Aims to be the canonical non-vendor-specific documentation of web features. The major browser vendors are on board. It's in "alpha" so things are a little rough.
A noble goal, to be sure, but it feels strange to me. If Mozilla is on board, why aren't the MDN docs moved over into it? Is MDN going to shut down and point everything to this? If not (likely), which are we supposed to be contribute directly to and do new contributions get ported to the other one?
I hope they have some full timers on this and they are masters of content strategy. This project will need ongoing content curation more than anything.