Fixed-Layout Books: Font Recommendation
For fixed-layout books, 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).
Read Aloud Books: CSS Styling of Media Overlays
When styling media overlays using CSS, follow these best practices:
Use the same color consistently throughout; do not change the color of the highlight unless it serves a specific purpose (for example, to distinguish a sound, such as an onomatopoeia, from the rest of the text).
Use a color that contrasts both font and background colors; the ideal color will depend on the design or background of each book.
Avoid pastels or light colors in favor of high-chroma colors.