Found an alternative that does validate:
removed the inline style and added this to CSS:
html{ font-size:1.136rem;}
Also, all the fontsizes in the CSS were in rem; changed these to em and the pages seem unchanged.
I believe that there is a difference if you have nested styles, e.g. applied by <div>s around slabs of text, but this book had none of those.
Throwing out an idea for a plugin: a tool that can multiply all font-sizes in the CSS by a stated amount; which would be cleaner than these hacks.
It is fairly common in ePubs converted from DTP that the base font size is small, which is easily adjusted in the reader, but when I switch to another book I have to reduce size.
A bit like when I play a video with the sound level much lower than normal, turn it up, and then get blasted when I play another with normal volume.
This book has numeric sizes, but others have stuff like
p {font-size: small;}
and other styles with x-small, xx-small, etc, making bumping everything up consistently a bit tricky.