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Primer CSS Development

If you've made it this far, thank you! We appreciate your contribution, and hope that this document helps you along the way. If you have any questions or problems, don't hesitate to file an issue.


Primer CSS is published to npm as @primer/css. Each of Primer CSS's "modules" lives in a subfolder under src/ with an index.scss in it. Generally speaking, the styles are divided into three primary themes:

  • Core styles (in core/) are common dependencies, which include support variables, native element and typography styles, buttons, navigation, tooltips, etc.
  • Product styles (in product/) are specific to, and include components such as avatars, labels, markdown styles, popovers, and progress indicators.
  • Marketing styles (in marketing/) are specific to GitHub marketing efforts, including international and event-focused sites as well as the more design-heavy feature pages on Marketing styles include new colors and button styles, and extend the core typography and whitespace scales.


Here's what you need to know about how the files are structured in both git and in the published npm module:

  • In git, all of the SCSS source files live in the src/ directory.

  • When published, all of the files in src/ are "hoisted" to the package root so that you can import, say, utilities with:

    @import "@primer/css/utilities/index.scss";
  • All bundle interdependencies within Primer CSS are defined as relative imports (e.g. with ../), so everything should work fine as long as the @primer/css directory is in one of your Sass include paths (i.e. node_modules).


The typical Primer workflow looks something like this:

  1. npm install to install the development dependencies.
  2. Start Storybook
  3. Navigate to the module you're working on and modify the SCSS and/or markdown files.
  4. Test your changes in Storybook.
  5. Push your work to a new branch.
  6. Request a review from one of the Primer "core" team members.


Run npm install to install the npm dependencies.

Docs site

The Primer CSS docs are built with React using Primer Components and automatically deployed on every push to this repo using our primer/deploy action. You can run the server locally with:

npm start

Then visit http://localhost:3000/css to view the site.

🚨 Warning: Next.js has a long-running issue with trailing slashes in URLs. Avoid visiting http://localhost:3000/ if possible, as this may cause your development server to fail in less-than-graceful ways.

The pages directory

The pages directory contains all of the documentation files that map to URLs on the site. Because we host the site at (and because of the way that Now's path aliasing feature works), we nest all of our documentation under the css subdirectory.

URL tests

We have a script that catches inadvertent URL changes caused by renaming or deleting Markdown docs:

npm run test-urls

This script includes some exceptions for URLs that have been intentionally moved or removed in the process of moving away from the GitHub Style Guide, and which you will need to modify if you rename or remove either Markdown docs or their path frontmatter. See #641 for more information.


To borrow a metaphor from Brad Frost, the docs site is Primer CSS's storefront, and Storybook is its workshop.

Our Storybook setup allows you to view every HTML code block in Primer CSS's Markdown docs in isolation. To get started, run the Storybook server with:

npm run start-storybook

This should open up the site in your browser (if not, navigate to http://localhost:8001).

Code blocks

All html fenced code blocks in src/**/*.md will be rendered as stories and listed under the relevant module's name in the left-hand nav. File changes should trigger a live reload automatically (after a brief delay).

Note: At this time, we do not load any stories from src/**/stories.js.


Our package.json houses a collection of run-scripts that we use to maintain, test, build, and publish Primer CSS, notably:

  • dist runs script/dist, which creates CSS bundles of all the index.scss files in src/.
  • check-links runs a link checker on your local development server (localhost:3000, started with npm start).
  • lint lints all of our SCSS source files.
  • lint-js lints the docs site and supporting scripts.
  • now-build and now-start are run on [Now] to build and start the docs site server. now-test runs them both in order.
  • test-urls compares a (pre-generated) list of paths from the Primer Style Guide to files in pages/css, and lets us know if we've inadvertently deleted or renamed anything.
  • test-migrate tests the primer-migrate command line utility.
  • watch runs the sync script in watch mode, copying files as they're changed from src/ to pages/css/.

You can list all of the available scripts with:

npm run
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