XML declaration

One thing I forgot about a well-formed document, is that it should have an XML declaration.

• An XML declaration is not mandatory
• It have 3 attributes that have a specific order
• version - Must always be included, currently the only possible value is "1.0"
  
• encoding - Specifies the character set use in the document, defaults to "UTF-8"
  
• standalone - Specifies if the document relys on extanal documents, such as a DTD or XML schema. Defaults to "no"
  
• Must start at the first character of the XML document

Example of XML declaration
<?xml version="1.0" encoding="iso-8859-1" standalone="no"?>

Valid

A XML document is valid when it

• conforms to a DTD or a XML Schema
• is well-formed

Well-formed

A well-formed document conforms to XML's syntax rules.

• XML tags is case sensitive
• All XML elements need a matching end tag
• All XML elements need to be correctly nested
• An XML document needs a root element
• All attributes should be qouted

Well-formed and valid

A XML document should be well-formed and valid.

• A well-formed document is syntactically correct
• A valid document is well-formed
  and needs
  
• a DTD (document type definition)
  or
  
• a XML Schema