XML Copy Editor supports DTD, XML Schema and RELAX NG validation.
To validate against a DTD or XML Schema, the document needs to be attached to the relevant definition or schema.
DTDs are stored locally. A number of DTDs are installed by default (e.g. the public identifier ‘-//W3C/DTD XHTML 1.0 Strict//EN’ is keyed to a local copy of the XHTML 1.0 Strict DTD), but you can also add new DTDs by editing the file ‘catalog’ in the ‘catalog’ directory and saving copies of the DTDs themselves in the ‘dtd’ directory.
As RELAX NG does not define an association mechanism, you can either select one of a number of RELAX NG preset grammars (XHTML, DocBook, XSLT, etc.) or select a grammar using a file dialog. You can supply additional preset grammars by copying them to the ‘rng’ directory.