Nesting is a high maintenance feature for tooling (transpilers, linters, ...) and code editors.

With each new at-rule that can be nested these tools need to be updated to specifically allow both declarations and style rules.

This is different from before where allowing only style rules was a sane default for unknown at-rules.

It would be helpful if there was an authoritative and exhaustive list of all at-rules that are allowed in nesting so that maintainers do not have to read every single specification to gather the complete list.

example :
css-transitions-2 introduces @starting-style which can be nested : https://drafts.csswg.org/css-transitions-2/#example-8d9992e9

I think it would even be reasonable if only css-nesting defines which at-rules can be nested.