I don't think I have an html editor. Office 2007 does not seem to include one. I'd try what you suggested in an attempt to narrow things down.
But that still involves extra steps that I want to avoid for regular use. Easier to use the Search & Replace workaround.
But, like you, I'd like to know how to get this to work through Extra CSS. The Reader finds the font files just fine once the correct font name is in the body statement.
The whole problem is that darn unorthodox body tag form Book Designer. (Although I claim almost no knowledge of things html.)
My purpose is to be able to select any of several fonts I have installed on the PRS-350 when I am about to begin reading a new book. I have the Extra CSS text for each font saved in a text file. The Book Designer htm files for all books are in Calibre. So I just want to decide on a font, copy the related Extra CSS to Calibre, and quickly convert the book from the html zip file to epub. Send to Reader. Read.