Stefan> Of course, maybe you can come up with file formats where trailing
Stefan> whitespaces are seriously problematic as well. I just can't think
Stefan> of any off the top of my head.
One seen recently -- on the groff list, IIRC -- involved continuation
lines. <\><NL> works, <\><SP><NL> does not.
That also affects C, C++, Elisp, and many other formats. The fact that we
haven't mentioned them indicates that it's not considered "seriously
problematic" enough.
Actually, font-lock can also be used in those modes to highlight such
suspicious forms: it's more useful than show-trailing-whitespace since
it can more specifically target those trailing whitespaces that are
semantically significant.