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I'm still working out what to do if no argument is specified... It may be an idea to do nothing visible and/or insert an html comment with "unknown markup fooasdfbar".
Sort of Off-Topic: I have a little pet peeve when "Unknown Markup" is displayed on the page (because I'm usually updating the site while it's live) though I know that any typo resulting in that error is 100% my fault. A comment instead of displaying the error would be kinder on my sanity