XML 1.0 options

'Acceptably different results'

Skipping external entities and DTD subsets Non-validating XML parsers may include external entities and the external DTD subset, but are not required to do so.
Different constraints Throughout the specification, two different types of constraints are defined - well-formedness constraints and validity constraints. Violations of well-formedness constraints should cause fatal errors and halt processing, while validity constraint violations may only raise a warning. (In reality, many parsers halt processing on validation errors.)
All perfectly legal None of this in any way makes these parsers outlaws. It's perfectly legimitate behavior, endorsed explicitly by the XML 1.0 specification. And non-validating parsers (like AElfred) are of course free to collect those external entities and DTDs if their designers want...