there is no mention of the "commonly used" X-Device-* in checkpoint
3.6.1's "how to do it" section, which I think is an editorial omission it
would be helpful to fix.
(I'm not wild about the suggestion to select on the server side by
preference, but since it recognises taht this is in many cases just
hopeful, I can live with it I guess, since it doesn't mandate people to
detect capabilities they don't care about).