I don't want a printer to pretend to support A4 paper if it doesn't
have and support that medium, but if there were a standard way to say
"I don't have A4 paper but if someone tries to print an A4 size page I
will shrink it to US letter size" (perhaps with a well-defined way to
do the shrinking) that would be a major win. Similarly for printing
USletter images on A4 media.
Ideally this would happen automatically, without ever causing problems
by having something shrink unexpectedly :)
Keith