>> Initially? The Wayland dev are still for CSD.
> IIRC, implementation of wayland in kwin used server-side decorations from the very beginning.
Sure but KWin isn't made by Wayland developers, it use Wayland which is different, Weston made in the Wayland project is still CSD only: the Wayland protocol supports both type of design.
> But wayland compositor can't do even that.
Weston cannot do it currently which isn't a big issue: the main issue is that if this 'decoration negotiation' part isn't part of the basic protocol (I don't know if it is or not: I haven't checked) then this will make interoperability complicated.