• Proposal
• Dart Sass (Add support for nesting in plain CSS dart-sass#2198)
• Tests (Add specs for plain CSS nesting sass-spec#1963)
• Documentation is probably not necessary, since this is covered by the general expectation that plain CSS parsing supports all plain CSS features
• Update canonical spec (Flush plain CSS nesting changes to the spec #3841)

Although we're preserving Sass's current nesting behavior for now (per #3030), we can support native CSS nesting when parsing plain CSS. We shouldn't do any resolution here, just pass it through as-is.