XML Validation

XML with correct syntax is Well Formed XML.

XML validated against a DTD is Valid XML.

"Well Formed" XML documents

A "Well Formed" XML document has correct XML syntax.

A "Well Formed" XML document is a document that conforms to the XML syntax rules that were described in the previous chapters:
  
<?xml version="1.0"?>

<note>

<to>Tove</to>

<from>Jani</from>

<heading>Reminder</heading>

<body>Don't forget me this weekend!</body>

</note> 

"Valid" XML documents

A "Valid" XML document also conforms to a DTD.

A "Valid" XML document is a "Well Formed" XML document, which also conforms to the rules of a Document Type Definition (DTD):
  
<?xml version="1.0"?>

<!DOCTYPE note SYSTEM "InternalNote.dtd">

<note>

<to>Tove</to>

<from>Jani</from>

<heading>Reminder</heading>

<body>Don't forget me this weekend!</body>

</note> 

XML DTD

A DTD defines the legal elements of an XML document.

The purpose of a DTD is to define the legal building blocks of an XML document. It defines the document structure with a list of legal elements. You can read more about DTD, and how to validate your XML documents in W3Schools' DTD School.

XML Schema

XSchema is an XML based alternative to DTD.

W3C supports an alternative to DTD called XML Schema. You can read more about XML Schema in W3Schools' Schema School.

Errors will Stop you

Errors in XML documents will stop the XML program.

The W3C XML specification states that a program should not continue to process an XML document if it finds a validation error. The reason is that XML software should be easy to write, and that all XML documents should be compatible.

With HTML it was possible to create documents with lots of errors (like when you forget an end tag). One of the main reasons that HTML browsers are so big and incompatible, is that they have their own ways to figure out what a document should look like when they encounter an HTML error.

With XML this should not be possible.