****************************************************************************** Extensions of the XML specification ****************************************************************************** ============================================================================== This document ============================================================================== This parser has some options extending the XML specification. Here, the options are explained. ============================================================================== Optional declarations instead of mandatory declarations ============================================================================== The XML spec demands that elements, notations, and attributes must be declared. However, there are sometimes situations where a different rule would be better: If there is a declaration, the actual instance of the element type, notation reference or attribute must match the pattern of the declaration; but if the declaration is missing, a reasonable default declaration should be assumed. I have an example that seems to be typical: The inclusion of HTML into a meta language. Imagine you have defined some type of "generator" or other tool working with HTML fragments, and your document contains two types of elements: The generating elements (with a name like "gen:xxx"), and the object elements which are HTML. As HTML is still evolving, you do not want to declare the HTML elements; the HTML fragments should be treated as well-formed XML fragments. In contrast to this, the elements of the generator should be declared and validated because you can more easily detect errors. The following two processing instructions can be included into the DTD: - References to unknown element types and notations no longer cause an error. The element may contain everything, but it must be still well-formed. It may have arbitrary attributes, and every attribute is treated as an #IMPLIED CDATA attribute. - References to unknown attributes inside one of the enumerated elements no longer cause an error. Such an attribute is treated as an #IMPLIED CDATA attribute. If there are several "optional-attribute-declarations" PIs, they are all interpreted (implicitly merged).