2.1.5 [W3C-XSS] Section 3.11.4, Identity-constraint Definition Validation Rules


The specification states:

 4 [Definition:]  Call the subset of the ·target node set· for which all the 
 {fields} evaluate to a node-set with exactly one member which is an element or 
 attribute node with a simple type the qualified node set...
 4.2 If the {identity-constraint category} is key, then all of the following 
 must be true: ...
 4.2.3 No element member of the ·key-sequence· of any member of the ·qualified 
 node set· was assessed as ·valid· by reference to an element declaration 
 whose {nillable} is true.


If an element of the key sequence of a qualified node set has a {nillable} attribute of value true, it can be valid.