Document CSS being removed or force CSS to be effectful

[Timer]

We need to make a decision on #5140.

I'm leaning towards forcing *.{css,sass,scss} files to be effectful.

[edmorley]

A downside to forcing CSS to be effectful, is that it wallpapers over mis-configured packages, and means non-CRA tooling (eg Neutrino) will appear to be broken to end users unless we also adopt the workaround.

[gaearon]

I guess let's mention it in migration notes prominently, and direct to webpack/webpack#6571 for discussion.

[gaearon]

On the other hand I don't quite feel comfortable with production breakages because of that. I'd feel safer switching to new webpack behavior after webpack adds a warning or an error for this.

[Timer]

Do we know of a scenario where importing CSS is not effectful?

[gaearon]

CSS modules isn't. But @sokra said CSS itself is always effectful.

[Timer]

Right -- he made it sound like we can apply it at the loader level, which means we can apply it to .css but not .module.css.

[gaearon]

Yep. Let's do it for now and we can decide to break in a future major once they have a warning.