I'm no expert but I'd say that if you really want to be semantic, you should use vocabularies (RDFa).
This should result in something like that:
<em property="italic" href="http://url/to/a/definition_of_italic"> Your text </em>
em
is used for the presentation (humans will see it in italic) and the property
and href
attributes are linking to a definition of what italic is (for machines).
You should check if there's a vocabulary for that kind of thing, maybe properties already exist.
More info about RDFa here: http://www.alistapart.com/articles/introduction-to-rdfa/