We’ve discussed this a bit internally, and @pmusaraj has an experimental theme component that takes advantage of this (maybe he can share it here on Meta? not sure what state it’s in).
The spec is likely a bit too early for us to adapt in Discourse by default… as far as I know it’s not even finalized yet, and we don’t support beta versions of browsers, so we’re talking about only desktop Safari 12.1 users… which is a very very small number of people when you look at overall browser share.
I do agree, it’s a very cool feature that would be nice to utilize.