Fonts are defined in the opf and CSS just as in flowing books. In addition to the font recommendations in Fonts, the following apply to working with fonts in fixed-layout books:
Use pixels instead of ems for the font-size.
Text placement should be specified in pixels.
Do not use fractional measurements for letter spacing, font size, and text placement. An example of a fractional measurement is 5.255px.
The ideal font-size (in pixels) is ~3% of viewportheight. For example, if <meta name="viewport" content="width=600, height=1000” />, Apple Books recommends the font-size to be 30px (1000 x .03).