add frontend skills

skills/frontend-design/reference/interaction-design.md

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+# Interaction Design
+
+## The Eight Interactive States
+
+Every interactive element needs these states designed:
+
+| State | When | Visual Treatment |
+|-------|------|------------------|
+| **Default** | At rest | Base styling |
+| **Hover** | Pointer over (not touch) | Subtle lift, color shift |
+| **Focus** | Keyboard/programmatic focus | Visible ring (see below) |
+| **Active** | Being pressed | Pressed in, darker |
+| **Disabled** | Not interactive | Reduced opacity, no pointer |
+| **Loading** | Processing | Spinner, skeleton |
+| **Error** | Invalid state | Red border, icon, message |
+| **Success** | Completed | Green check, confirmation |
+
+**The common miss**: Designing hover without focus, or vice versa. They're different. Keyboard users never see hover states.
+
+## Focus Rings: Do Them Right
+
+**Never `outline: none` without replacement.** It's an accessibility violation. Instead, use `:focus-visible` to show focus only for keyboard users:
+
+```css
+/* Hide focus ring for mouse/touch */
+button:focus {
+  outline: none;
+}
+
+/* Show focus ring for keyboard */
+button:focus-visible {
+  outline: 2px solid var(--color-accent);
+  outline-offset: 2px;
+}
+```
+
+**Focus ring design**:
+- High contrast (3:1 minimum against adjacent colors)
+- 2-3px thick
+- Offset from element (not inside it)
+- Consistent across all interactive elements
+
+## Form Design: The Non-Obvious
+
+**Placeholders aren't labels**—they disappear on input. Always use visible `<label>` elements. **Validate on blur**, not on every keystroke (exception: password strength). Place errors **below** fields with `aria-describedby` connecting them.
+
+## Loading States
+
+**Optimistic updates**: Show success immediately, rollback on failure. Use for low-stakes actions (likes, follows), not payments or destructive actions. **Skeleton screens > spinners**—they preview content shape and feel faster than generic spinners.
+
+## Modals: The Inert Approach
+
+Focus trapping in modals used to require complex JavaScript. Now use the `inert` attribute:
+
+```html
+<!-- When modal is open -->
+<main inert>
+  <!-- Content behind modal can't be focused or clicked -->
+</main>
+<dialog open>
+  <h2>Modal Title</h2>
+  <!-- Focus stays inside modal -->
+</dialog>
+```
+
+Or use the native `<dialog>` element:
+
+```javascript
+const dialog = document.querySelector('dialog');
+dialog.showModal();  // Opens with focus trap, closes on Escape
+```
+
+## The Popover API
+
+For tooltips, dropdowns, and non-modal overlays, use native popovers:
+
+```html
+<button popovertarget="menu">Open menu</button>
+<div id="menu" popover>
+  <button>Option 1</button>
+  <button>Option 2</button>
+</div>
+```
+
+**Benefits**: Light-dismiss (click outside closes), proper stacking, no z-index wars, accessible by default.
+
+## Destructive Actions: Undo > Confirm
+
+**Undo is better than confirmation dialogs**—users click through confirmations mindlessly. Remove from UI immediately, show undo toast, actually delete after toast expires. Use confirmation only for truly irreversible actions (account deletion), high-cost actions, or batch operations.
+
+## Keyboard Navigation Patterns
+
+### Roving Tabindex
+
+For component groups (tabs, menu items, radio groups), one item is tabbable; arrow keys move within:
+
+```html
+<div role="tablist">
+  <button role="tab" tabindex="0">Tab 1</button>
+  <button role="tab" tabindex="-1">Tab 2</button>
+  <button role="tab" tabindex="-1">Tab 3</button>
+</div>
+```
+
+Arrow keys move `tabindex="0"` between items. Tab moves to the next component entirely.
+
+### Skip Links
+
+Provide skip links (`<a href="#main-content">Skip to main content</a>`) for keyboard users to jump past navigation. Hide off-screen, show on focus.
+
+## Gesture Discoverability
+
+Swipe-to-delete and similar gestures are invisible. Hint at their existence:
+
+- **Partially reveal**: Show delete button peeking from edge
+- **Onboarding**: Coach marks on first use
+- **Alternative**: Always provide a visible fallback (menu with "Delete")
+
+Don't rely on gestures as the only way to perform actions.
+
+---
+
+**Avoid**: Removing focus indicators without alternatives. Using placeholder text as labels. Touch targets <44x44px. Generic error messages. Custom controls without ARIA/keyboard support.