All beginning tags must have an ending tag. The data is displayed in a hierarchical form. XML is not as forgiving as html. When the browser encounters something it does not understand it just stops. One mistake ruins the entire document.
Error messages contain information that is very useful when trouble-shooting problems.
The DTD can be a part of the original document or a completely separate document. One DTD can be used with an unlimited amount of XML documents. Library of Congress could make one DTD for all libraries to use for cataloging. The DTD for my page is very simple. It contains one line.