an <abbr> can be marked as a abbreviation without providing the long form via a title, if you don't know the long form but want to note it as an abbr, or if the

an <abbr> can be marked as a abbreviation without providing the long form via a title, if you don't know the long form but want to note it as an abbr, or if the abbr is titled previously and you're reusing the same abbr multiple times in the same context. not sure about <dfn> without a title though.