It depends what you mean by "standard". There are as many DTDs as there are types of data, and they just need to be agreed between users of the data. I don't think there can be anything like "the standard DTDs". Can you give us more of an idea of what you are looking for ?
I think each industry should be trying to standardise on a particular DTD. If they did and presented their XML files to the world which conformed to this DTD, it would be easier for agents to search through the xml files and find what they want. An example would be a mechanic looking for a certain spare part; (s)he goes to a web site which runs scripts from various motor factor web sites and returns the motor factors, and associated information, which have that particular spare part. The above scenario would only be achieved if the xml files conformed to a particular DTD. frank