Origins of em?

First of all, sorry for my bad English; I’m not a native speaker.

I look for historical information about origins of the term em (typographic unit of measurement).

Many sources state that it descends from the letter M, since sometime and somewhere it had exactly square dimensions. This sounds credible, but I fail to find any reliable historical evidences.

For instance, when and where was the term em used for the first time? Who has invented it?

HTML Quote, Blockquote, and Cite elements.

I hate to bring this up, but it has always bothered me about Typophile:

Quotations are the backbone of discussion, but two positively mundane HTML elements, <q> and <blockquote>, are disallowed?

Because of this, the poor <cite> element is used and abused, although it is not the most appropriate markup for quotations.

According to the W3C, "<cite>" is markup for text which "contains a citation or a reference to other sources." (HTML 4.01 Specification &sect; 9.2.1 Phrase elements)

Allowing <cite> is fine, even giving it a little semantic wiggle room, but not allowing <q> or <blockquote> is just plain silly.

Given that this is a forum for discussion, shouldn't the proper markup be allowed?