[pssquirrel]

Another option that doesn't require duplicating/renaming font files, is to use the same source file for both normal and bold.

Spoiler:

@font-face { 
   font-family: serif; 
   font-weight: normal; 
   font-style: normal; 
   src: url(res:///Data/fonts/NimbusMod.ttf); 
} 

@font-face { 
   font-family: serif; 
   font-weight: bold; 
   font-style: normal; 
   src: url(res:///Data/fonts/NimbusMod.ttf); 
} 

@font-face { 
   font-family: serif; 
   font-weight: normal; 
   font-style: italic; 
   src: url(res:///Data/fonts/NimbusMod_I.ttf); 
} 

@font-face { 
   font-family: serif; 
   font-weight: bold; 
   font-style: italic; 
   src: url(res:///Data/fonts/NimbusMod_I.ttf); 
}

Then, to distinguish between normal and bold text when there's no bold font, you could add "text-decoration: underline" However, this only works when you know what tag the epub uses to call bold. In other words, if an epub uses <span class="bold"> or <b> to call for bold-faced text, you could add:

span.bold {text-decoration: underline; }
b {text-decoration: underline; }

Then all text that's supposed to be bold (but which can't be bold because there's no font) will be underlined instead.