2.1.5 [W3C-XSS] Section 3.11.4, Identity-constraint Definition Validation Rules
V0006:
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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.
MSXML6
If an element of the key sequence of a qualified node set
has a {nillable} attribute of value true,
it can be valid.