Opening per action item in today’s telcon ( #8738 (comment) )

Problem

We currently desugar every nested rule using :is(), even when it’s completely redundant. E.g. this:

.foo, #bar {
	& {
		color: red;
	}
}

apparently becomes:

:is(.foo, #bar) {
	color: red;
}

This is not only redundant, it actually has observable side effects: since :is() has a flat specificity equal to the specificity of its argument with the highest specificity, the second rule is not equivalent to the first.

But even worse, because :is() cannot do pseudo-elements, it means this doesn’t work at all:

.foo::before {
	padding: 1em;
	@media (width < 500px) {
		padding: .5em;
	}
}

It would be rewritten like:

.foo::before {
	padding: 1em;
}

@media (width < 500px) {
	:is(.foo::before) {
		padding: .5em;
	}
}

which is invalid.

It also means we cannot fix #8738 by wrapping bare declarations after rules in & {} because in pseudo-elements they could be ignored.

Proposed solution

Edit: I'm not in favor of this any more, see #9492 (comment)

& {} should have no observable side effects. There is never a case where & needs :is() to be rewritten, so adding it not only introduces the problems discussed above, but is entirely redundant.

So a quick fix to the pressing issues could be to simply special case &.

Later down the line, we may want to consider expanding the set of cases that don’t require :is() because there is no danger of combinatorial explosion. From a quick call with @fantasai it seems to us that e.g. cases where there are only simple selectors all the way up would not need :is(), but more research is needed.