"em" is a unit for vertical length (height), not horizontal one (width), so we have to use HTML's features (or, maybe we could learn how HTML does it, and compute the same way)

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@36879 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e