That's a bit confusing to me, I don't understand. I think that type of tagging can be ambiguous. Just to suggest a better construct:

<tag type="tag" class="tag" purpose="tag" tag_subtype="empty" description="this is a emptytag, a subtype of tag" empty="true"></tag>

Now, that's not perfect, I would even describe it as minimalist, but I hope it sets you in the right direction!