What

We've talked a few times about the overhead it takes to develop components twice for rails architecture. Once in primer/css and again in primer/view_components. This disconnect can lead to multiple versions and iterations happening in primer/css before any component is actually ready. ^{1 2 3 4 5 6}

This pull aims to mirror what we did for delivering web-components with PVC but for CSS. We worked on this in another PR to see if we could render a SegmentedControl #1315 to add a postcss pipeline and compile the segmented_control.css. Ideally I would like to leave the bundle in this PR empty, and add the component css in my segmented_control branch.

How

Because this is essentially a new build system, we want to be more forward thinking with the code written here and opt for using PostCSS with the preset-env plugin that allows us to write more CSS spec features like, nested selectors, match functions, container queries etc.

1. 👀 We made a PR for testing with a docs build that has SegmentedControl rendered from the PVC package. What you're seeing on that page is the component rendered with the CSS provided by the bundle.
2. 👀 We also test published a pre version from this pull request that contains an empty for now css bundle file compiled from the primer.css source.

Currently this runs on the build:css package.json script. Running a postcss-cli command. I intentionally left this pretty simple because I figured we might want to integrate this into rollup or webpack later.

In our github.com production bundle we have a few options:

1. We could include the compiled css file in our main bundle by adding a @import "@primer/view-components/app/assets/styles/primer_view_components.css" import line. This is most likely what we'll start with.
2. We could use css modules to load specific bundles for each component dynamically when the component is included on the page. This is more work, but would be worth the reward.