I don't know about KFX, but for KF8, I add a media query so that it doesn't affect epub renderering engines.
Code:
@media amzn-kf8 {
html {margin-left: -36px; margin-right: -36px; }
}
Also, more specific margin-left and margin-right rules will override the above. So you need to look for and revise/delete things like:
Code:
p.para {margin-left: 0; margin-right: 0}
p.txt {margin:0}
This can get tedious.
To normalize line-height, I delete most, if not all, line-height rules to allow the Kindle to set the line-height per user settings.
ETA: Updated this post using <html> instead of <p>. When I first started playing with negative margins, <p> was the only thing that worked for me, but now <html> is working, not sure what changed. But I'm happy about it!