Frontend Ui Engineering

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

  
    Tasks
  
  
    
  

// Avoid: Over-configured
}
/>

Keep components focused:

// Good: Does one thing
export function TaskItem({ task, onToggle, onDelete }: TaskItemProps) {
  return (
    
       onToggle(task.id)} />
      {task.title}
       onDelete(task.id)}>
        
      
    
  );
}

Separate data fetching from presentation:

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

  if (isLoading) return ;
  if (error) return ;
  if (tasks.length === 0) return ;

  return ;
}

// Presentation: handles rendering
export function TaskList({ tasks }: { tasks: Task[] }) {
  return (
    
      {tasks.map(task => )}
    
  );
}

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 | Production Quality | |---|---| | Purple/indigo everything | Use the project's actual color palette | | Excessive gradients | Flat or subtle gradients matching the design system | | Rounded everything (rounded-2xl) | Consistent border-radius from the design system | | Generic hero sections | Content-first layouts | | Lorem ipsum-style copy | Realistic placeholder content | | Oversized padding everywhere | Consistent spacing scale | | Stock card grids | Purpose-driven layouts | | Shadow-heavy design | 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
Click me        // ✓ Focusable by default
Click me               // ✗ Not focusable

     onKeyDown={e => e.key === 'Enter' && handleClick()}>
  Click me

ARIA Labels

// Label interactive elements that lack visible text

// Label form inputs
Email

// Or use aria-label when no visible label exists

Focus Management

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

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

  // Trap focus inside dialog when open
  return (
    
      Close
      {/* dialog content */}
    
  );
}

Meaningful Empty and Error States

// Don't show blank screens
function TaskList({ tasks }: { tasks: Task[] }) {
  if (tasks.length === 0) {
    return (
      
        
        No tasks
        Get started by creating a new task.
        Create Task
      
    );
  }

  return ...;
}

Responsive Design

Design for mobile first, then expand:

// Tailwind: mobile-first responsive

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

Loading and Transitions

// Skeleton loading (not spinners for content)
function TaskListSkeleton() {
  return (
    
      {Array.from({ length: 3 }).map((_, i) => (
        
      ))}
    
  );
}

// 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);
    },
  });
}

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

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: LLl0k0laD
• Source: LLl0k0laD/agent-skills
• License: MIT

Install and usage instructions live in the source repository linked above.