[JSWolf]

Originally Posted by Quoth

Nothing should use rem or pica

Edit:
Text, margins or padding should not use px or %
The px or % are for images on ebooks.

I agree that rem should never be used even in cases where it works because there are too many cases where it doesn't work.

As for px or % for margins or padding, there is one case where I use % for margins and it's OK that the line changes size based on the screen. I use this code for section breaks. It gives a 2px 20% wide line.

But for things like chapter headers, I see a lof of nonsense such as a 15% top margin. For places where you don't want the space to change based on the screen size, you don't want % or px.

hr {
  margin-top: 0.9em;
  margin-right: 40%;
  margin-bottom: 0.9em;
  margin-left: 40%;
  border-top: 2px solid;
}