Make selectors imported from .css files extendable

[hab25]

The problem is well explained by the question, and pentzzsolt's answer, here: https://stackoverflow.com/questions/48624359/how-to-extend-a-class-from-a-css-file-in-sass

[nex3]

This does currently work, according to my tests, in both Dart Sass and LibSass. There's not currently a spec for it, which is an oversight; I've just put out sass/sass-spec#1463 to fix that.

[hab25]

@nex3 Dart sass doesn't work for me. Here's a MRE: https://github.com/vinayluzrao/sass-issue-2743

[nex3]

When you write @import "css-importee.css", you aren't actually importing that CSS; you're writing a plain CSS import. To import a CSS stylesheet, you need to leave off the extension and write @import "css-importee".

[hab25]

Ah I see, thanks a lot for clearing it up