Originally Posted by JSWolf
I can understand indents shifting. But why would you want the margins shifting? In fact, just do away with the margins and you'll have a better reading experience (IMHO).
I think you might want it if you also specify the text width in em. Then the font choice will decide the size of the paper. Of course you do not want it for text on an E-book.
1 em is also the width of the letter m in the chosen font. It is not the height.