### 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"
+>
+```
+
+**How it works**:
+- `srcset` lists available images with their actual widths (`w` descriptors)
+- `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—hamburger + drawer on mobile, horizontal compact on tablet, full with labels on desktop. **Tables**: Transform to cards on mobile using `display: block` and `data-label` attributes. **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.