It's now 2019, and the latest draft of the CSS Nesting Module actually has something like this. Introducing @nest at-rules.
3.2. The Nesting At-Rule: @nest
While direct nesting looks nice, it is somewhat fragile. Some valid nesting selectors, like .foo &, are disallowed, and editing the selector in certain ways can make the rule invalid unexpectedly. As well, some people find the nesting challenging to distinguish visually from the surrounding declarations.
To aid in all these issues, this specification defines the @nest rule, which imposes fewer restrictions on how to validly nest style rules. Its syntax is:
@nest = @nest <selector> { <declaration-list> }The @nest rule functions identically to a style rule: it starts with a selector, and contains declarations that apply to the elements the selector matches. The only difference is that the selector used in a @nest rule must be nest-containing, which means it contains a nesting selector in it somewhere. A list of selectors is nest-containing if all of its individual complex selectors are nest-containing.
(Copy and pasted from the URL above).
Example of valid selectors under this specification:
.foo {
color: red;
@nest & > .bar {
color: blue;
}
}
/* Equivalent to:
.foo { color: red; }
.foo > .bar { color: blue; }
*/
.foo {
color: red;
@nest .parent & {
color: blue;
}
}
/* Equivalent to:
.foo { color: red; }
.parent .foo { color: blue; }
*/
.foo {
color: red;
@nest :not(&) {
color: blue;
}
}
/* Equivalent to:
.foo { color: red; }
:not(.foo) { color: blue; }
*/