[DNSB]

Have you tried wrapping your extra CSS for KF8/azw3 in @media amzn-kf8 { .... } selectors? The CSS will only apply to KF8 format ebooks, anything else will just skip it. For example:

@media amzn-kf8 {
   html {  
     margin-right: -47px;
     margin-left: -47px;
     break-inside: avoid !important;
   }
}