This is an archived snapshot of W3C's public bugzilla bug tracker, decommissioned in April 2019. Please see the home page for more details.
It doesn't seem like anybody has a use case for them, and it would simplify the model somewhat.
Fixed by the introduction of mixins: https://github.com/heycam/webidl/commit/45e8173d40ddff8dcf81697326e094bcf8b92920