Validating vs non validating parsers with xml
An example at the end of 5.4.2 also refers to this, drawing a distinction between "validating parsers" and "backwards compatibility." " = " ; for validating parsers / " ; for backward compatibility Read literally, the spec could be interpreted to mean that a new implementation of a nonvalidating parser should actually produce "backwards-compatible" behaviour.I'm wondering whether the intent is really to suggest that the behaviour that produces " is deprecated, but that implementors should be aware of this behaviour in older implementations.A well-formed XML document may in addition be valid if it meets certain further constraints. This makes sense because following the well-formedness rules only indicates adherence to basic syntactical constraints; it says nothing about meeting the more stringent requirements of a specific structural model.To reinforce the difference, let's take another look at the Employees example from chapter 2.Because validation can significantly impact performance, especially when long and complex DTDs are involved.Some developers find that while enabling validation during development and test phases is crucial, it's sometimes beneficial to surpress validation in production systems where document throughput is most valued and the reliability of the data is already known.If the parser encounters a well-formedness problem, it should only report the problem to the calling application.It explicitly must not attempt to correct what might be missing, overlapping, or misspelled.
According to the XML 1.0 specification ( TR/REC-xml#proc-types), Validating processors must, at user option, report violations of the constraints expressed by the declarations in the DTD, and failures to fulfill the validity constraints given in this specification.
However, because it does not contain a document type declaration, the parser has no DTD to compare the document instance against in order to determine validity. Element ' Projects' is not expected here, expecting ' Title'  : Error: premature end to content of element ' Phone Numbers'.
Therefore, it is well-formed, but not valid, or at least its validity cannot be determined. Expecting child element ' Cell' This document is well-formed and valid because it matches the structure defined by the DTD, which is referenced in the document type declaration.
Ignoring for a moment the potential importance of validity to data-oriented applications, you might wonder why even when an XML document does not require a DTD (i.e., is standalone), it still must be well-formed.
In fact, if a document is not well-formed, it cannot even be called an XML document.
Consult the documentation of prospective parsers to determine how to toggle this switch, and which is the default mode.