I've just come across this issue and filed https://github.com/web-platform-tests/wpt/issues/22824 about what appears to be missing test coverage for some of this feature. I'm not sure my issue is correct, but hoping that tests can be added either from the Chromium side or when implementing this in Gecko.
I presume the URL here is to some sort of design document? Would it be possible to make that document public? Thanks!