"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" encoding="ISO-8859-1"?>
<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" encoding="ISO-8859-1"?>
<!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.

XML Schema

XML Schema is an XML based alternative to DTD.

Errors will Stop you

Errors in XML documents will stop the XML program.

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.

XML in Data Islands

XML Embedded in HTML

The unofficial <xml> tag is used to embed XML data within HTML.

XML data can be embedded directly in an HTML page like this:

<xml id="note">
<note>
  <to>Tove</to>
  <from>Jani</from>
  <heading>Reminder</heading>
  <body>Don't forget me this weekend!</body>
</note>
</xml>

Or a separate XML file can be embedded like this:

<xml id="note" src="note.xml">
</xml>

Note that the <xml> tag is an HTML element, not an XML element.

Data Binding

Data Islands can be bound to HTML elements (like HTML tables).

In the example below, an XML Data Island with an ID "cdcat" is loaded from an external XML file. An HTML table is bound to the Data Island with a data source attribute, and finally the tabledata elements are bound to the XML data with a data field attribute inside a span.

<html>
<body>

<xml id="cdcat" src="cd_catalog.xml"></xml>

<table border="1" datasrc="#cdcat">
<tr>
<td><span datafld="ARTIST"></span></td>
<td><span datafld="TITLE"></span></td>
</tr>
</table>

</body>
</html>