DESIGN.md Skill

DESIGN.md is Google's open spec (Apache-2.0, google-labs-code/design.md) for describing a visual identity to coding agents. One file combines:

• YAML front matter — machine-readable design tokens (normative values)
• Markdown body — human-readable rationale, organized into canonical sections

Tokens give exact values. Prose tells agents why those values exist and how to apply them. The CLI (npx @google/design.md) lints structure + WCAG contrast, diffs versions for regressions, and exports to Tailwind or W3C DTCG JSON.

When to use this skill

• User asks for a DESIGN.md file, design tokens, or a design system spec
• User wants consistent UI/brand across multiple projects or tools
• User pastes an existing DESIGN.md and asks to lint, diff, export, or extend it
• User asks to port a style guide into a format agents can consume
• User wants contrast / WCAG accessibility validation on their color palette

For purely visual inspiration or layout examples, use popular-web-designs instead. For process and taste when designing a one-off HTML artifact from scratch (prototype, deck, landing page, component lab), use claude-design. This skill is for the formal spec file itself.

File anatomy

---
version: alpha
name: Heritage
description: Architectural minimalism meets journalistic gravitas.
colors:
  primary: "#1A1C1E"
  secondary: "#6C7278"
  tertiary: "#B8422E"
  neutral: "#F7F5F2"
typography:
  h1:
    fontFamily: Public Sans
    fontSize: 3rem
    fontWeight: 700
    lineHeight: 1.1
    letterSpacing: "-0.02em"
  body-md:
    fontFamily: Public Sans
    fontSize: 1rem
rounded:
  sm: 4px
  md: 8px
  lg: 16px
spacing:
  sm: 8px
  md: 16px
  lg: 24px
components:
  button-primary:
    backgroundColor: "{colors.tertiary}"
    textColor: "#FFFFFF"
    rounded: "{rounded.sm}"
    padding: 12px
  button-primary-hover:
    backgroundColor: "{colors.primary}"
---

## Overview

Architectural Minimalism meets Journalistic Gravitas...

## Colors

- **Primary (#1A1C1E):** Deep ink for headlines and core text.
- **Tertiary (#B8422E):** "Boston Clay" — the sole driver for interaction.

## Typography

Public Sans for everything except small all-caps labels...

## Components

`button-primary` is the only high-emphasis action on a page...

Token types

Component property whitelist: backgroundColor, textColor, typography, rounded, padding, size, height, width. Variants (hover, active, pressed) are separate component entries with related key names (button-primary-hover), not nested.

Canonical section order

Sections are optional, but present ones MUST appear in this order. Duplicate headings reject the file.

1. Overview (alias: Brand & Style)
2. Colors
3. Typography
4. Layout (alias: Layout & Spacing)
5. Elevation & Depth (alias: Elevation)
6. Shapes
7. Components
8. Do's and Don'ts

Unknown sections are preserved, not errored. Unknown token names are accepted if the value type is valid. Unknown component properties produce a warning.

Workflow: authoring a new DESIGN.md

1. Ask the user (or infer) the brand tone, accent color, and typography direction. If they provided a site, image, or vibe, translate it to the token shape above.
2. Write DESIGN.md in their project root using write_file. Always include name: and colors:; other sections optional but encouraged.
3. Use token references ({colors.primary}) in the components: section instead of re-typing hex values. Keeps the palette single-source.
4. Lint it (see below). Fix any broken references or WCAG failures before returning.
5. If the user has an existing project, also write Tailwind or DTCG exports next to the file (tailwind.theme.json, tokens.json).

Workflow: lint / diff / export

The CLI is @google/design.md (Node). Use npx — no global install needed.

# Validate structure + token references + WCAG contrast
npx -y @google/design.md lint DESIGN.md

# Compare two versions, fail on regression (exit 1 = regression)
npx -y @google/design.md diff DESIGN.md DESIGN-v2.md

# Export to Tailwind theme JSON
npx -y @google/design.md export --format tailwind DESIGN.md > tailwind.theme.json

# Export to W3C DTCG (Design Tokens Format Module) JSON
npx -y @google/design.md export --format dtcg DESIGN.md > tokens.json

# Print the spec itself — useful when injecting into an agent prompt
npx -y @google/design.md spec --rules-only --format json

All commands accept - for stdin. lint returns exit 1 on errors. Use the --format json flag and parse the output if you need to report findings structurally.

Lint rule reference (what the 7 rules catch)

• broken-ref (error) — {colors.missing} points at a non-existent token
• duplicate-section (error) — same ## Heading appears twice
• invalid-color, invalid-dimension, invalid-typography (error)
• wcag-contrast (warning/info) — component textColor vs backgroundColor ratio against WCAG AA (4.5:1) and AAA (7:1)
• unknown-component-property (warning) — outside the whitelist above

When the user cares about accessibility, call this out explicitly in your summary — WCAG findings are the most load-bearing reason to use the CLI.

Pitfalls

• Don't nest component variants. button-primary.hover is wrong; button-primary-hover as a sibling key is right.
• Hex colors must be quoted strings. YAML will otherwise choke on # or truncate values like #1A1C1E oddly.
• Negative dimensions need quotes too. letterSpacing: -0.02em parses as a YAML flow — write letterSpacing: "-0.02em".
• Section order is enforced. If the user gives you prose in a random order, reorder it to match the canonical list before saving.
• version: alpha is the current spec version (as of Apr 2026). The spec is marked alpha — watch for breaking changes.
• Token references resolve by dotted path. {colors.primary} works; {primary} does not.

Spec source of truth

• Repo: https://github.com/google-labs-code/design.md (Apache-2.0)
• CLI: @google/design.md on npm
• License of generated DESIGN.md files: whatever the user's project uses; the spec itself is Apache-2.0.