2.2.8 [XML] Section 3.2, Element Type Declarations
The element structure of an XML document may, for validation purposes, be constrained using element type and attribute-list declarations. An element type declaration constrains the element's content. Element type declarations often constrain which element types can appear as children of the element. At user option, an XML processor MAY issue a warning when a declaration mentions an element type for which no declaration is provided, but this is not an error.
MSXML3 and MSXML6
The following clarifications apply:
The element structure is constrained when defined in the DTD or when the validateOnParse property is set to
true; otherwise, validation does not occur.
An error is raised when an element type is used in the XML document but not defined. No error is raised if the element type is not used.