Interaction Design

Animation Timing Reference

Research-backed durations. The most common mistake is animations that are too slow.

Easing rules:

• ease-out (cubic-bezier(0, 0, 0.2, 1)): For elements entering the screen — starts fast, decelerates. Feels responsive.
• ease-in (cubic-bezier(0.4, 0, 1, 1)): For elements leaving — starts slow, accelerates. Feels natural.
• ease-in-out (cubic-bezier(0.4, 0, 0.2, 1)): For elements transforming within the screen.
• Never use linear for UI — looks mechanical.

:root {
  --ease-out:     cubic-bezier(0, 0, 0.2, 1);
  --ease-in:      cubic-bezier(0.4, 0, 1, 1);
  --ease-in-out:  cubic-bezier(0.4, 0, 0.2, 1);

  --duration-fast:    100ms;
  --duration-normal:  200ms;
  --duration-slow:    300ms;
}

prefers-reduced-motion (SAFETY-CRITICAL)

Parallax, large-scale animations, and auto-playing effects can cause nausea, dizziness, and seizures in users with vestibular disorders. This is a safety concern, not just a preference.

/* Global reduced-motion reset — add to your base CSS */
@media (prefers-reduced-motion: reduce) {
  *,
  *::before,
  *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
    scroll-behavior: auto !important;
  }
}

For animations that serve a purpose (progress feedback), provide a reduced alternative:

.spinner {
  animation: spin 1s linear infinite;
}

@media (prefers-reduced-motion: reduce) {
  .spinner {
    animation: none;
    /* Show a static indicator instead */
    opacity: 0.5;
  }
}

In React/Tailwind:

import { useReducedMotion } from "@/hooks/use-reduced-motion";

function AnimatedModal({ children }: { children: React.ReactNode }) {
  const reduced = useReducedMotion();
  return (
    <div
      className={cn(
        "transition-all",
        reduced ? "duration-0" : "duration-200 ease-out"
      )}
    >
      {children}
    </div>
  );
}

// Hook
function useReducedMotion() {
  return window.matchMedia("(prefers-reduced-motion: reduce)").matches;
}

Loading States Decision Matrix

Skeleton screens feel 20–30% faster than spinners for the same wait time. They set layout expectations and eliminate "jumping" content.

Skeleton screen pattern (Tailwind)

function Skeleton({ className }: { className?: string }) {
  return (
    <div
      className={cn("animate-pulse rounded-md bg-muted", className)}
      aria-hidden="true"
    />
  );
}

// Usage — mirror the actual layout
function CardSkeleton() {
  return (
    <div className="rounded-lg border p-4 space-y-3">
      <div className="flex items-center gap-3">
        <Skeleton className="h-10 w-10 rounded-full" />
        <div className="space-y-1.5 flex-1">
          <Skeleton className="h-4 w-3/4" />
          <Skeleton className="h-3 w-1/2" />
        </div>
      </div>
      <Skeleton className="h-20 w-full" />
      <Skeleton className="h-4 w-2/3" />
    </div>
  );
}

Optimistic UI pattern

function useLikePost(postId: string) {
  const [liked, setLiked] = useState(false);

  async function toggleLike() {
    // 1. Update UI immediately (optimistic)
    setLiked((prev) => !prev);
    try {
      // 2. Sync with server
      await api.toggleLike(postId);
    } catch {
      // 3. Revert on error
      setLiked((prev) => !prev);
      toast.error("Failed to update. Please try again.");
    }
  }

  return { liked, toggleLike };
}

Use optimistic UI for: liking, bookmarking, toggling settings, marking items complete. Not suitable for: payments, deletions, irreversible actions.

CLS Prevention (Cumulative Layout Shift)

Target: CLS < 0.1 (Google Core Web Vitals "Good" threshold).

60% of CLS is caused by images without dimensions.

/* Always reserve space for images */
.image-wrapper {
  aspect-ratio: 16 / 9;        /* reserves correct proportional space */
  background-color: hsl(var(--muted));
  overflow: hidden;
}

.avatar {
  width: 40px;
  height: 40px;
  border-radius: 50%;
  flex-shrink: 0;
}

/* Responsive images without CLS */
img {
  max-width: 100%;
  height: auto;
  display: block;
}

<!-- Always include width + height -->
<img src="hero.jpg" width="1200" height="630" alt="..." loading="lazy" />

For dynamic content (ads, async components), reserve fixed space:

.ad-slot { min-height: 250px; }
.async-card { min-height: 120px; }

Form UX Patterns

Research: Single-column layouts complete 15.4% faster than multi-column. Removing one optional field at Expedia increased annual revenue by $12M.

Core rules

Accessible form pattern

function FormField({
  id,
  label,
  error,
  children,
}: {
  id: string;
  label: string;
  error?: string;
  children: React.ReactNode;
}) {
  return (
    <div className="space-y-1.5">
      <label htmlFor={id} className="text-sm font-medium text-foreground">
        {label}
      </label>
      {/* Clone child with aria props */}
      {React.cloneElement(children as React.ReactElement, {
        id,
        "aria-describedby": error ? `${id}-error` : undefined,
        "aria-invalid": error ? true : undefined,
        className: cn((children as React.ReactElement).props.className,
          error && "border-destructive focus-visible:ring-destructive"
        ),
      })}
      {error && (
        <p id={`${id}-error`} className="text-sm text-destructive" role="alert">
          {error}
        </p>
      )}
    </div>
  );
}

Mobile UX — Touch Targets & Thumb Zones

Touch target sizes

/* Expand tap area without changing visual size */
.icon-button {
  position: relative;
  padding: 12px;  /* 48px total for a 24px icon */
}

/* Or use the ::after pseudo-element */
.small-tap-target::after {
  content: "";
  position: absolute;
  inset: -8px;
}

Thumb zones (one-handed use)

┌─────────────────────┐
│  ✗ Hard to reach    │  ← Top corners, especially top-right
│─────────────────────│
│  ~ Stretch zone     │  ← Middle of screen, slight reach
│─────────────────────│
│  ✓ Natural zone     │  ← Bottom 40% — thumb reaches easily
│  ✓✓ Primary zone   │  ← Bottom bar — most comfortable
└─────────────────────┘

Decisions based on thumb zones:

• Primary actions (CTA, submit): bottom third
• Navigation: bottom bar over hamburger menu
• Destructive actions: middle or top (requires deliberate reach)
• Secondary actions: anywhere, but avoid top-right corner

Bottom navigation vs hamburger

Use bottom navigation for 3–5 primary destinations. For 6+ items, use a hybrid (bottom bar for top 4–5, slide-out drawer for the rest).

Dark Pattern Quick Reference

2024 FTC sweep: 76% of apps use at least one dark pattern. Risk: FTC enforcement, EU EAA, state law violations.

Related Skills

• styling/tailwindcss — Tailwind animation utilities (animate-pulse, transitions)
• accessibility/wcag — focus management in modals/dialogs
• best-practices/performance — Core Web Vitals and CLS measurement
• ux/visual-hierarchy — typography and spacing foundations
• ux/design-systems — animation tokens (duration, easing variables)

Deep Knowledge

Load via mcp__documentation__fetch_docs:

• ux-interaction-design — extended animation research, microinteraction patterns, loading state psychology
• ux-mobile — thumb zone research data, touch target guidelines, navigation pattern studies
• ux-forms — form conversion research, validation UX studies, accessible error pattern library
• ux-ethical-design — full dark pattern catalog, FTC/EU EAA legal context