FPKit Developer

A Claude Code skill for building applications with @fpkit/acss components.

---

Purpose

This skill helps developers:

• Compose custom components from fpkit primitives
• Validate CSS variables against fpkit conventions
• Maintain accessibility when building with fpkit
• Follow best practices for component composition

This skill is for: Developers using @fpkit/acss in their applications

Not for: Developing the @fpkit/acss library itself (use

fpkit-component-builder

for that)

---

Workflow

When a user requests a new component or feature, follow this workflow:

Step 1: Analyze the Request

Understand what the user needs:

• What is the component supposed to do?
• What user interactions are required?
• Are there similar fpkit components?

Ask clarifying questions if the requirements are unclear.

---

Step 2: Check for Existing fpkit Components

Review the @fpkit/acss component catalog or Storybook:

Available components:

• Buttons: Button (with

  aria-disabled

  pattern, focus management, performance optimized)
• Links: Link (auto security for external links, ref forwarding, prefetch support)
• Cards: Card, CardHeader, CardTitle, CardContent, CardFooter
• Forms: Input, Field, FieldLabel, FieldInput
• Layout: Header, Main, Footer, Aside, Nav
• Typography: Heading, Text
• Dialogs: Dialog, Modal
• Feedback: Alert, Badge, Tag
• Data: Table, List
• Interactive: Details, Popover
• Icons: Icon library

Decision tree:

1. CHECK: Does fpkit have the exact component?
   ✓ YES → Use it directly, customize with CSS variables
           (Skip to Step 6: Accessibility)
   ✗ NO  → Continue to next check

2. CHECK: Can it be built by composing 2+ fpkit components?
   ✓ YES → Proceed to Step 3 (Composition)
   ✗ NO  → Continue to next check

3. CHECK: Can an existing fpkit component be extended/wrapped?
   ✓ YES → Proceed to Step 4 (Extension)
   ✗ NO  → Continue to next check

4. BUILD: Component requires custom implementation
   → Proceed to Step 5 (Custom Implementation)

---

Step 3: Compose from fpkit Components

Create a new component file that imports and combines fpkit components:

// components/StatusButton.tsx
import { Button, Badge } from '@fpkit/acss'

export interface StatusButtonProps extends React.ComponentProps<typeof Button> {
  status: 'active' | 'inactive' | 'pending'
}

export const StatusButton = ({ status, children, ...props }: StatusButtonProps) => {
  return (
    <Button {...props}>
      {children}
      <Badge variant={status}>{status}</Badge>
    </Button>
  )
}

Guidelines:

• Import fpkit components using named imports
• Extend fpkit component prop types with TypeScript
• Spread

  ...props

  to preserve all fpkit functionality
• Keep composition simple (≤ 3 levels deep)

Reference: See

references/composition.md

for patterns and examples

---

Step 4: Extend fpkit Components

Wrap an fpkit component to add custom behavior:

// components/LoadingButton.tsx
import { Button, type ButtonProps } from '@fpkit/acss'
import { useState } from 'react'

export interface LoadingButtonProps extends ButtonProps {
  loading?: boolean
  onClickAsync?: (e: React.MouseEvent) => Promise<void>
}

export const LoadingButton = ({
  loading,
  onClickAsync,
  children,
  ...props
}: LoadingButtonProps) => {
  const [isLoading, setIsLoading] = useState(loading)

  const handleClick = async (e: React.MouseEvent) => {
    if (onClickAsync) {
      setIsLoading(true)
      try {
        await onClickAsync(e)
      } finally {
        setIsLoading(false)
      }
    }
  }

  return (
    <Button {...props} disabled={isLoading || props.disabled} onClick={handleClick}>
      {isLoading ? 'Loading...' : children}
    </Button>
  )
}

Guidelines:

• Extend fpkit prop types (don't replace them)
• Preserve all fpkit functionality (aria-disabled, focus management, etc.)
• Add custom logic around fpkit components
• Maintain accessibility - fpkit Button automatically handles:

  • aria-disabled

    pattern for better keyboard accessibility
  • Focus management (stays in tab order when disabled)
  • Event prevention when disabled
  • Automatic className merging for

    .is-disabled

    styling

---

Step 5: Custom Implementation

If the component is truly custom and can't use fpkit:

1. Follow fpkit patterns:

   • Use semantic HTML
   • Add proper ARIA attributes
   • Support keyboard navigation
   • Use rem units (not px)
   • Define CSS variables for theming

2. Create styles (if needed):

   // components/CustomComponent.scss
   .custom-component {
     padding: var(--custom-padding, 1rem);
     border-radius: var(--custom-radius, 0.375rem);
     background: var(--custom-bg, white);
   
     // Use rem units only!
     margin-bottom: 1rem; // NOT 16px
     gap: 0.5rem; // NOT 8px
   }
   

3. Validate CSS variables (for custom styles):

   python scripts/validate_css_vars.py components/
   

Reference: See

references/css-variables.md

for naming conventions

---

Step 6: Ensure Accessibility

Check accessibility compliance:

• Uses semantic HTML (

  <button>

  ,

  <nav>

  , etc.)
• All interactive elements are keyboard accessible
• Proper ARIA attributes (

  aria-label

  ,

  aria-describedby

  , etc.)
• Visible focus indicators
• Color contrast meets WCAG AA (4.5:1 for normal text)
• Screen reader friendly (meaningful labels)

Test:

• Navigate with keyboard only (Tab, Enter, Escape)
• Use automated testing (jest-axe)
• Check Storybook a11y addon (if documenting in Storybook)

Reference: See

references/accessibility.md

for patterns

---

Step 7: Document the Component (Optional)

When creating a reusable component for a team:

Create a Storybook story:

// components/StatusButton.stories.tsx
import type { Meta, StoryObj } from '@storybook/react'
import { StatusButton } from './StatusButton'

const meta = {
  title: 'Components/StatusButton',
  component: StatusButton,
  tags: ['autodocs'],
} satisfies Meta<typeof StatusButton>

export default meta
type Story = StoryObj<typeof meta>

export const Active: Story = {
  args: {
    status: 'active',
    children: 'Server Status',
  },
}

export const Inactive: Story = {
  args: {
    status: 'inactive',
    children: 'Server Status',
  },
}

---

Reference Documentation

Quick Reference Search

When searching reference files for specific patterns, use these grep commands:

# Find composition patterns by type
grep -i "pattern.*:" references/composition.md

# Find specific composition examples
grep -i "iconbutton\|confirmbutton\|taginput" references/composition.md

# Find CSS variables by component
grep -i "\-\-btn\|--alert\|--card" references/css-variables.md

# Find CSS variable naming rules
grep -i "naming\|convention\|pattern" references/css-variables.md

# Find ARIA patterns and attributes
grep -i "aria-\|role=" references/accessibility.md

# Find WCAG requirements
grep -i "wcag\|contrast\|level aa" references/accessibility.md

# Find testing patterns
grep -i "describe\|test\|expect\|render" references/testing.md

# Find Storybook configuration
grep -i "meta\|story\|args" references/storybook.md

# Find architecture patterns
grep -i "polymorphic\|as prop\|compound" references/architecture.md

---

Composition Patterns

Read

references/composition.md

when composing components. Covers:

• Decision tree (compose vs extend vs create)
• 5 common composition patterns with code examples
• Real-world examples (IconButton, ConfirmButton, TagInput)
• Anti-patterns to avoid (nesting depth, polymorphism misuse)
• TypeScript prop type patterns

Common questions → Reference sections:

• "How do I combine multiple fpkit components?" → Patterns 1-3
• "Should I compose or extend?" → Decision Tree
• "How deep can I nest components?" → Anti-Patterns (≤3 levels)
• "How do I type extended props?" → TypeScript Support

CSS Variables

Read

references/css-variables.md

for customization guidance. Covers:

• Naming convention (

  --component-property

  )
• Discovery techniques (DevTools inspection, IDE autocomplete)
• Customization strategies (global theme, scoped overrides)
• Complete variable reference for Button, Alert, Card, and other components
• Framework integration patterns (React inline styles, CSS Modules, styled-components)

Common questions → Reference sections:

• "What CSS variables does Button support?" → Component Reference: Button
• "How do I customize globally vs per-component?" → Customization Strategies
• "How do I find available variables?" → Discovery Techniques
• "Can I use variables with styled-components?" → Framework Integration

Accessibility

Read

references/accessibility.md

for WCAG compliance guidance. Covers:

• WCAG 2.1 Level AA compliance requirements
• ARIA patterns and attributes for interactive components
• Keyboard navigation requirements (Tab, Enter, Escape, Arrow keys)
• Form accessibility (labels, error messages, validation)
• Color contrast requirements (4.5:1 normal text, 3:1 large text)
• Testing approaches (manual keyboard testing + automated jest-axe)

Common questions → Reference sections:

• "What ARIA attributes do I need?" → ARIA Patterns
• "How do I make keyboard navigation work?" → Keyboard Navigation
• "What's the minimum color contrast?" → Color Contrast (4.5:1)
• "How do I test accessibility?" → Testing with jest-axe

Architecture

Read

references/architecture.md

to understand fpkit design patterns. Covers:

• Polymorphic UI component foundation (render as different elements)
• Understanding the

  as

  prop for semantic flexibility
• Simple vs compound component patterns (when to use each)
• TypeScript support and prop type inference
• Styling architecture (data attributes for variants, CSS variables for theming)
• Props patterns and conventions (spreading, forwarding refs)

Common questions → Reference sections:

• "What is the

  as

  prop?" → Polymorphic Components
• "When should I use compound components?" → Component Patterns
• "How does prop spreading work?" → Props Patterns
• "How are variants implemented?" → Styling Architecture

Testing

Read

references/testing.md

for component testing strategies. Covers:

• Vitest and React Testing Library setup and configuration
• Testing composed components (integration vs unit tests)
• Query best practices (prefer getByRole, getByLabelText over getByTestId)
• Event testing patterns (user clicks, keyboard interactions, form submissions)
• Accessibility testing with jest-axe (automated a11y checks)
• Async testing and loading states (waitFor, findBy queries)

Common questions → Reference sections:

• "How do I test composed components?" → Testing Composed Components
• "What query should I use?" → Query Best Practices (prefer getByRole)
• "How do I test keyboard events?" → Event Testing
• "How do I test accessibility?" → jest-axe Integration

Storybook

Read

references/storybook.md

for documentation strategies. Covers:

• Storybook setup and configuration (addons, plugins)
• Story structure and patterns (CSF 3.0 format)
• Documenting composed components (props tables, descriptions)
• CSS variable customization stories (controls, args)
• Interactive testing with play functions (user-event)
• Accessibility testing in Storybook (a11y addon)

Common questions → Reference sections:

• "How do I document a composed component?" → Documenting Composed Components
• "How do I show CSS variable options?" → CSS Variable Customization
• "How do I add interactive tests?" → Play Functions
• "How do I test accessibility in Storybook?" → A11y Addon

---

Tools

CSS Variable Validation

Validate custom CSS variables against fpkit conventions:

# Validate a file
python scripts/validate_css_vars.py components/Button.scss

# Validate a directory
python scripts/validate_css_vars.py styles/

# Validate current directory
python scripts/validate_css_vars.py

What it checks:

• ✅ Naming pattern:

  --{component}-{property}

• ✅ rem units (not px)
• ✅ Approved abbreviations: bg, fs, fw, radius, gap
• ✅ Full words for: padding, margin, color, border, display, width, height

---

Examples

Quick reference examples demonstrating key patterns. For detailed implementations and advanced patterns, see

references/composition.md

.

Example: Simple Composition (StatusButton)

Combining Button + Badge to show status:

import { Button, Badge } from '@fpkit/acss'

export interface StatusButtonProps extends React.ComponentProps<typeof Button> {
  status: 'active' | 'inactive' | 'pending'
}

export const StatusButton = ({ status, children, ...props }: StatusButtonProps) => {
  return (
    <Button {...props}>
      {children}
      <Badge variant={status}>{status}</Badge>
    </Button>
  )
}

Key patterns demonstrated:

• Import fpkit components with named imports
• Extend fpkit prop types with TypeScript
• Spread

  ...props

  to preserve fpkit functionality
• Simple composition (2 components combined)

---

More Complex Examples

For production-ready implementations with state management, validation, and advanced patterns:

• IconButton (Container + Content Pattern) See

  references/composition.md

  → Pattern 1: Container Component with Content Demonstrates: Icon positioning, flexible layout, semantic structure

• ConfirmButton (Stateful Wrapper Pattern) See

  references/composition.md

  → Pattern 4: Enhanced Wrapper with State Demonstrates: Dialog integration, async actions, confirmation flows

• LoadingButton (Async State Management) See SKILL.md → Step 4: Extend fpkit Components (lines 117-156) Demonstrates: Loading states, async handlers, disabled state management

• TagInput (Compound Composition Pattern) See

  references/composition.md

  → Pattern 5: Compound Components Demonstrates: Multi-component orchestration, keyboard events, array state

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Best Practices

✅ Do

• Compose from fpkit - Start with fpkit components to inherit built-in accessibility
• Extend prop types - Use TypeScript to extend fpkit types (preserves type safety)
• Preserve accessibility - fpkit uses

  aria-disabled

  for better keyboard accessibility
• Use

  onClick

  for events - Captures keyboard, mouse, touch, and assistive tech
• Use CSS variables - Customize with variables, not hardcoded styles
• Validate CSS - Run validation script on custom styles
• Document compositions - Document fpkit components used in JSDoc
• Test integration - Test how composed components work together
• Add tooltips to disabled buttons - fpkit's

  aria-disabled

  pattern allows this!

❌ Don't

• Don't duplicate fpkit - If it exists in fpkit, use it
• Don't break accessibility - Maintain ARIA and keyboard navigation
• Don't use

  onPointerDown

  alone - It doesn't fire for keyboard users
• Don't override

  disabled

  handling - Trust fpkit's

  aria-disabled

  pattern
• Don't use px units - Always use rem
• Don't over-compose - Keep composition depth ≤ 3 levels
• Don't nest interactive elements - No

  <button>

  inside

  <a>

• Don't ignore polymorphism - Use

  as

  prop instead of wrapping
• Don't manually add

  rel="noopener"

  to external links - fpkit Link does this automatically

---

Troubleshooting

CSS Variables Not Applying

1. Check specificity - ensure the selector has equal or higher specificity
2. Check cascade order - import fpkit CSS before custom overrides
3. Check typos - variable names are case-sensitive

Component Not Keyboard Accessible

1. Ensure using semantic HTML (

   <button>

   , not

   <div>

   )
2. Add

   role

   ,

   tabIndex

   , and keyboard handlers if needed
3. Check focus indicators are visible
4. Test with Tab, Enter, Space, Escape keys

TypeScript Errors

1. Extend fpkit prop types:

   interface MyProps extends ButtonProps

2. Import types:

   import { Button, type ButtonProps } from '@fpkit/acss'

3. Spread props correctly:

   <Button {...props}>

---

Resources

• fpkit Documentation - Complete guides
• Storybook - Component examples
• npm Package - Installation and API
• GitHub - Source code and issues

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Compatible with @fpkit/acss v0.1.x

This skill is designed for applications using

@fpkit/acss

version 1.x. For version-specific documentation, check the npm package documentation in

node_modules/@fpkit/acss/docs/

.