Originally Posted by Linus
Em unit is the following. The unit is the width of the widest character in the font that you are using. Typically it is an M or W. The two used Em items are the Em Space and the Em dash. Both are the width of an M. Halft of the Em is called an En space or dash.
Wikipedia says that's not entirely correct; an em unit = point size
. (So 1 em for a 16-pt font is 16 pts.) It also says that "width of an M" is the former definition, because with printing presses they were the same, but with modern fonts they don't always match up.
Okay . . . again, I have never seen anyone use Em spaces to set up a margin. I would use an Em space for an endent rather than using the tab.
Apparently, it's suggested for webpages
, where what the viewer sees depends as much on what kind of filters they're using (browser, plugins, converters, etc.) as the raw html.
I suppose some ebook publishers have decided that, since ebooks are a "scrollable" medium, they should be treated exactly like webpages, and get em-based margins.