Many thanks, kiwidude.
Originally Posted by kiwidude
...leaving such orphaned declarations in the css file does absolutely no harm, and the reader will just fall back to using its default font which is what people wanted.
The only downside is that calibre removes these font declarations from the css and puts them in every html/xhtml file when doing a epub -> epub conversion - I don't know why calibre does this. So I usually open the css file after removing the fonts and delete the font declarations.