I have no idea, it's not something important to me. They respect the exact same CSS that I force every book to have manually in Calibre and that's all I care about. I don't use overrides because I already destructively fixed the original file
(I know, I know, but overrides weren't a thing on Books and I got used to it.)
(For reading on a small screen, I prefer to optimize the CSS to use the whole width of the screen at all times, skip lines between paragraphs and to have no embedded font.)
I never actually got foliate to work for me unfortunately. I can test kybooks3 for CSS behavior if I knew what to look for.