For twenty years the brand guidelines PDF had one reliable reader: the designer in their first week on the job. After that it lived in a shared drive and got opened mostly during arguments.
That reader has been replaced. In April, Google Labs open-sourced DESIGN.md, a format its Stitch design tool had been using for roughly a year. The public repository, google-labs-code/design.md, is Apache 2.0 licensed and still marked alpha. It describes one markdown file that sits at the root of a project. YAML front matter at the top holds the tokens: colors, type sizes, spacing, corner radii, component properties. Ordinary markdown sections below explain how to apply them, in a fixed order starting with Overview, Colors, Typography and Components. A command-line tool lints the file, checks token references and WCAG contrast, diffs two versions, and exports to Tailwind or the W3C design token format.
A coding agent reads that file on every task, top to bottom, which is more attention than any brand book has received from a human since the launch presentation.
The split inside the file is where most teams will get it wrong. Tokens are the part everyone already has. Any working design system has hex values and a type scale sitting in Figma or a JSON export, and converting them takes an afternoon. The prose sections ask for something most brands never produced, which is the reasoning: why the accent color appears once per screen, why the display face never drops below a certain size, which rule bends for a campaign and which never bends. When an agent hits a decision the tokens do not cover, and it hits one on nearly every task, that prose is the only brand judgment it has.
My own system runs on this split: tokens as data, components as code, prose reserved for judgment calls. The prose layer is the one that keeps growing. Every time an agent makes a defensible choice I would never make, the fix turns out to be a sentence explaining a rule I had enforced for years and never written down, usually one I would have sworn was obvious to anyone who had looked at the work for five minutes.
Ownership is the governance question hiding in the file format. If DESIGN.md lives in the engineering repo, then whoever approves pull requests now approves brand changes, and most companies will make that decision by accident. The diff command helps here. It reports token-level changes between versions, so a shifted brand color shows up in code review, where someone can be asked to justify it before it ships to every screen at once.
The limits are real and I do not have answers for most of them. The spec may change before it settles. The linter can tell you a color pair fails contrast, and it has no opinion about whether a layout feels cheap. Nobody I know has a credible way to lint tone of voice, and some part of every brand will keep living in a person who is willing to say no in a meeting.
Write the reasons down anyway. The machine is the first reader that will follow them exactly, including the parts you got wrong.