Rules for constructing well-formed XML Documents

Wel-formedness is essential in XML. The W3C instructs us that violations of well-formedness constraints are fatal errors. Documents that are not well-formed will not load in a browser or will not be processed by an XML parser, according to the XML Recommendation.

Five basic rules will help you construct well-formed XML documents. You should commit these rules to memory:

1. XML uses elements to markup content.

XML elements consist of a start tag and an end tag. Start tags begin with < and end with >. End tags begin with </ and end with >. Element names in XML are case sensitive. They may start with a letter, an underscore character, or a colon character. The next characters in an element name may be letters, digits, underscores, hyphens, periods, and colons but not white space. Spaces, carriage returns, line feeds, and tabs are all treated as white space in XML.

1. Tags cannot be inferred and must be Tags cannot be inferred and must be.

All start tags must have corresponding ending tags. All ending tags must have corresponding start tags.
For the document to be well-formed, it must be written in the following way:

<NAME><FIRST>John</FIRST></NAME>

1. An empty element must be closed with />.

Empty elements may be used for elements that have no content.
You may be familiar with the <IMG> and <BR> empty tags from HTML. In HTML, empty tags are not required to have closing tag in the form />. In XML, empty elements must be closed with />. For example

<PURCHASE-ORDER NUMBER="1234"/>

1. XML elements that have name-value pair attributes must enclose attribute values in single or double quotation marks.

<BOOK ISBN="345671">
      <AUTHOR>James Gosling</AUTHOR>
</BOOK>

In this example, the <BOOK> element has an attribute named ISBN with the value 345671.
Note that the attribute value is enclosed in double quotation marks for the document to be well-formed.

1. XML elements must nest and un-nest in reverse order.

For example, the following XML document is not well-formed because it violates the rule for correctly nesting elements:

<NAME><FIRST>John</NAME></FIRST>

<NAME><FIRST>John</FIRST></NAME>

Well Formed XML Document
The next lesson shows you how to determine the inherent structure of information within XML documents.