2.2.3 [XML] Section 3.2, Element Type Declarations

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C0009:

The specification states:

 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.

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The element structure of an XML document is not constrained for validation purposes.