CSS imports

See webpack-contrib/css-loader#1233 for discussion.

To build:

yarn install
yarn run build:dev

then open the dist/index.html file.

Notice that in this example, the CSS from module D is included as many different style tags because there is no option in css-loader to deduplicate the CSS imports (since version 3.3.1). In more complicated cases (such as JupyterLab), we have dozens of modules, with a common set of framework modules that nearly every package is importing. We end up with over 21,000 style DOM elements in our core application, but CSS @import deduplication brings this down to just 175 style DOM elements. Having 21,000 style DOM elements is causing our application to have 10x the start time (on the order of 20-30 seconds) and seems to slow down the application in general.

The idea here is that:

• We want to leave it up to the user how CSS gets into the browser, so we do not hardcode a css import in each module's Javascript (in this toy example, we've left the js out completely). This means that the CSS stays completely separate from the module's javascript files, which means that any dependencies in the CSS must be expressed through @import statements.
• Also, each package stands on its own, and should be easy to include in a web application by itself, so each package's CSS file @imports its dependency's CSS files

Our system basically sets up a dependency graph between css files using @import that mirrors the dependency graph of the actual packages. An application that wants to include any module just has to get the main CSS file for that module onto the page, and any required dependency CSS will automatically get loaded through the CSS @import dependency graph.

Note that if we threw away our first requirement above, and had each module's main js file include its main CSS file, the webpack module dependency deduplication would do the deduplication we want (e.g., module D's css would be on the page once, since module D's javascript is executed once). However, this would require that the module must be used with a bundler that understands css files, and we'd rather not make that assumption hardcoded in the module.

While CSS in general has issues with ordering and the global namespace, in this restricted scenario where we are mimicking the package dependency graph and curating our CSS to not have conflicts, it would be really helpful to have deduplicating.