Wayland is a replacement for the X11 window system protocol and architecture with the aim to be easier to develop, extend, and maintain.
A simplified way of looking at it is that Wayland is a decentralised approach versus the X centralised approach. As such, the dev-libs/wayland ebuild provides methods and functions for writing and managing various components.
One main aspect of Wayland implementation is the Wayland Compositor (display server), and a list of them can be found at Wikipedia List of Wayland compositors.
For clients that do not natively display in Wayland, there is Xwayland, an Xorg server running on top of the wayland server.
How does one start Wayland?
Starting a Wayland session equates to starting the relevant Wayland Compositor. For GNOME, this is usually done via the display manager GDM. Instructions can be found at [GNOME Optional Wayland Support].
For KDE, this is usually done via the display manager SDDM.
What graphics cards are supported?
Support for GPUs is via Mesa (either GBM (Generic Buffer Management) or EGL (Embedded-System Graphics Library)). Using the Funtoo [graphics mix-ins] should work (to be tested).
How does one see if it's working?
The simplest way to verify your session type is via
KDE kwin has a debug console that shows whether each window is displaying natively in Wayland or using xwayland:
qdbus org.kde.KWin /KWin org.kde.KWin.showDebugConsole
What is and isn't supported compared to X?
Tips for running specific applications in Wayland mode can be found at Gentoo wiki.