Wayland might be taking over from X11 on a widely used Linux desktop as early as 6 months from now (Fedora 23):
https://blogs.gnome.org/mclasen/2015/01/19/gnome-wayland-por...
Obviously X11 will not just disappear, but is targeting Servo at X11 at this early stage in it's development like writing a Python library targeting Python 2?