Building a component library is hard, and a big part of that comes from the tooling needed to make it work.

If your component library has 100 components, but the project you're working on only needs 3 or 4 of them, should you have to import the JavaScript and CSS for all of it? No.

This is an example project with two components:

• component-library: a toy component library with two atomic components: Button and Checkbox. It uses TypeScript, React, and CSS Modules, and it's compiled into tree-shakable ESM, with TypeScript definitions and source maps in the distribution.
• example-consumer: A minimal React app, compiled with Webpack, and configured so that only the components you use end up in your production bundle.

.
├── global.d.ts # Default declaration for CSS files. (typed-css-modules can be used instead of this.)
├── package.json # contains babelrc, browserslist, peerDependencies, sideEffects and module configuration.
├── rollup.config.js #
├── src
│   ├── index.ts # Entrypoint. Exports all modules. Can be tree-shaken.
│   ├── base.css # Base CSS, includes a reset (modern-css-reset), and color variables.
│   │── variables
│   │   └── colors.css # CSS variable example, sets colors on :root.
│   ├── components
│   │   └── atoms
│   │       ├── Button
│   │       │   ├── Button.module.css
│   │       │   └── Button.tsx
│   │       └── Checkbox
│   │           ├── Checkbox.module.css
│   │           └── Checkbox.tsx
├── tsconfig.json # TypeScript configuration, only emits declaration files

Usually, a component library would be built and the dist folder released in an NPM module. But in the interest of demonstration, both projects can be compiled and linked using the script below.

cd component-library
npm i
npm run build
npm link
cd -

cd example-consumer
npm i
npm link component-library
cd -

Run the consumer example with npm start. Build a production bundle with npm run build -- --mode=production. To debug what's in the bundle, use npm run build and look at the output.

I had to fork rollup-plugin-postcss because it doesn't respect the preserveModules option.

rollup-plugin-postcss is in search of a new maintainer, so it may be a while before any PRs get merged.