I’ll criticize it.

I recognize this is a preview and I desperately hope this implementation isn’t kept around and treated as a quirk.

This implementation is extremely unintuitive given their explanation of the expected behavior of CSS Nesting and the & symbol.

To quote:

    The & signals to the browser “this is where I want the selector from outside this nest to go”.

Their explanation and the actual implementation result in a majorly different CSS selector.

The implemented functionality, however useful, makes no sense as a default if one can explicitly use :is to achieve this behavior like below.

    .foo .bar {
        .baz :is(&) {
        }
    }

The default should behave like they claim it does; simply replace & with the “outside” selector.