UI Design

Opinionated constraints for building better interfaces with agents.

When to Use

• Building UI components with Tailwind CSS
• Implementing animations or transitions
• Adding interactive elements with keyboard/focus behavior
• Reviewing frontend code for accessibility
• Designing layouts with proper z-index and spacing
• Working with loading states, error handling, or empty states

Stack

Requirement	Rule
Tailwind CSS	MUST use defaults (spacing, radius, shadows) before custom values
Animation library	MUST use motion/react (formerly framer-motion) for JS animation
CSS animation	SHOULD use tw-animate-css for entrance and micro-animations
Class logic	MUST use cn utility (clsx + tailwind-merge)

Components

Requirement	Rule
Interactive primitives	MUST use accessible primitives (Base UI, React Aria, Radix) for keyboard/focus behavior
Existing components	MUST use project's existing primitives first
Consistency	NEVER mix primitive systems within the same interaction surface
New primitives	SHOULD prefer Base UI if compatible with stack
Icon buttons	MUST add aria-label to icon-only buttons
Custom behavior	NEVER rebuild keyboard or focus behavior by hand unless explicitly requested

Interaction

Requirement	Rule
Destructive actions	MUST use AlertDialog for destructive or irreversible actions
Loading states	SHOULD use structural skeletons
Viewport height	NEVER use h-screen, use h-dvh
Fixed elements	MUST respect safe-area-inset
Error display	MUST show errors next to where the action happens
Input behavior	NEVER block paste in input or textarea elements

Animation

Requirement	Rule
Default	NEVER add animation unless explicitly requested
Compositor props	MUST animate only transform, opacity
Layout props	NEVER animate width, height, top, left, margin, padding
Paint props	SHOULD avoid background, color except for small, local UI (text, icons)
Entrance easing	SHOULD use ease-out on entrance
Feedback timing	NEVER exceed 200ms for interaction feedback
Looping	MUST pause looping animations when off-screen
Accessibility	MUST respect prefers-reduced-motion
Custom easing	NEVER introduce custom easing curves unless explicitly requested
Large surfaces	SHOULD avoid animating large images or full-screen surfaces

Typography

Requirement	Rule
Headings	MUST use text-balance
Body text	MUST use text-pretty for paragraphs
Data	MUST use tabular-nums
Dense UI	SHOULD use truncate or line-clamp
Letter spacing	NEVER modify letter-spacing (tracking-) unless explicitly requested

Layout

Requirement	Rule
Z-index	MUST use a fixed scale (no arbitrary z-x)
Square elements	SHOULD use size-x instead of w-x + h-x

Performance

Requirement	Rule
Blur effects	NEVER animate large blur() or backdrop-filter surfaces
Will-change	NEVER apply will-change outside an active animation
useEffect	NEVER use for anything expressible as render logic

Design

Requirement	Rule
Gradients	NEVER use unless explicitly requested
Purple/multicolor gradients	NEVER use
Glow effects	NEVER use as primary affordances
Shadows	SHOULD use Tailwind CSS default scale unless explicitly requested
Empty states	MUST give one clear next action
Accent colors	SHOULD limit to one per view
Color tokens	SHOULD use existing theme or Tailwind CSS tokens before introducing new ones

Common Pitfalls

The most common UI design failures occur in spacing, color contrast, mobile responsiveness, state management, and visual hierarchy. These gotchas are discovered through repeated agent mistakes when building interfaces.

1. Inconsistent Spacing (Arbitrary Margins, No System)

Problem: Using an inconsistent mix of Tailwind spacing values without a deliberate system makes layouts feel incoherent.

// ❌ WRONG - arbitrary pixel values, no spacing system
<div className="ml-[13px] mr-[22px] pt-[7px]">Content</div>

// ✅ CORRECT - use system scales
<div className="ml-4 mr-4 pt-4">Content</div>

Why it fails:

• Designs look accidental (some spacing is 2px, some is 16px, no pattern)
• Mobile layouts break (spacing doesn't scale proportionally)
• Alignment is impossible (gaps don't align to grid)

Fix:

• ALWAYS use Tailwind spacing scale (0, 1, 2, 3, 4, 6, 8, 12, 16, 24, 32, etc.)
• Define spacing systems for common patterns:
  • Button padding: px-4 py-2
  • Card padding: p-6
  • Gap between sections: gap-8
  • Nested spacing: ml-6 (not ml-3)
• Group related elements with consistent gaps (use same gap-x for all horizontal lists)

2. Poor Color Contrast (Accessibility Violations)

Problem: Color combinations that fail WCAG AA contrast ratios (4.5:1 for text, 3:1 for UI components) making content illegible for low-vision users.

// ❌ WRONG - gray-400 text on gray-100 background fails contrast
<p className="text-gray-400 bg-gray-100">Important content</p>

// ✅ CORRECT - gray-700 on gray-100 passes WCAG AA
<p className="text-gray-700 bg-gray-100">Important content</p>

Why it fails:

• Approximately 4-5% of users have color blindness, and a broader 15%+ have some form of visual impairment
• Users on bright displays (phones) can't read low-contrast text
• Legal liability (WCAG compliance required in many jurisdictions)

Fix:

• Use color pairs with sufficient contrast:
  • text-gray-900/text-white on any background (always safe)
  • text-gray-700 on light backgrounds
  • text-gray-300 on dark backgrounds
• Test colors: Use online contrast checkers (https://webaim.org/resources/contrastchecker/)
• Avoid problematic pairs:
  • gray-400 on gray-100 ❌
  • red-500 on pink-100 ❌
  • Disabled states: use text-gray-400 on white, text-gray-600 on color
• For colored text (not gray): Use darker tints (red-700 not red-300)

3. Mobile-Unfriendly Layouts (Small Touch Targets, Horizontal Scroll)

Problem: Touch targets smaller than 44×44px, horizontal scrolling, or layouts that don't adapt to narrow viewports.

// ❌ WRONG - button too small, no responsive layout
<button className="px-2 py-1 text-xs">Click me</button>
<div className="flex gap-2">
  <div className="w-80">Sidebar</div>
  <div className="w-80">Content</div>
</div>

// ✅ CORRECT - hit target size, responsive stack
<button className="px-4 py-3 md:px-6 md:py-3">Click me</button>
<div className="flex flex-col md:flex-row gap-4">
  <div className="w-full md:w-80">Sidebar</div>
  <div className="flex-1">Content</div>
</div>

Why it fails:

• 40% of traffic is mobile; small buttons are impossible to tap
• Horizontal scrolling is a UX nightmare on phones
• Layouts that work on desktop can be unusable on mobile

Fix:

• Minimum touch target: 44×44px
  • Buttons: at least px-4 py-3 (standard: px-6 py-3)
  • Links: wrap in larger clickable area
• Stack on mobile: Use flex-col by default, md:flex-row for desktop
• Use responsive widths:
  • w-full on mobile, md:w-80 or md:w-1/3 on desktop
  • Use flex-1 instead of fixed widths for flexible layouts
• Test responsiveness: Check at 375px (iPhone SE) and 768px (tablet) breakpoints
• Avoid horizontal scroll: Never force users to scroll sideways

4. Missing States (Loading, Error, Empty, Disabled)

Problem: Interfaces that only show the happy path, failing silently when loading, erroring, or empty.

// ❌ WRONG - no feedback states
<button onClick={save}>Save</button>
<div>{data.map(item => <ItemCard item={item} />)}</div>

// ✅ CORRECT - all states accounted for
<button onClick={save} disabled={loading}>
  {loading ? 'Saving...' : 'Save'}
</button>

{loading && <LoadingSkeleton />}
{error && <ErrorAlert message={error} />}
{!loading && data.length === 0 && <EmptyState action="Create your first item" />}
{!loading && !error && data.map(item => <ItemCard item={item} />)}

Why it fails:

• Users don't know if action worked (no feedback = perceived failure)
• Empty lists look broken (not intentionally empty)
• No recovery path from errors

Fix:

• Loading states: Use structural skeletons (not spinners)
  • Layout remains stable while content loads
  • Users see what's coming, not just spinning
• Error states: Show next to the failed action
  • Explain what happened and how to fix it
  • Include retry button
• Empty states: Make intentional and actionable
  • "No items yet" → "Create your first item" (button)
  • Show what empty state means, how to populate
• Disabled states: Always provide visual feedback
  • disabled={true} + opacity-50 or cursor-not-allowed
  • Never silently disable without explaining why

5. Unclear Hierarchy (Everything Looks Equally Important)

Problem: All text the same size, all colors the same weight, all spacing the same distance — nothing stands out.

// ❌ WRONG - flat hierarchy
<div className="text-lg text-gray-700">
  <div className="text-lg text-gray-700">Subheading</div>
  <div className="text-lg text-gray-700">Subtitle</div>
  <button className="text-lg px-4 py-2">Action</button>
</div>

// ✅ CORRECT - clear hierarchy
<div>
  <h1 className="text-3xl font-bold text-gray-900">Title</h1>
  <h2 className="text-xl font-semibold text-gray-700 mt-6">Subheading</h2>
  <p className="text-base text-gray-600 mt-2">Subtitle</p>
  <button className="mt-6 px-6 py-3 bg-blue-600 text-white font-semibold rounded">
    Primary Action
  </button>
</div>

Why it fails:

• Users don't know where to look first
• Scanning pages is slower (no skimmable structure)
• Important actions get missed

Fix:

• Size hierarchy:
  • Headings: text-3xl (h1) → text-2xl (h2) → text-xl (h3)
  • Body text: text-base (paragraphs) → text-sm (secondary)
• Weight hierarchy:
  • Primary actions: font-bold or font-semibold
  • Important text: font-semibold
  • Body: font-normal
  • Secondary: font-normal with lighter color
• Color hierarchy:
  • Primary text: text-gray-900
  • Secondary text: text-gray-600
  • Tertiary: text-gray-500
• Spacing hierarchy:
  • Section gaps: gap-8 (largest)
  • Component gaps: gap-4
  • Dense content: gap-2
• One accent color per view: Use a single color for CTAs, not multiple colors fighting for attention

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Quick Reference

Allowed Animation Properties

transform, opacity

Forbidden Animation Properties

width, height, top, left, margin, padding, blur(), backdrop-filter

Required Accessibility Patterns

// Icon button - always add aria-label
<button aria-label="Close dialog">
  <XIcon />
</button>

// Respect reduced motion
@media (prefers-reduced-motion: reduce) {
  * { animation: none !important; }
}

Class Utility Pattern

import { clsx } from 'clsx';
import { twMerge } from 'tailwind-merge';

export function cn(...inputs: ClassValue[]) {
  return twMerge(clsx(inputs));
}