attribute attribute-name element-qualifier* (is | isnt ) defaulted. 

An operator for testing where the value of an attribute is set. It is always available, and no error is generated if the specified attribute doesn't exist or was not given a value.

The test succeeds when:

• an element attribute has a default value given in the element declaration, but has no value set in the start tag of the element.
• a data attribute has a default value given in the declaration for the external entity's notation, but has no value set in the entity declaration.