Who's defining standard DTDs? What are these x MLs anyway?
It seems like almost every kind human endeavor has some kind of group that is standardizing on DTDs that are relevant to them. By defining DTDs, they are able to exchange tagged data. The tags represent something about the semantics of the data. Often they name the resulting language defined by the standard DTD as something-ML. For example, a group defining exchange of real estate data (listings of houses, vacation properties, etc.) defined some DTDs and called the resulting language relML.
A partial list of efforts to standardize DTDs is available here.