This is a follow up ticket for the discussion started in xmlunit/user-guide#9

It would be nice to have an API that allowed the DTD of a document to get manipulated or to inject an internal DTD. The legacy module contains an support for replacing a DTD in https://github.com/xmlunit/xmlunit/blob/master/xmlunit-legacy/src/main/java/org/custommonkey/xmlunit/DoctypeInputStream.java

One approach could be a custom Source implementation inside the input package. I'm not convinced we'd need support inside the Input builder.