XML Document Types

An XML document with correct syntax is called "Well Formed".

A "Valid" XML document must also conform to a specified document type.

Well Formed XML Documents

An XML document with correct syntax is "Well Formed".

The syntax rules were described in the previous chapters:

  • XML documents must have a root element
  • XML elements must have a closing tag
  • XML tags are case sensitive
  • XML elements must be properly nested
  • XML attribute values must be quoted
<?xml version="1.0" encoding="UTF-8"?>
<body>Don't forget me this weekend!</body>

An XML Validator

To help you check the syntax of your XML files, we have created an XML validator to syntax-check your XML.

Please see the next chapter.

Valid XML Documents

A valid XML document is not the same as a well formed XML document.

The first rule, for a valid XML document, is that it must be well formed (see previous paragraph).

The second rule is that a valid XML document must conform to a document type.

Rules that defines legal elements and attributes for XML documents are often called document definitions, or document schemas.

When to Use a Document Definition?

A document definition is the easiest way to provide a reference to the legal elements and attributes of a document.

A document definition also provides a common reference that many users (developers) can share.

A document definition provides a standardization that makes life easier.

When NOT to Use a Document Definition?

XML does not require a document definition.

When you are experimenting with XML, or when you are working with small XML files, creating document definitions may be a waste of time.

If you develop applications, wait until the specification is stable before you add a document definition. Otherwise, your software might stop working because of validation errors.

Document Definitions

There are different types of document definitions that can be used with XML:

  • The original Document Type Definition (DTD)
  • The newer, and XML based, XML Schema