@RbnJrg: Couldn’t really believe what you said about the Pocketbook, because I always found their renderer rather compatible. I own a Pocketbook Era (PB700), Firmware U700.6.8.4183.
I took an older test file and quickly put your ".bigMargin" example in, setting the "div" background to green so we can see what’s happening. As far as I can see, all is as intended. I enclose the test EPUB so you can try for yourself.
Sigil, Thorium, Foliate and the actual device show it works fine. Oh, and KOReader, too.
Just in case anyone wonders: Block element vertical margins usually
collapse, that’s why we don’t see "more margin" in the example.