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 } = 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} />;
}

// Presentation: handles rendering
export function TaskList({ tasks }: { tasks: Task[] }) {
  return (
    <ul role="list" className="divide-y">
      {tasks.map(task => <TaskItem key={task.id} task={task} />)}
    </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

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 }: { tasks: Task[] }) {
  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" 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);
    },
  });
}

See Also

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

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, error, and empty states all handled
• ] Follows the project's design system (spacing, colors, typography)
• ] No accessibility warnings in dev tools or axe-core

Limitations

• Use this skill only when the task clearly matches its upstream source and local project context.
• Verify commands, generated code, dependencies, credentials, and external service behavior before applying changes.
• Do not treat examples as a substitute for environment-specific tests, security review, or user approval for destructive or costly actions.