design-tokens

What this skill does

# Design Tokens

Generate type scales, color palettes, spacing systems, and dark mode derivations with math — not guessing. Includes WCAG contrast checking, systematic spacing grids, and production-ready CSS/Tailwind output.

## When to Use

- User is setting up a new project's design system
- User asks for a type scale, color palette, or spacing system
- User needs WCAG-compliant color combinations
- User wants dark mode colors derived from a light palette
- User asks for "design tokens" or "theme setup"
- Building a Tailwind config or CSS custom properties

## Core Philosophy

- **Math over taste.** Scales should follow ratios, not arbitrary values.
- **Accessibility by default.** Every text/background combo must pass WCAG AA.
- **Systematic.** Every value should be derivable from a base + ratio.
- **Portable.** Output as CSS custom properties, Tailwind config, or JSON tokens.

---

## Type Scale Generation

### The Formula

```
fontSize = baseFontSize × ratio^step
```

### Recommended Ratios

| Ratio | Name | Value | Best for |
|-------|------|-------|----------|
| Minor Second | 1.067 | Tight, minimal difference | Dense UI, dashboards |
| Major Second | 1.125 | Subtle progression | Apps, data-heavy interfaces |
| Minor Third | 1.200 | Balanced, versatile | Most websites, SaaS |
| Major Third | 1.250 | Clear hierarchy | Marketing sites, blogs |
| Perfect Fourth | 1.333 | Strong contrast | Editorial, landing pages |
| Augmented Fourth | 1.414 | Dramatic | Bold designs, portfolios |
| Perfect Fifth | 1.500 | Very dramatic | Hero-heavy designs |

### Generating the Scale

Given a base size (typically 16px) and a ratio:

```
Step -2: 16 / ratio² = xs
Step -1: 16 / ratio  = sm
Step  0: 16          = base
Step  1: 16 × ratio  = lg
Step  2: 16 × ratio² = xl
Step  3: 16 × ratio³ = 2xl
Step  4: 16 × ratio⁴ = 3xl
Step  5: 16 × ratio⁵ = 4xl
Step  6: 16 × ratio⁶ = 5xl
```

Round to nearest 0.5px or convert to rem (÷ 16).

### Line Height Rules

| Font Size | Line Height | Use |
|-----------|-------------|-----|
| ≤ 14px | 1.6–1.7 | Small text, captions |
| 16–20px | 1.5–1.6 | Body text |
| 20–32px | 1.3–1.4 | Subheadings |
| 32–48px | 1.1–1.2 | Headings |
| 48px+ | 1.0–1.1 | Display/hero text |

**Rule of thumb:** As font size increases, line height decreases.

### Letter Spacing Rules

| Size | Letter Spacing | Why |
|------|---------------|-----|
| Small text (≤14px) | `0.01–0.02em` | Slightly open for readability |
| Body text | `0em` (normal) | Don't touch it |
| Subheadings | `-0.01em` | Slightly tighten |
| Headings | `-0.02em` to `-0.03em` | Tighten as size grows |
| Display text | `-0.03em` to `-0.05em` | Tight tracking at large sizes |

### Font Weight Pairing

| Role | Weight | Tailwind |
|------|--------|----------|
| Body | 400 (Regular) | `font-normal` |
| Body emphasis | 500 (Medium) | `font-medium` |
| Subheading | 600 (Semibold) | `font-semibold` |
| Heading | 700 (Bold) | `font-bold` |
| Display | 800 (Extrabold) | `font-extrabold` |

### Output Example (CSS Custom Properties)

```css
:root {
  --font-size-xs: 0.694rem;    /* 11.1px */
  --font-size-sm: 0.833rem;    /* 13.3px */
  --font-size-base: 1rem;      /* 16px */
  --font-size-lg: 1.2rem;      /* 19.2px */
  --font-size-xl: 1.44rem;     /* 23px */
  --font-size-2xl: 1.728rem;   /* 27.6px */
  --font-size-3xl: 2.074rem;   /* 33.2px */
  --font-size-4xl: 2.488rem;   /* 39.8px */
  --font-size-5xl: 2.986rem;   /* 47.8px */
}
```

---

## Color Palette Generation

### Step 1: Choose Base Colors

Every palette needs:

| Token | Purpose | Example |
|-------|---------|---------|
| `primary` | Main brand color, CTAs | Your brand color |
| `neutral` | Text, borders, backgrounds | Gray (warm/cool/pure) |
| `success` | Positive states | Green |
| `warning` | Caution states | Amber/yellow |
| `error` | Destructive states | Red |
| `info` | Informational | Blue (can overlap primary) |

### Step 2: Generate Shade Scale (50–950)

For each base color, generate a 10-step shade scale. The base color is typically the 500 step.

**Method: HSL manipulation**

Starting from the base HSL:

| Step | Lightness Adjustment | Saturation Adjustment |
|------|---------------------|----------------------|
| 50 | +45% | -30% |
| 100 | +38% | -25% |
| 200 | +28% | -15% |
| 300 | +18% | -5% |
| 400 | +8% | 0% |
| 500 | 0% (base) | 0% (base) |
| 600 | -8% | +5% |
| 700 | -18% | +5% |
| 800 | -28% | 0% |
| 900 | -38% | -10% |
| 950 | -45% | -20% |

Clamp all values: lightness 0–100%, saturation 0–100%.

### Step 3: Semantic Token Mapping

Map shade steps to semantic roles:

```css
/* Light mode */
--color-bg: var(--neutral-50);
--color-bg-subtle: var(--neutral-100);
--color-bg-muted: var(--neutral-200);
--color-border: var(--neutral-200);
--color-border-strong: var(--neutral-300);
--color-text-muted: var(--neutral-500);
--color-text-subtle: var(--neutral-600);
--color-text: var(--neutral-900);
--color-text-heading: var(--neutral-950);

--color-primary: var(--primary-600);
--color-primary-hover: var(--primary-700);
--color-primary-bg: var(--primary-50);
--color-primary-text: white;
```

---

## WCAG Contrast Checking

### The Formula

Contrast ratio = (L1 + 0.05) / (L2 + 0.05)

Where L1 = lighter relative luminance, L2 = darker.

### Relative Luminance

```
For each channel (R, G, B):
  sRGB = channel / 255
  linear = sRGB <= 0.04045 ? sRGB / 12.92 : ((sRGB + 0.055) / 1.055) ^ 2.4

L = 0.2126 × R_linear + 0.7152 × G_linear + 0.0722 × B_linear
```

### WCAG Requirements

| Level | Ratio | Applies to |
|-------|-------|-----------|
| AA Normal Text | ≥ 4.5:1 | Body text (< 18px or < 14px bold) |
| AA Large Text | ≥ 3:1 | ≥ 18px regular or ≥ 14px bold |
| AAA Normal Text | ≥ 7:1 | Enhanced accessibility |
| AA UI Components | ≥ 3:1 | Borders, icons, focus rings |

### Quick Reference (Neutral on White #FFFFFF)

| Shade | Approx Contrast | Passes |
|-------|-----------------|--------|
| 300 | ~2.5:1 | ❌ Decorative only |
| 400 | ~3.5:1 | ✅ Large text, UI |
| 500 | ~4.5:1 | ✅ AA body text |
| 600 | ~6:1 | ✅ AA comfortable |
| 700 | ~8:1 | ✅ AAA body text |

### Checking Contrast Programmatically

When generating palettes, always verify:
1. `text` (900) on `bg` (50) → must be ≥ 4.5:1
2. `text-muted` (500) on `bg` (50) → must be ≥ 4.5:1
3. `primary` (600) on `white` → must be ≥ 4.5:1
4. `primary-text` on `primary` (600) → must be ≥ 4.5:1
5. `border` (200) on `bg` (50) → must be ≥ 3:1

If a combo fails, adjust the darker color one step darker until it passes.

---

## Spacing System

### Base-4 Scale (Recommended)

Everything is a multiple of 4px. Predictable, consistent, works with most font sizes.

```css
:root {
  --space-0: 0px;
  --space-0.5: 2px;
  --space-1: 4px;
  --space-1.5: 6px;
  --space-2: 8px;
  --space-3: 12px;
  --space-4: 16px;
  --space-5: 20px;
  --space-6: 24px;
  --space-8: 32px;
  --space-10: 40px;
  --space-12: 48px;
  --space-16: 64px;
  --space-20: 80px;
  --space-24: 96px;
  --space-32: 128px;
}
```

This matches Tailwind's default scale exactly.

### Spacing Usage Guide

| Context | Spacing | Values |
|---------|---------|--------|
| Inline icon gap | space-1 to space-2 | 4–8px |
| Button padding | space-2 × space-4 | 8px 16px |
| Card padding | space-4 to space-6 | 16–24px |
| Section gap (between elements) | space-6 to space-8 | 24–32px |
| Section padding (container) | space-12 to space-16 | 48–64px |
| Page section vertical rhythm | space-16 to space-24 | 64–96px |

### The Container Width Scale

| Token | Width | Use |
|-------|-------|-----|
| `sm` | 640px | Narrow content (auth forms) |
| `md` | 768px | Blog posts, documentation |
| `lg` | 1024px | App layouts |
| `xl` | 1280px | Wide layouts |
| `2xl` | 1536px | Full-width dashboards |

---

## Dark Mode Derivation

### The Inversion Pattern

Don't manually pick dark mode colors. Derive them systematically:

```
Light mode          →  Dark mode
neutral-50  (bg)    →  neutral-950 (bg)
neutral-100 (bg-subtle) → neutral-900 (bg-subtle)
neutral-2