Anyway, I've used negative margins on KF8 and for one KFX and I did not notice any issue with the KFX.
Code:
html {
margin-right: -45px;
margin-left: -45px;
}
That's for a 300dpi Kindle.
I'll give it another try and double check the results.
Also, what version Kindle Previewer are you using? Is it the current version (3.46)?