3.3 Attribute-List Declarations [XML Rec 1.0]
At user option, an XML processor may issue a warning if attributes
are declared for an element type not itself declared, but this is
not an error.
3.2 Element Type Declarations [XML Rec 1.0]
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.
3.3 Attribute-List Declarations [XML Rec 1.0]
For interoperability, an XML processor may at user option issue a
warning when more than one attribute-list declaration is provided
for a given element type, or more than one attribute definition is
provided for a given attribute, but this is not an error.