Determines whether DTD validation is applied to documents loaded using this
DocumentBuilder.
By default, no DTD validation takes place.
Note that with the Microsoft System.Xml parser, if no DTD validation
takes place, then ID and IDREF attributes declared in the DTD are not
reported (and are therefore not accessible to the id() and idref() functions).
Also, default and fixed values of attributes defined in the DTD will not be reported.