Google Labs open-sourced the DESIGN.md format in April 2026. It is a single plain-text file, checked into your repository, that encodes your visual identity in two zones: YAML frontmatter up top with typed tokens for colors, typography, spacing, and corner radii, and Markdown prose below explaining why those values exist and when to break them. The analogy to README.md is exact. Every engineer reads README.md first. Every agent will read DESIGN.md first.
The problem this solves is specific and costly. Ask a coding agent to build a UI component today and it returns a generic shadcn card or a gray Material button, not because it failed, but because your brand lives in Figma files and three senior designers' heads, neither of which the model can access at build time. With a DESIGN.md in the repo, the same request returns your exact hex value, your corner radius, and the reasoning behind the one-action-per-screen rule. The tokens tell the agent what. The prose tells it why. It needs both.
The article is worth reading in full because it walks through the actual construction of the file, from writing your first color token to running an audit against your live site. It also covers the eight required sections in order, shows two of Google's reference examples side by side, a pet-services app and a glassmorphism weather tool, and explains how the specification handles unknown tokens and headings. The format is open. The argument for adopting it now, before agents ship your next screen without it, is the real content.
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