Since this program told me that the Cochin font I like is on my system in 500 weight, I edited the code in stylesheet.css.
I changed
font-weight: normal
to
font-weight: 500
Looking towards a solution to this, please forgive me for reiterating: I did not initially specify a font weight. All I did was convert a Word docx file to epub. The only check boxes of consequence that I have checked in the conversion process are Embed all fonts in document and Subset all fonts in document.
Based on the error messages I've gotten, I am guessing most programs understand font-weight: normal to = 400.
Is that the spec?
Yes it is:
Per:
https://www.w3.org/wiki/CSS/Properties/font-weight
The keyword 'normal' is synonymous with '400', and 'bold' is synonymous with '700'.