Embedded means the font EXISTS inside the book.
Sigil keeps those in the Fonts sub-folder as shown in the book browser.
You can have a Font USAGE (@font) that simply relies on the CURRENT device to supply at view time. That may work in Sigil, as it has the Computers Font collection to draw from
Not Working:
'fontfamily' in the styles, must match the @font
Sigil does not care what the familyname is (some devices DO require this match the internal name of the font

) as long as they match the usage.
CASE MATTERS. use the same everyplace. Spaces in the Family require that be quoted "Has Space", serif;

I saw a book that the whole string was quoted
"Has Space, serif" ; the commas are alternate separators. they must be outside quotes