frontend-ui-engineering

SKILL.md

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:

// 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:

// 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:

// 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:

/* 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

// 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

// 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

// 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

// 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:

// 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

// 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