[2] The 'em' unit is equal to the computed value of the 'font-size' property of the element on which it is used. The exception is when 'em' occurs in the value of the 'font-size' property itself, in which case it refers to the font size of the parent element. It may be used for vertical or horizontal measurement.
I'm the author of the article (the reponse not the original). I didn't say em is based on "m". All I said about em is:
"(For completeness, I should mention here that the "em" and "ex" units are exceptions to the above. The lengths 1em and 1ex do vary relative to the other units, because they depend on the font in use.)"
I'll update this to say "font size" as I didn't mean to give the impression that it was dependent the choice of font.
[1] http://stackoverflow.com/a/4531441/1971539
[2] The 'em' unit is equal to the computed value of the 'font-size' property of the element on which it is used. The exception is when 'em' occurs in the value of the 'font-size' property itself, in which case it refers to the font size of the parent element. It may be used for vertical or horizontal measurement.