---
name: frontend-ui-engineering
description: Builds production-quality, accessible, responsive user-facing UIs. Use when building or modifying interfaces and pages, creating components, implementing layouts, meeting WCAG accessibility requirements, managing state, or when the output needs to look and feel production-quality rather than AI-generated.
---

# Frontend UI Engineering

## Overview

Build production-quality user interfaces that are accessible, performant, and visually polished. The goal is UI that looks like it was built by a design-aware engineer at a top company, not like it was generated by an AI. This means real design system adherence, proper accessibility, thoughtful interaction patterns, and no generic "AI aesthetic."

## When to Use

- Building new UI components or pages
- Modifying existing user-facing interfaces
- Implementing responsive layouts
- Adding interactivity or state management
- Fixing visual or UX issues

## Component Architecture

### File Structure

Colocate everything related to a component:

```
src/components/
  TaskList/
    TaskList.tsx          # Component implementation
    TaskList.test.tsx     # Tests
    TaskList.stories.tsx  # Storybook stories (if using)
    use-task-list.ts      # Custom hook (if complex state)
    types.ts              # Component-specific types (if needed)
```

### Component Patterns

**Prefer composition over configuration:**

```tsx
// Good: Composable
<Card>
  <CardHeader>
    <CardTitle>Tasks</CardTitle>
  </CardHeader>
  <CardBody>
    <TaskList tasks={tasks} />
  </CardBody>
</Card>

// Avoid: Over-configured
<Card
  title="Tasks"
  headerVariant="large"
  bodyPadding="md"
  content={<TaskList tasks={tasks} />}
/>
```

**Keep components focused:**

```tsx
// Good: Does one thing
export function TaskItem({ task, onToggle, onDelete }: TaskItemProps) {
  return (
    <li className="flex items-center gap-3 p-3">
      <Checkbox checked={task.done} onChange={() => onToggle(task.id)} />
      <span className={task.done ? 'line-through text-muted' : ''}>{task.title}</span>
      <Button variant="ghost" size="sm" onClick={() => onDelete(task.id)}>
        <TrashIcon />
      </Button>
    </li>
  );
}
```

**Separate data fetching from presentation:**

```tsx
// Container: handles data
export function TaskListContainer() {
  const { tasks, isLoading, error, refetch, toggleTask, deleteTask } = useTasks();

  if (isLoading) return <TaskListSkeleton />;
  if (error) return <ErrorState message="Failed to load tasks" retry={refetch} />;
  if (tasks.length === 0) return <EmptyState message="No tasks yet" />;

  return <TaskList tasks={tasks} onToggle={toggleTask} onDelete={deleteTask} />;
}

// Presentation: handles rendering
export function TaskList({ tasks, onToggle, onDelete }: { tasks: Task[] } & Pick<TaskItemProps, 'onToggle' | 'onDelete'>) {
  return (
    <ul role="list" className="divide-y">
      {tasks.map(task => <TaskItem key={task.id} task={task} onToggle={onToggle} onDelete={onDelete} />)}
    </ul>
  );
}
```

## State Management

**Choose the simplest approach that works:**

```
Local state (useState)           → Component-specific UI state
Lifted state                     → Shared between 2-3 sibling components
Context                          → Theme, auth, locale (read-heavy, write-rare)
URL state (searchParams)         → Filters, pagination, shareable UI state
Server state (React Query, SWR)  → Remote data with caching
Global store (Zustand, Redux)    → Complex client state shared app-wide
```

**Avoid prop drilling deeper than 3 levels.** If you're passing props through components that don't use them, introduce context or restructure the component tree.

## Design System Adherence

### Reference-led UI quality

When the product needs a distinct visual direction, collect evidence before choosing a layout:

1. Search a trusted reference catalogue or use references supplied by the product team.
2. Study two or three relevant screens. Record decisions about hierarchy, density, navigation, controls, responsive behavior, and interaction states.
3. Turn those decisions into a short design contract before implementation. Name the screen's job, primary action, required states, responsive rules, and patterns to reject.
4. Rebuild the useful structure in the product's own components, tokens, content, and visual language. Never copy another product's branding, proprietary text, imagery, or exact layout.

Use references as evidence, not as templates. If references are unavailable, document the assumptions and verify the result against the product's existing design system.

### Avoid the AI Aesthetic

AI-generated UI has recognizable patterns. Avoid all of them:

| AI Default | Why It Is a Problem | Production Quality |
|---|---|---|
| Purple/indigo everything | Models default to visually "safe" palettes, making every app look identical | Use the project's actual color palette |
| Excessive gradients | Gradients add visual noise and clash with most design systems | Flat or subtle gradients matching the design system |
| Rounded everything (rounded-2xl) | Maximum rounding signals "friendly" but ignores the hierarchy of corner radii in real designs | Consistent border-radius from the design system |
| Generic hero sections | Template-driven layout with no connection to the actual content or user need | Content-first layouts |
| Lorem ipsum-style copy | Placeholder text hides layout problems that real content reveals (length, wrapping, overflow) | Realistic placeholder content |
| Oversized padding everywhere | Equal generous padding destroys visual hierarchy and wastes screen space | Consistent spacing scale |
| Stock card grids | Uniform grids are a layout shortcut that ignores information priority and scanning patterns | Purpose-driven layouts |
| Shadow-heavy design | Layered shadows add depth that competes with content and slows rendering on low-end devices | Subtle or no shadows unless the design system specifies |

### Spacing and Layout

Use a consistent spacing scale. Don't invent values:

```css
/* Use the scale: 0.25rem increments (or whatever the project uses) */
/* Good */  padding: 1rem;      /* 16px */
/* Good */  gap: 0.75rem;       /* 12px */
/* Bad */   padding: 13px;      /* Not on any scale */
/* Bad */   margin-top: 2.3rem; /* Not on any scale */
```

### Typography

Respect the type hierarchy:

```
h1 → Page title (one per page)
h2 → Section title
h3 → Subsection title
body → Default text
small → Secondary/helper text
```

Don't skip heading levels. Don't use heading styles for non-heading content.

### Color

- Use semantic color tokens: `text-primary`, `bg-surface`, `border-default`, not raw hex values
- Ensure sufficient contrast (4.5:1 for normal text, 3:1 for large text)
- Don't rely solely on color to convey information (use icons, text, or patterns too)

## Accessibility (WCAG 2.1 AA)

Every component must meet these standards:

### Keyboard Navigation

```tsx
// Every interactive element must be keyboard accessible
<button onClick={handleClick}>Click me</button>        // ✓ Focusable by default
<div onClick={handleClick}>Click me</div>               // ✗ Not focusable
<div role="button" tabIndex={0} onClick={handleClick}    // ✓ But prefer <button>
     onKeyDown={e => {
       if (e.key === 'Enter') handleClick();
       if (e.key === ' ') e.preventDefault();
     }}
     onKeyUp={e => {
       if (e.key === ' ') handleClick();
     }}>
  Click me
</div>
```

### ARIA Labels

```tsx
// Label interactive elements that lack visible text
<button aria-label="Close dialog"><XIcon /></button>

// Label form inputs
<label htmlFor="email">Email</label>
<input id="email" type="email" />

// Or use aria-label when no visible label exists
<input aria-label="Search tasks" type="search" />
```

### Focus Management

```tsx
// Move focus when content changes
function Dialog({ isOpen, onClose }: DialogProps) {
  const closeRef = useRef<HTMLButtonElement>(null);

  useEffect(() => {
    if (isOpen) closeRef.current?.focus();
  }, [isOpen]);

  // Trap focus inside dialog when open
  return (
    <dialog open={isOpen}>
      <button ref={closeRef} onClick={onClose}>Close</button>
      {/* dialog content */}
    </dialog>
  );
}
```

### Meaningful Empty and Error States

```tsx
// Don't show blank screens
function TaskList({ tasks, onCreateTask }: { tasks: Task[]; onCreateTask: () => void }) {
  if (tasks.length === 0) {
    return (
      <div role="status" className="text-center py-12">
        <TasksEmptyIcon className="mx-auto h-12 w-12 text-muted" />
        <h3 className="mt-2 text-sm font-medium">No tasks</h3>
        <p className="mt-1 text-sm text-muted">Get started by creating a new task.</p>
        <Button className="mt-4" onClick={onCreateTask}>Create Task</Button>
      </div>
    );
  }

  return <ul role="list">...</ul>;
}
```

## Responsive Design

Design for mobile first, then expand:

```tsx
// Tailwind: mobile-first responsive
<div className="
  grid grid-cols-1      /* Mobile: single column */
  sm:grid-cols-2        /* Small: 2 columns */
  lg:grid-cols-3        /* Large: 3 columns */
  gap-4
">
```

Test at these breakpoints: 320px, 768px, 1024px, 1440px.

## Loading and Transitions

```tsx
// Skeleton loading (not spinners for content)
function TaskListSkeleton() {
  return (
    <div className="space-y-3" role="status" aria-busy="true" aria-label="Loading tasks">
      {Array.from({ length: 3 }).map((_, i) => (
        <div key={i} className="h-12 bg-muted animate-pulse rounded" />
      ))}
    </div>
  );
}

// Optimistic updates for perceived speed
function useToggleTask() {
  const queryClient = useQueryClient();

  return useMutation({
    mutationFn: toggleTask,
    onMutate: async (taskId) => {
      await queryClient.cancelQueries({ queryKey: ['tasks'] });
      const previous = queryClient.getQueryData(['tasks']);

      queryClient.setQueryData(['tasks'], (old: Task[]) =>
        old.map(t => t.id === taskId ? { ...t, done: !t.done } : t)
      );

      return { previous };
    },
    onError: (_err, _taskId, context) => {
      queryClient.setQueryData(['tasks'], context?.previous);
    },
  });
}
```

## Building UI Before the Backend Exists

When the feature follows an API contract (`../../references/parallel-frontend-backend.md`), the UI does not wait for the backend:

1. Write the runtime schema (for example `zod`) for each response from the contract.
2. Add a unit test that the schema parses every fixture example the screen uses.
3. Serve fixture examples in end-to-end tests through the shared fake-route helper. Never call the browser test API's route interception directly for the feature's paths.
4. Mark criteria that need the real backend with "Verify end to end in <E2E task>". The UI task passes when they work against fixtures.
5. Remove a fake route when the helper reports that the backend now has it.

Treat a response that fails the schema as a load error with a retry action, so a contract drift shows up as a clear error instead of a broken screen.

## See Also

For detailed accessibility requirements and testing tools, see `../../references/accessibility-checklist.md`.

For comments, UI copy punctuation and the completion check, see `../../references/house-rules.md`. UI copy follows the same punctuation rules as code: no em dash or en dash.

## Common Rationalizations

| Rationalization | Reality |
|---|---|
| "Accessibility is a nice-to-have" | It's a legal requirement in many jurisdictions and an engineering quality standard. |
| "We'll make it responsive later" | Retrofitting responsive design is 3x harder than building it from the start. |
| "The design isn't final, so I'll skip styling" | Use the design system defaults. Unstyled UI creates a broken first impression for reviewers. |
| "This is just a prototype" | Prototypes become production code. Build the foundation right. |
| "The AI aesthetic is fine for now" | It signals low quality. Use the project's actual design system from the start. |

## Red Flags

- Components with more than 200 lines (split them)
- Inline styles or arbitrary pixel values
- Missing error states, loading states, or empty states
- No keyboard navigation testing
- Color as the sole indicator of state (red/green without text or icons)
- Generic "AI look" (purple gradients, oversized cards, stock layouts)

## Verification

After building UI:

- [ ] Component renders without console errors
- [ ] All interactive elements are keyboard accessible (Tab through the page)
- [ ] Screen reader can convey the page's content and structure
- [ ] Responsive: works at 320px, 768px, 1024px, 1440px
- [ ] Loading, empty, error, success, and permission states handled when applicable
- [ ] Follows the project's design system (spacing, colors, typography)
- [ ] The rendered result passes a final UI-specific finish-gate review
- [ ] No accessibility warnings in dev tools or axe-core
- [ ] Runtime schemas parse every contract fixture the screen uses, when the feature has an API contract
- [ ] End-to-end tests install fake routes only through the shared helper