@only directive

[meefox]

Idea is to define a block which will be used in particular selector if rule is true.
And all other properties in whole selector should be skipped.
As a simple example:
main.scss

body {
    background: #000;
    @only $forIE8 {
        background: #FFF;
    }
    @only $forIE9 {
        background: #CCC;
    }
}

ie8.scss

$forIE8: true;
@import "main";

ie9.scss

$forIE9: true;
@import "main";

all.scss

$forIE8: false;
@import "main";

Output:
ie8.css

body {
  background: #FFF;
}

ie9.css

body {
  background: #CCC;
}

all.css

body {
  background: #000;
}

[lolmaus]

Why aren't you using @if?

[cimmanon]

Probably because he doesn't want to have to write an else.

body {
    @if $forIE8 {
        background: #FFF;
    } @else if $forIE9 {
        background: #CCC;
    } @else {
        background: #000;
    }
}

It is worth noting that Sass doesn't eliminate duplicate property declarations. If it did, we would have problems creating simple fallbacks like this:

.foo {
    background: blue;
    background: rgba(blue, .5);
}

[meefox]

This is different :)
Because more important part is content above of @only

body {
    background: #000; <---
    @only {...
}

This should be a part of all.css and should't be in ie.css

So with @only you could avoid this necessary @if in each selectors when you don't have IE fixes.

h1 {
    @if $modern {
        color: #DDD;
    }
}

Main point is to have something which will give possibility to split one selector between files.

[Snugug]

@meefox if that is indeed the expected result, this sounds like it's related to, if not a duplicate of, #241

[lolmaus]

Also this: #116 (comment)

[nex3]

@if and @else are sufficient for this use case.

[wheeyls]

FWIW, I thought this directive would be helpful as well. I've written a postcss plugin based on this syntax here: https://github.com/wheeyls/postcss-only-directive.