@MrWarper
Thanks for advice.
Im affraid you missed my original post.
I created the ePub from RTF file using Calibre automatic conversion.
Additionaly, The conversion process there, when using custom settings, has an extensive settings, but I didnt find any that would let me disable/control the way Calibre adds the font specification in the output ePub.
Removing the font (specification ?) tag did somehow solve the problem.
But I (as software dev) can imagine how Calibre, iRex reader FW and ePub are all very compliacted systems, and I have little understanding of any of them, so Im still far from being sure what caused what and why, if you understand me.
If you have a good knowledge of any of those, you and try help me decipher the mess