The xml-common package serves the following purpose:
If you are not personally going to be doing development in the various XML packages (such as the xml-keyval package), then you can consider yourself an end user of the product. From the end user perspective, this package only provides documentation (so you won't need to install anything). You will, however, want to review the most helpful End User's Guide section for some general tips on the requirements and use of the other XML packages which are derived works.
After spending some time going over the details on involved in preparing a system for processing XML documents, I'm considering adding the following two utlities to this package (which means you will probably want to install this package in the future):
A script which checks the user's system to see if it is properly configured for processing XML documents (offering suggestions and the ability to configure as well).
Once you have a system setup for XML processing, it doesn't take much to create a script to translate DocBook.org style documents to HTML.
If you are a developer of XML software, you will want a better understanding of the foundation which this package provides. To keep the size of this document to a minimum, the Developer's Guide has its own manual.