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I showed johnsonm the output of xmlto man foo.xml, and he suggested that you line wrap long lines inside <para>s so all viewers can display them properly.
The reason it doesn't do this at the moment (except for it being 'out of the way') is that it would require the stylesheet to be quite smart about it. I don't know groff well enough to know what things at the beginning of the line are 'special'. As it is, we don't escape things like '\' in a para, which we ought to. I'd rather not have to go hunting for '.' and whatever else as well if I can help it. :-) Which viewers in particular don't display these long lines properly?
Closing, no input.