Originally Posted by Tex2002ans
The most commonly used units are em, px, and % (same as em. 1em=100%, 1.5em=150%). In ebooks, using em is the recommended way.
Percents and ems are only equivalent when they're used in font sizes (or in line-height, I believe). In margins, indents, widths, etc. they are two very different things. Percent values are measured relative to the container block's width (generally), while ems are referred to the current font size.