| t is illegal to omit the closing tag.
In HTML some elements do not have to have a closing tag. The following code is legal in HTML:
<p>This is a paragraph
<p>This is another paragraph
In XML all elements must have a closing tag, like this:
<p>This is a paragraph</p>
<p>This is another paragraph</p>
Note: You might have noticed from the previous example that the XML declaration did not have a closing tag. This is not an error. The declaration is not a part of the XML document itself. It is not an XML element, and it should not have a closing tag.
XML tags are case sensitive
Unlike HTML, XML tags are case sensitive.
With XML, the tag <Letter> is different from the tag <letter>.
Opening and closing tags must therefore be written with the same case:
<Message>This is incorrect</message>
<message>This is correct</message>
All XML elements must be properly nested
Improper nesting of tags makes no sense to XML.
In HTML some elements can be improperly nested within each other like this:
<b><i>This text is bold and italic</b></i>
In XML all elements must be properly nested within each other like this:
<b><i>This text is bold and italic</i></b>
All XML documents must have a root element
All XML documents must contain a single tag pair to define a root element.
All other elements must be within this root element.
All elements can have sub elements (child elements). Sub elements must be correctly nested within their parent element:
<root>
<child>
<subchild>.....</subchild>
</child>
</root>
Attribute values must always be quoted
With XML, it is illegal to omit quotation marks around attribute values.
XML elements can have attributes in name/value pairs just like in HTML. In XML the attribute value must always be quoted. Study the two XML documents below. The first one is incorrect, the second is correct:
<?xml version="1.0" encoding="ISO-8859-1"?>
<note date=12/11/2002>
<to>Tove</to>
<from>Jani</from>
</note>
<?xml version="1.0" encoding="ISO-8859-1"?>
<note date="12/11/2002">
<to>Tove</to>
<from>Jani</from>
</note>
The error in the first document is that the date attribute in the note element is not quoted.
This is correct: date="12/11/2002". This is incorrect: date=12/11/2002.
With XML, white space is preserved
With XML, the white space in your document is not truncated.
This is unlike HTML. With HTML, a sentence like this:
Hello my name is Tove,
will be displayed like this:
Hello my name is Tove,
because HTML reduces multiple, consecutive white space characters to a single white space.
With XML, CR / LF is converted to LF
With XML, a new line is always stored as LF.
Do you know what a typewriter is? Well, a typewriter is a mechanical device which was used last century to produce printed documents. :-)
After you have typed one line of text on a typewriter, you have to manually return the printing carriage to the left margin position and manually feed the paper up one line.
In Windows applications, a new line is normally stored as a pair of characters: carriage return (CR) and line feed (LF). The character pair bears some resemblance to the typewriter actions of setting a new line. In Unix applications, a new line is normally stored as a LF character. Macintosh applications use only a CR character to store a new line.
Comments in XML
The syntax for writing comments in XML is similar to that of HTML.
<!-- This is a comment -->
There is nothing special about XML
There is nothing special about XML. It is just plain text with the addition of some XML tags enclosed in angle brackets.
Software that can handle plain text can also handle XML. In a simple text editor, the XML tags will be visible and will not be handled specially.
In an XML-aware application however, the XML tags can be handled specially. The tags may or may not be visible, or have a functional meaning, depending on the nature of the application.
XML Elements 
XML Elements are extensible and they have relationships.
XML Elements have simple naming rules.
XML Elements are Extensible
XML documents can be extended to carry more information.
Look at the following XML NOTE example:
<note>
<to>Tove</to>
<from>Jani</from>
<body>Don''''t forget me this weekend!</body>
</note>
Let''''s imagine that we created an application that extracted the <to>, <from>, and <body> elements from the XML document to produce this output:
MESSAGE
To: Tove
From: Jani
Don''''t forget me this weekend!
Imagine that the author of the XML document added some extra information to it:
<note>
<date>2002-08-01</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don''''t forget me this weekend!</body>
</note>
Should the application break or crash?
No. The application should still be able to find the <to>, <from>, and <body> elements in the XML document and produce the same output.
XML documents are Extensible.
XML Elements have Relationships
Elements are related as parents and children.
To understand XML terminology, you have to know how relationships between XML elements are named, and how element content is described.
Imagine that this is a description of a book:
My First XML
Introduction to XML
XML Syntax
- Elements must have a closing tag
- Elements must be properly nested
Imagine that this XML document describes the book:
<book>
<title>My First XML</title>
<prod id="33-657" media="paper"></prod> <chapter>Introduction to XML
<para>What is HTML</para>
<para>What is XML</para>
</chapter>
<chapter>XML Syntax
<para>Elements must have a closing tag</para>
<para>Elements must be properly nested</para>
</chapter>
</book>
Book is the root element. Title, prod, and chapter are child elements of book. Book is the parent element of title, prod, and chapter. Title, prod, and chapter are siblings (or sister elements) because they have the same parent.
Elements have Content
Elements can have different content types.
An XML element is everything from (including) the element''''s start tag to (including) the element''''s end tag.
An element can have element content, mixed content, simple content, or empty content. An element can also have attributes.
In the example above, book has element content, because it contains other elements. Chapter has mixed content because it contains both text and other elements. Para has simple content (or text content) because it contains only text. Prod has empty content, because it carries no information.
In the example above only the prod element has attributes. The attribute named id has the value "33-657". The attribute named media has the value "paper".
Element Naming
XML elements must follow these naming rules:
- Names can contain letters, numbers, and other characters
- Names must not start with a number or punctuation character
- Names must not start with the letters xml (or XML or Xml ..)
- Names cannot contain spaces
Take care when you "invent" element names and follow these simple rules:
Any name can be used, no words are reserved, but the idea is to make names descriptive. Names with an underscore separator are nice.
Examples: <first_name>, <last_name>.
Avoid "-" and "." in names. For example, if you name something "first-name," it could be a mess if your software tries to subtract name from first. Or if you name something "first.name," your software may think that "name" is a property of the object "first."
Element names can be as long as you like, but don''''t exaggerate. Names should be short and simple, like this: <book_title> not like this: <the_title_of_the_book>.
XML documents often have a corresponding database, in which fields exist corresponding to elements in the XML document. A good practice is to use the naming rules of your database for the elements in the XML documents.
Non-English letters like éòá are perfectly legal in XML element names, but watch out for problems if your software vendor doesn''''t support them.
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