The following video is from an intensive (albeit introductory) course that I’ve been teaching on web technologies — XML, XHTML and CSS. It discusses how to validate the structure of an XML file using DocType definitions (DTDs).
DocType definitions are sets of instructions that tell a document parser whether a particular document is well formed or not. This allows for the parser to recognize potential errors.
I’m posting the video here in the hope that it will be helpful to a larger audience than just those in the course. After all, DocTypes are an extremely useful tool.