I wrote about this whole family of files in my recent newsletter: DESIGN.md, SKILL.md, SOUL.md, the markdown artifacts you write so an agent can read them. Nick Babich has the practitioner walkthrough for the DESIGN.md flavor of it, specifically the version that Google Stitch reads when it generates a screen. He describes the format directly:

DESIGN.md is a markdown file with two layers: YAML front matter that contains machine-readable design tokens (exact hex values, font properties, spacing scales) and Body that features a human-readable design rationale.

The two-layer split is right. The YAML is the part the agent can’t argue with: primary: "#d97706" is #d97706. The body is where you tell the agent why, and it has to be written like prose, not a config file. Babich’s philosophy section is where I’d point a designer who’s about to write their first one:

Unlike a traditional specification that often has very specific details that designers should follow when crafting a new design, DESIGN.md is less prescriptive in its nature. It creates a solution foundation for AI tools (colors, typography, corner radius) while providing enough freedom to alter the format for domain-specific needs. Another thing is that DESIGN.md is a living artifact, not a static config file. It should evolve as your design evolves.

The “less prescriptive” line is counterintuitive. You’d think the whole point of feeding rules to an agent is to be more prescriptive, not less. But Babich is right about the shape: pin down the tokens, leave the application loose, refine the file as the agent surfaces edge cases you didn’t think about. These files hold what we used to keep in our heads and call taste, and you don’t write taste like a requirements doc. You write it like a brief, and you keep editing it.