@mosho1 please add tests for this

That's a good idea. I played around with it (had to make the dev env run in windows) and it turns out the option isn't really useful outside setting it to '' on windows. I didn't want to break any existing functionality so I didn't change the default from '/' to an empty string.

The real problem, is that in css-module-loader-core/src/file-system-loader.j there is this line:

fileRelativePath = path.resolve( path.join( this.root, relativeDir ), newPath )

And for me, on windows, relativeDir (which comes from postcss's css.source.input.from) is actually an absolute path. I'm not sure how that works on other systems, but if the path.join is required on unix systems, maybe the better solution would be to detect windows and set the right value ('' rather than '/' automatically).