Thanks for starting this :)
FYI, here's our tests: https://github.com/w3c/web-platform-tests/pull/2169
Can we get an update from the DOM team on the progress here?
William told me he's progressing on this but recently he's been helping with some more urgent Push bugs. William, do you have more detail to add?
FYI, William told me he's getting close to a set of reviewable patches here.