Took a look at the coding and I must admit I find it a real mess (not your fault, Dave, but the program's). It uses CSS but then puts single cell tables inside DIV tags, which is bad practice, and also uses unclosed BR tags (i.e.
instead of the correct
).

The fonts are all defined at the top, and are pretty standard (Verdana, Times New Roman, and Arial), but no alternatives are specified, also bad practice.

Not really surprising you get odd results on some browser/OS combinations. Hard thing to check completely, but next time you might want to try a different generator.