To clarify a bit further than what the other comments said (abc.localhost. isn't implemented on macOS): The RFC draft is talking about domains ending in ".loca

To clarify a bit further than what the other comments said (abc.localhost. isn't implemented on macOS): The RFC draft is talking about domains ending in ".localhost.", not any domain containing ".localhost." (so abc.localhost.example.com wouldn't be localhost). All domains have to end with a period, or they're not fully qualified. "example.com" could mean "example.com.my.domain." if whatever you're using uses "my.domain." as the root. But "example.com." is always "example.com." Nowadays that feature isn't used very often, as the root of domains not ending in a period is usually assumed to be in the root zone.