If the parameter isId is true, this method declares the specified
attribute to be a user-determined ID attribute. This affects the value of
Attr.isId and the behavior of Document.getElementById, but does not
change any schema that may be in use, in particular this does not affect
the Attr.schemaTypeInfo of the specified Attr node. Use the value false
for the parameter isId to undeclare an attribute for being a user-determined ID attribute.
If the parameter isId is true, this method declares the specified attribute to be a user-determined ID attribute. This affects the value of Attr.isId and the behavior of Document.getElementById, but does not change any schema that may be in use, in particular this does not affect the Attr.schemaTypeInfo of the specified Attr node. Use the value false for the parameter isId to undeclare an attribute for being a user-determined ID attribute.