Accelint Design Foundation

Expert knowledge for styling with @accelint/design-foundation and @accelint/design-toolkit — opinionated Tailwind conventions that differ from vanilla implementations.

NEVER Do When Styling with Design Foundation

NEVER use numeric spacing classes as first choice - Strongly prefer the semantic scale: p-xxs, gap-m, m-l. Numeric classes like p-4, gap-6 work with a 1:1 relationship (p-1 = 1px, NOT 4px like vanilla Tailwind), but should only be used for rare cases where implementing non-conforming designs. The semantic scale provides design system consistency.
NEVER use manual theme handling with raw color values - Avoid dark:bg-gray-900 or className={theme === 'dark' ? 'bg-black' : 'bg-white'}. Use semantic color classes like bg-surface-default and fg-primary-bold that automatically adapt to light/dark themes.
NEVER use borders for sizing elements - Use outline instead of border classes. Borders add to element dimensions (breaks layouts), while outlines overlay without affecting size. Elements should size consistently based on content and padding only.
NEVER use arbitrary Tailwind variants - Arbitrary values like hover:[&>svg]:opacity-50 break the design system. Use supported React Aria variants or conditional class rendering with clsx.
NEVER bypass CSS layers when styling components - Use @layer components.l1, @layer components.l2 for cascade hierarchy. Bypassing layers causes specificity wars and makes overrides unpredictable.
NEVER use primitive/domain tokens as first choice - Strongly prefer semantic tokens (bg-surface-default, fg-primary-bold) in components. The utility classes (bg-*, fg-*, icon-*, outline-*) provide fallback access to domain-* and primitive-* tokens for rare cases where designs go beyond the design system, but this should be exceptional. Semantic tokens provide theming flexibility and design system consistency.
NEVER use inline Tailwind classes for component styling - Component styles belong in CSS modules, not inline className props. Inline Tailwind should only be used for minor one-off overrides. Using inline classes for all component styling creates unmaintainable code and loses the benefits of CSS modules (scoping, organization, reusability).
NEVER use multiple @apply directives in a single CSS rule - Group all Tailwind classes into a single @apply statement. Multiple @apply directives prevent Tailwind IDE plugins from sorting classes and identifying issues. Write @apply bg-surface-default outline-1 outline-interactive p-m; not separate @apply lines.
NEVER use attribute selectors for variants in CSS modules - Use @variant directive blocks, not attribute selectors like [data-size="small"]. Write @variant size-small { @apply p-s; } not .button[data-size="small"] { @apply p-s; }. The @variant directive automatically applies styles when the matching data attribute is present.
NEVER use Tailwind's default theme values - The design foundation removes and replaces Tailwind defaults. Relying on default shadows, font sizes, or colors will break. Use only the semantic classes provided by the design system.
NEVER omit @reference directive in CSS modules - Every CSS module file must include @reference '#globals'; (if custom entrypoint exists) or @reference '@accelint/design-foundation/styles'; at the top. Without this, semantic tokens and @variant blocks are undefined, causing build errors.
NEVER skip PostCSS configuration - The @accelint/postcss-tailwind-css-modules plugin is required in postcss.config.mjs. Without it, named group selectors (like group-hover/button:) and @variant selectors fail to resolve in CSS modules.
NEVER import clsx directly from 'clsx' package - Always import from @accelint/design-foundation/lib/utils instead: import { clsx } from '@accelint/design-foundation/lib/utils';. The design foundation re-exports clsx with additional type support and design system integration. Importing directly bypasses these enhancements.

Before Styling a Component, Ask

Apply these tests to ensure styling aligns with the design system:

Theme Compatibility

Will this work in both light and dark themes? Use semantic color tokens that adapt automatically. Test by toggling between @variant light and @variant dark.
Am I using raw color values? If yes, replace with semantic tokens. Raw values don't theme.

Token System

Am I using the correct token type? Strongly prefer semantic tokens (bg-surface-default, fg-primary-bold). Only use domain-* or primitive-* fallbacks for exceptional cases where design goes beyond the system.
Is there a semantic token for this? Check the token catalog first. If no semantic token exists and the design genuinely requires it, fallback to domain-* or primitive-* is acceptable but rare.

Token Selection Framework

When choosing a token, follow this decision tree:

Identify element purpose - Is this a surface, text, icon, or outline?
Determine hierarchy - Primary, secondary, or tertiary emphasis?
Consider state - Default, hover, active, disabled?
Check status - Info, success, warning, danger?

Example: "I need text color for a primary heading" → Purpose: text (fg-*) → Hierarchy: primary with emphasis (primary-bold) → Result: fg-primary-bold

Example: "I need background for an interactive button in hover state" → Purpose: background (bg-*) → State: interactive hover (interactive-bold-hover) → Result: bg-interactive-bold-hover

Spacing System

Does this spacing value exist in the semantic scale? Use xxs/xs/s/m/l/xl/xxl/oversized scale. If a value isn't in the scale, question whether it's needed or use the closest semantic value.
Need a non-standard spacing value? Numeric classes (p-1, m-12) work with 1:1 relationship (p-1 = 1px, NOT 4px), but only use for rare non-conforming design cases. Semantic scale is strongly preferred for consistency.

Variant Usage

Can this be expressed with data attributes? Use data-color="info" with supported variants instead of arbitrary classes.
Am I overriding Design Toolkit components correctly? Use className or classNames props, not custom CSS files.

Layout Impact

Will outlines work here or do I need borders? Outlines work for most cases. Borders are only needed when the border must affect layout dimensions.

Styling Approach

Should this be in CSS modules or inline? Component styles belong in CSS modules (.module.css). Only use inline Tailwind classes for minor one-off overrides or adjustments.
Is this a reusable component or one-off instance? Reusable components require CSS modules. One-off instances can use inline classes for small tweaks.

Setup Verification

Is PostCSS configured correctly? Check that @accelint/postcss-tailwind-css-modules plugin is in postcss.config.mjs. Without it, named groups and @variant selectors won't work in CSS modules.
Does every CSS module have @reference? Each .module.css file must reference either '#globals' (custom entrypoint) or '@accelint/design-foundation/styles' at the top.
Is the CSS entrypoint imported first? Custom globals.css (or design-foundation/styles) must be the first import in the root layout.

Setup Requirements

CRITICAL: Design foundation requires specific PostCSS and CSS module configuration to work correctly.

PostCSS Configuration

Create or update postcss.config.mjs in project root:

export default {
  plugins: {
    '@tailwindcss/postcss': {},
    '@accelint/postcss-tailwind-css-modules': {}, // Required for CSS modules
  },
};

Why: The @accelint/postcss-tailwind-css-modules plugin fixes named group resolution in CSS module selectors (e.g., group-hover/button:) and @variant selectors. Without it, these selectors fail to resolve correctly in CSS module files.

Package.json Imports (If Custom CSS Entrypoint Exists)

If the project implements a custom CSS entrypoint for token/utility configuration:

{
  "imports": {
    "#globals": "./src/styles/globals.css"
  }
}

Purpose: Allows CSS modules to reference the custom entrypoint via @reference '#globals';

CSS Module Reference Pattern

Every CSS module file must include a reference directive:

/* If project has custom CSS entrypoint (defined in package.json imports): */
@reference '#globals';

@layer components.l1 {
  .button {
    @apply px-m py-xs;
  }
}

/* If NO custom CSS entrypoint, reference design-foundation directly: */
@reference '@accelint/design-foundation/styles';

@layer components.l1 {
  .button {
    @apply px-m py-xs;
  }
}

Why: The @reference directive imports the design system's tokens, utilities, and variant definitions. Without it, semantic tokens and @variant blocks are undefined.

Custom CSS Entrypoint (Optional)

If implementing custom tokens or utilities, create a CSS entrypoint (e.g., src/styles/globals.css):

/* Import design foundation base */
@import "@accelint/design-foundation/styles";

/* Add custom token overrides or utilities here */
@theme {
  --custom-brand-color: #ff0000;
}

Then: Import this file as the first import in your root layout component:

import './styles/globals.css'; // First import
import { ReactNode } from 'react';

export default function RootLayout({ children }: { children: ReactNode }) {
  return <html>{children}</html>;
}

How to Use

This skill uses progressive disclosure to minimize context usage:

1. Start with Core Patterns (SKILL.md)

Follow the styling patterns and token usage below for consistent implementation.

2. Reference Token Catalog (AGENTS.md)

Load AGENTS.md for quick reference of available tokens, spacing scale, and variant patterns.

3. Load Detailed References as Needed

MANDATORY loading triggers - Load these references in specific scenarios:

Setting up design foundation for the first time:

MANDATORY: Load references/setup.md (~8.9K) completely when user says "setup design foundation", "configure design foundation", "install design foundation", or encounters build errors like "undefined variable" or "@variant not found"
Do NOT Load: token-reference.md, variant-system.md, spacing-scale.md, migration-guide.md

Choosing tokens or understanding token hierarchy:

MANDATORY: Load references/token-reference.md (~6.5K) when uncertain which semantic token to use or when user needs complete token catalog
Do NOT Load: setup.md (unless build errors), migration-guide.md (unless migrating)

Working with @variant system or component variants:

MANDATORY: Load references/variant-system.md (~5.8K) when implementing data attribute variants or working with React Aria states
Do NOT Load: setup.md (unless build errors), migration-guide.md (unless migrating)

Understanding spacing scale or numeric fallbacks:

MANDATORY: Load references/spacing-scale.md (~8.5K) when confused about semantic vs numeric spacing or need complete spacing catalog
Do NOT Load: token-reference.md (unless also working with colors), setup.md (unless build errors)

Migrating from vanilla Tailwind:

MANDATORY: Load references/migration-guide.md (~4.2K) when converting existing Tailwind code to design foundation conventions
Do NOT Load: Other references unless specific issues arise after migration

Troubleshooting build errors or setup issues:

MANDATORY: Load references/setup.md completely when encountering errors like "undefined variable", "@variant not found", "group-hover/button: not working"
Do NOT Load: Other references until setup is confirmed working

Styling Patterns

CSS Modules for Component Styling

Default approach: Component styles in CSS modules with @apply directives.

user-card.module.css:

@layer components.l1 {
  /* ✅ Single @apply per rule - enables IDE plugin support */
  .card {
    @apply bg-surface-default outline-1 outline-interactive shadow-elevation-raised-muted p-m;
  }

  .header {
    @apply flex items-center justify-between mb-s;
  }

  .title {
    @apply fg-primary-bold text-body-l;
  }

  .content {
    @apply space-y-xs mb-m;
  }

  .email {
    @apply fg-primary-bold text-body-m;
  }

  .role {
    @apply fg-primary-muted text-body-s;
  }

  .actions {
    @apply flex gap-s;
  }
}

/* ❌ Wrong - multiple @apply directives break IDE plugins */
/*
.card {
  @apply bg-surface-default;
  @apply outline-1 outline-interactive;
  @apply shadow-elevation-raised-muted;
  @apply p-m;
}
*/

user-card.tsx:

import styles from './user-card.module.css';

export function UserCard({ name, email, role }) {
  return (
    <article className={styles.card}>
      <header className={styles.header}>
        <h3 className={styles.title}>{name}</h3>
      </header>
      <div className={styles.content}>
        <p className={styles.email}>{email}</p>
        <p className={styles.role}>{role}</p>
      </div>
    </article>
  );
}

Inline classes only for one-off overrides:

// ✅ Correct - CSS module + inline override for specific instance
<UserCard className="mb-xl" /> {/* One-off spacing adjustment */}

// ❌ Wrong - all styling inline
<div className="bg-surface-default outline-1 outline-interactive shadow-elevation-raised-muted p-m">
  <h3 className="fg-primary-bold text-body-l">{name}</h3>
</div>

Conditional classes with clsx:

// ✅ Correct - import clsx from design foundation
import { clsx } from '@accelint/design-foundation/lib/utils';
import styles from './Button.module.css';

export function Button({ variant, isActive }) {
  return (
    <button className={clsx(styles.button, isActive && styles.active)}>
      Click me
    </button>
  );
}

// ❌ Wrong - importing directly from clsx package
import clsx from 'clsx';

Token Categories

Background tokens (bg-*):

bg-surface-default - Primary surface (page/card background)
bg-surface-raised - Raised/elevated surface
bg-interactive-bold - Primary action background
bg-info-muted - Info message backgrounds
bg-advisory-muted - Warning message backgrounds
bg-critical-muted - Error/danger message backgrounds
bg-normal-muted - Success message backgrounds

Foreground tokens (fg-*):

fg-primary-bold - Primary text (headlines, body text)
fg-primary-muted - Subtle text (captions, secondary content)
fg-inverse-bold - Text on dark/colored backgrounds
fg-accent-primary-bold - Interactive text/links
fg-critical-bold - Error/danger text

Outline tokens (outline-*):

outline-interactive - Interactive outlines (buttons, inputs)
outline-static - Static outlines (borders, dividers)
outline-info-bold - Info state outlines
outline-advisory-bold - Warning state outlines
outline-critical-bold - Error/danger state outlines

Token fallback pattern:

// ✅ Preferred - semantic tokens
<div className="bg-surface-default fg-primary-bold">Content</div>

// ⚠️  Acceptable but rare - domain/primitive fallback for edge cases
<div className="bg-domain-brand fg-primitive-blue-500">
  {/* Use only when design exceeds system */}
</div>

// ❌ Wrong - using fallbacks when semantic tokens exist
<div className="bg-primitive-neutral-100 fg-primitive-neutral-900">
  {/* Should use bg-surface-default fg-primary-bold */}
</div>

Spacing Scale

Eight-step semantic scale: xxs → xs → s → m → l → xl → xxl → oversized

Think semantic meaning, not pixels. m means "medium for this context", not a specific pixel value.

CSS module usage:

.button {
  @apply px-m py-xs; /* ✅ Semantic spacing - preferred */
}

.card {
  @apply p-m space-y-s; /* ✅ Semantic scale for consistency */
}

/* ⚠️  Acceptable but rare - numeric fallback for edge cases */
.nonStandardLayout {
  @apply p-13; /* 13px exactly - use only for non-conforming designs */
  /* Note: p-1 = 1px (not 4px like vanilla Tailwind) */
}

Spacing fallback pattern:

Preferred: Use semantic scale (p-m, gap-s, mb-l)
Fallback: Numeric classes available with 1:1 relationship (p-1 = 1px, p-12 = 12px)
When to use numeric: Only for non-conforming designs where semantic scale doesn't fit

Outlines vs Borders

Prefer outlines - they don't affect element dimensions.

/* ✅ Correct - outline doesn't impact layout */
.card {
  @apply outline-1 outline-interactive;
}

/* ❌ Wrong - border adds to dimensions */
.card {
  @apply border-2 border-gray-300;
}

Use borders only when:

Border must participate in layout (table cells)
Dimension impact is explicitly desired

Variant System

Use data attributes for component variants:

Button.module.css:

@layer components.l1 {
  .button {
    @apply px-m py-xs;
  }
}

@layer components.l2 {
  /* ✅ Use @variant blocks for conditional styling */
  .button {
    @variant color-info {
      @apply bg-interactive-bold fg-inverse-bold;
    }

    @variant color-critical {
      @apply outline-2 outline-critical-bold fg-critical-bold;
    }

    @variant size-large {
      @apply px-l py-s;
    }
  }
}

/* ❌ Wrong - attribute selectors */
/*
.button[data-color="primary"] {
  @apply bg-interactive-bold;
}
*/

Button.tsx:

import styles from './Button.module.css';

export function Button({ children, color = 'primary', size = 'medium' }) {
  return (
    <button
      className={styles.button}
      data-color={color}
      data-size={size}
    >
      {children}
    </button>
  );
}

Common variants:

data-color: info, success, warning, danger
data-size: small, medium, large
data-state: active, disabled, loading

CSS Layer Hierarchy

Always use layers in CSS modules:

/* ✅ Correct - layered for predictable specificity */
@layer components.l1 {
  .base-styles { /* Base component styles */ }
}

@layer components.l2 {
  .variant-styles { /* Variant styles */ }
}

@layer components.l3 {
  .state-styles { /* State styles */ }
}

/* ❌ Wrong - no layers causes specificity issues */
.component {
  background: var(--bg-surface-default);
}

Layer order: l1 (base) < l2 (variants) < l3 (states)

Overriding Design Toolkit Components

Use className prop for overrides:

// ✅ Correct - override with inline class
<DesignToolkitButton className="mb-l">
  Action
</DesignToolkitButton>

// ✅ Correct - use classNames prop for internal elements
<DesignToolkitButton classNames={{ icon: 'icon-override' }}>
  Action
</DesignToolkitButton>

Common Issues

Issue: "undefined variable --bg-surface-default"

Cause: Missing @reference directive in CSS module.

Fix: Add @reference '#globals'; or @reference '@accelint/design-foundation/styles'; at the top of the CSS module file.

/* ✅ Correct */
@reference '@accelint/design-foundation/styles';

@layer components.l1 {
  .card {
    @apply bg-surface-default; /* Now defined */
  }
}

Issue: "@variant directive not recognized"

Cause: Missing @reference directive or PostCSS plugin not configured.

Fix 1: Verify @reference directive exists at top of CSS module.

Fix 2: Check postcss.config.mjs includes @accelint/postcss-tailwind-css-modules plugin:

export default {
  plugins: {
    '@tailwindcss/postcss': {},
    '@accelint/postcss-tailwind-css-modules': {}, // Add this
  },
};

Issue: "group-hover/button: selector not working"

Cause: Missing @accelint/postcss-tailwind-css-modules plugin.

Fix: Add plugin to postcss.config.mjs (see above).

Issue: Styles not applying / tokens showing as raw values

Cause: CSS entrypoint not imported or imported too late.

Fix: Ensure globals.css (or @accelint/design-foundation/styles) is the FIRST import in root layout:

// ✅ Correct order
import './styles/globals.css'; // First
import './other-styles.css';
import Component from './Component';

// ❌ Wrong order
import Component from './Component';
import './styles/globals.css'; // Too late

Issue: "Cannot find module '#globals'"

Cause: Missing package.json imports field or path incorrect.

Fix: Add imports to package.json:

{
  "imports": {
    "#globals": "./src/styles/globals.css"
  }
}

Or change @reference to direct path:

@reference './styles/globals.css';

Or use design-foundation directly:

@reference '@accelint/design-foundation/styles';

Important Notes

Tailwind defaults are removed - The design foundation resets Tailwind's default theme. Don't rely on any default values for colors, spacing, shadows, or typography. Only use what's explicitly defined in the design system.
Semantic tokens are CSS variables - All semantic tokens compile to CSS custom properties like var(--bg-surface-default). They can be used in custom CSS when utilities aren't sufficient, but prefer utilities.
Token fallback chain exists - Utility classes (bg-*, fg-*, icon-*, outline-*) check semantic tokens first, then fall back to domain-* and primitive-* tokens. This fallback is intentional for rare cases where designs exceed the design system, but semantic tokens should be the default choice for consistency and theming.
Spacing scale is semantic, not pixel-mapped - Don't think "m = 16px". Think "m = medium spacing for this context". The scale adapts to different contexts (button padding vs section margin). Numeric classes work with 1:1 relationship (p-1 = 1px, NOT 4px), but are fallbacks for non-conforming designs only.
Theme variants use @ directives - The @theme static and @variant light/dark syntax is custom. Don't confuse with standard CSS media queries. Themes are applied via class/attribute on root element.
Design Toolkit components are React Aria - They use React Aria for accessibility. Don't fight the React Aria state management or variants. Work with className and classNames props.
Setup is NOT optional - The PostCSS plugin and @reference directives are required, not conveniences. Missing setup causes cryptic build errors ("undefined variable", "@variant not found"). Always verify setup before debugging styling issues.

accelint-design-foundation