I experimented for negative margins on Kindle PW3 for a while, purely for my own use. I'd never create a book for distribution like that as you don't know if Amazon would tomorrow delete the extra margin they add in firmware.
It makes no sense on any non-Amazon app or ereader. I don't bother now. I only use my three kindles as serial numbers and for testing how Kindle book versions of epub2 might look. Or really look by a test purchase of what we upload as epub2 to Amazon KDP. I don't trust the preview "mobi" they generate.
My 5" Sony, 7" Kobo and 7.8" Boyue Mars don't need special broken margins.
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