nostr:npub1j46f9va5qdrmamcwmncew7g8mwekgma386dhws6k04fsnlkqpcpsj23gm7 almost

It’s two rules:

a {}
a b {}

Take a look at the spec. It has some examples that might help.

https://drafts.csswg.org/css-nesting/#example-82c7d228