What is XML?
XML stands for EXtensible Markup Language
XML is a markup language much like HTML
XML was designed to describe data, not to display data
XML tags are not predefined. You must define your own tags
XML is designed to be self-descriptive
XML is a W3C Recommendation
The Difference Between XML and HTML
XML is not a replacement for HTML.
XML and HTML were designed with different goals:
XML was designed to describe data, with focus on what data is
HTML was designed to display data, with focus on how data looks
HTML is about displaying information, while XML is about carrying
XML Separates Data from HTML
If you need to display dynamic data in your HTML document, it will take a lot
of work to edit the HTML each time the data changes.
With XML, data can be stored in separate XML files. This way you can
concentrate on using HTML/CSS for display and layout, and be sure that
changes in the underlying data will not require any changes to the HTML.
With a few lines of JavaScript code, you can read an external XML file and
update the data content of your web page.
XML Simplifies Data Sharing
In the real world, computer systems and databases contain data in
incompatible formats.
XML data is stored in plain text format. This provides a software- and
hardware-independent way of storing data.
This makes it much easier to create data that can be shared by different
XML Simplifies Data Transport
One of the most time-consuming challenges for developers is to exchange
data between incompatible systems over the Internet.
Exchanging data as XML greatly reduces this complexity, since the data can
be read by different incompatible applications.
XML Simplifies Platform Changes
Upgrading to new systems (hardware or software platforms), is always time
consuming. Large amounts of data must be converted and incompatible data
is often lost.
XML data is stored in text format. This makes it easier to expand or upgrade
to new operating systems, new applications, or new browsers, without losing
XML Makes Your Data More Available
Different applications can access your data, not only in HTML pages, but also
from XML data sources.
With XML, your data can be available to all kinds of "reading machines"
(Handheld computers, voice machines, news feeds, etc.), and make it more
available for blind people, or people with other disabilities.
Internet Languages Written in XML
Several Internet languages are written in XML. Here are some examples:
XML Schema
XML Document Example
<?xml version="1.0" encoding="UTF-8"?>
<to> Tove</to>
<body>Don't forget me this weekend!</body>
XML Documents Form a Tree Structure
XML documents must contain a root element. This element is "the parent"
of all other elements.
The elements in an XML document form a document tree. The tree starts at
the root and branches to the lowest level of the tree.
All elements can have sub elements (child elements):
The terms parent, child, and sibling are used to describe the relationships
between elements. Parent elements have children. Children on the same
level are called siblings (brothers or sisters).
All elements can have text content and attributes (just like in HTML).
The image above represents one book in the XML below:
<book category="COOKING">
<title lang="en">Everyday Italian</title>
<author>Giada De Laurentiis</author>
<book category="CHILDREN">
<title lang="en">Harry Potter</title>
<author>J K. Rowling</author>
<book category="WEB">
<title lang="en">Learning XML</title>
<author>Erik T. Ray</author>
The root element in the example is <bookstore>. All <book> elements in the
document are contained within <bookstore>.
The <book> element has 4 children: <title>,< author>, <year>, <price>.
XML Syntax Rules
All XML Elements Must Have a Closing Tag
In HTML, some elements do not have to have a closing tag:
<p>This is a paragraph.
In XML, it is illegal to omit the closing tag. All elements must have a closing
<p>This is a paragraph.</p>
<br />
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, and it has no closing tag.
XML Tags are Case Sensitive
XML tags are case sensitive. The tag <Letter> is different from the tag
Opening and closing tags must be written with the same case:
<Message>This is incorrect</message>
<message>This is correct</message>
Note: "Opening and closing tags" are often referred to as "Start and end
tags". Use whatever you prefer. It is exactly the same thing.
XML Elements Must be Properly Nested
In HTML, you might see improperly nested elements:
<b><i>This text is bold and italic</b></i>
In XML, all elements must be properly nested within each other:
<b><i>This text is bold and italic</i></b>
In the example above, "Properly nested" simply means that since the <i>
element is opened inside the <b> element, it must be closed inside the <b>
XML Documents Must Have a Root Element
XML documents must contain one element that is the parent of all other
elements. This element is called the rootelement.
XML Attribute Values Must be Quoted
XML elements can have attributes in name/value pairs just like in HTML.
In XML, the attribute values must always be quoted.
<note date=12/11/2007>
<note date="12/11/2007">
The error in the first document is that the date attribute in the note element
is not quoted.
Entity References
Some characters have a special meaning in XML.
If you place a character like "<" inside an XML element, it will generate an
error because the parser interprets it as the start of a new element.
This will generate an XML error:
<message>if salary < 1000 then</message>
To avoid this error, replace the "<" character with an entity reference:
<message>if salary &lt; 1000 then</message>
There are 5 pre-defined entity references in XML:
less than
greater than
quotation mark
Note: Only the characters "<" and "&" are strictly illegal in XML. The greater
than character is legal, but it is a good habit to replace it.
Comments in XML
The syntax for writing comments in XML is similar to that of HTML.
<!-- This is a comment -->
White-space is Preserved in XML
XML does not truncate multiple white-spaces in a document (while HTML
truncates multiple white-spaces to one single white-space):
Hello Tove
Hello Tove
XML Stores New Line as LF
Windows applications store a new line as: carriage return and line feed
Unix and Mac OSX uses LF.
Old Mac systems uses CR.
XML stores a new line as LF.
Well Formed XML
XML documents that conform to the syntax rules above are said to be "Well
Formed" XML documents.
XML Elements
What is an XML Element?
An XML element is everything from (including) the element's start tag to
(including) the element's end tag.
An element can contain:
other elements
or a mix of all of the above...
<book category="CHILDREN">
<title>Harry Potter</title>
<author>J K. Rowling</author>
<book category="WEB">
<title>Learning XML</title>
<author>Erik T. Ray</author>
In the example above, <bookstore> and <book> have element contents,
because they contain other elements. <book> also has
an attribute (category="CHILDREN"). <title>, <author>, <year>, and
<price> have text contentbecause they contain text.
Empty XML Elements
An element with no content is said to be empty.
In XML, you can indicate an empty element like this:
or you can use an empty tag, like this (this sort of element syntax is called
<element />
The two forms above produce identical results in an XML parser.
Note: Empty elements do not have any content, but they can have
XML Naming Rules
XML elements must follow these naming rules:
Element names are case-sensitive
Element names must start with a letter or underscore
Element names cannot start with the letters xml (or XML, or Xml, etc)
Element names can contain letters, digits, hyphens, underscores, and
Element names cannot contain spaces
Any name can be used, no words are reserved (except xml).
Best Naming Practices
Create descriptive names, like this: <person>, <firstname>, <lastname>.
Create short and simple names, like this: <book_title> not like this:
Avoid "-". If you name something "first-name", some software may think you
want to subtract "name" from "first".
Avoid ".". If you name something "first.name", some software may think that
"name" is a property of the object "first".
Avoid ":". Colons are reserved for namespaces (more later).
Non-English letters like éòá are perfectly legal in XML, but watch out for
problems if your software doesn't support them.
Naming Styles
There are no naming styles defined for XML elements. But here are some
commonly used:
Lower case
Upper case
Pascal case
Camel case
If you choose a naming style, it is good to be consistent!
XML documents often have a corresponding database. A good practice is to
use the naming rules of your database for the elements in the XML
XML Elements are Extensible
XML elements can be extended to carry more information.
Look at the following XML example:
<body>Don't forget me this weekend!</body>
Let's imagine that we created an application that extracted the <to>,
<from>, and <body> elements from the XML document to produce this
To: Tove
From: Jani
Don't forget me this weekend!
Imagine that the author of the XML document added some extra information
to it:
<body>Don't forget me this weekend!</body>
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.
One of the beauties of XML, is that it can be extended without breaking
XML Attributes
XML elements can have attributes, just like HTML.
Attributes are designed to contain data related to a specific element.
XML Attributes Must be Quoted
Attribute values must always be quoted. Either single or double quotes can
be used.
For a person's gender, the <person> element can be written like this:
<person gender="female">
or like this:
<person gender='female'>
If the attribute value itself contains double quotes you can use single quotes,
like in this example:
<gangster name='George "Shotgun" Ziegler'>
or you can use character entities:
<gangster name="George &quot;Shotgun&quot; Ziegler">
XML Elements vs. Attributes
Take a look at these examples:
<person gender="female">
In the first example gender is an attribute. In the last, gender is an element.
Both examples provide the same information.
There are no rules about when to use attributes or when to use elements in
My Favorite Way
The following three XML documents contain exactly the same information:
A date attribute is used in the first example:
<note date="2008-01-10">
A <date> element is used in the second example:
An expanded <date> element is used in the third example: (THIS IS MY
Avoid XML Attributes?
Some things to consider when using attributes are:
attributes cannot contain multiple values (elements can)
attributes cannot contain tree structures (elements can)
attributes are not easily expandable (for future changes)
Don't end up like this:
<note day="10" month="01" year="2008"
to="Tove" from="Jani" heading="Reminder"
body="Don't forget me this weekend!">
XML Attributes for Metadata
Sometimes ID references are assigned to elements. These IDs can be used
to identify XML elements in much the same way as the id attribute in HTML.
This example demonstrates this:
<note id="501">
<body>Don't forget me this weekend!</body>
<note id="502">
<heading>Re: Reminder</heading>
<body>I will not</body>
The id attributes above are for identifying the different notes. It is not a part
of the note itself.
What I'm trying to say here is that metadata (data about data) should be
stored as attributes, and the data itself should be stored as elements.
XML Namespaces
XML Namespaces provide a method to avoid element name conflicts.
Name Conflicts
In XML, element names are defined by the developer. This often results in a
conflict when trying to mix XML documents from different XML applications.
This XML carries HTML table information:
This XML carries information about a table (a piece of furniture):
<name>African Coffee Table</name>
If these XML fragments were added together, there would be a name conflict.
Both contain a <table> element, but the elements have different content and
A user or an XML application will not know how to handle these differences.
Solving the Name Conflict Using a Prefix
Name conflicts in XML can easily be avoided using a name prefix.
This XML carries information about an HTML table, and a piece of furniture:
<f:name>African Coffee Table</f:name>
In the example above, there will be no conflict because the two <table>
elements have different names.
XML Namespaces - The xmlns Attribute
When using prefixes in XML, a so-called namespace for the prefix must be
The namespace is defined by the xmlns attribute in the start tag of an
The namespace declaration has the following syntax. xmlns:prefix="URI".
<h:table xmlns:h="http://www.w3.org/TR/html4/">
<f:table xmlns:f="http://www.w3schools.com/furniture">
<f:name>African Coffee Table</f:name>
In the example above, the xmlns attribute in the <table> tag give the h: and
f: prefixes a qualified namespace.
When a namespace is defined for an element, all child elements with the
same prefix are associated with the same namespace.
Namespaces can be declared in the elements where they are used or in the
XML root element:
<root xmlns:h="http://www.w3.org/TR/html4/"
<f:name>African Coffee Table</f:name>
Note: The namespace URI is not used by the parser to look up information.
The purpose is to give the namespace a unique name. However, often
companies use the namespace as a pointer to a web page containing
namespace information.
Uniform Resource Identifier (URI)
A Uniform Resource Identifier (URI) is a string of characters which
identifies an Internet Resource.
The most common URI is the Uniform Resource Locator (URL) which
identifies an Internet domain address. Another, not so common type of URI
is the Universal Resource Name (URN).
In our examples we will only use URLs.
Default Namespaces
Defining a default namespace for an element saves us from using prefixes in
all the child elements. It has the following syntax:
This XML carries HTML table information:
<table xmlns="http://www.w3.org/TR/html4/">
This XML carries information about a piece of furniture:
<table xmlns="http://www.w3schools.com/furniture">
<name>African Coffee Table</name>
Namespaces in Real Use
XSLT is an XML language that can be used to transform XML documents into
other formats, like HTML.
In the XSLT document below, you can see that most of the tags are HTML
The tags that are not HTML tags have the prefix xsl, identified by the
namespace xmlns:xsl="http://www.w3.org/1999/XSL/Transform":
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
<xsl:template match="/">
<h2>My CD Collection</h2>
<table border="1">
<th style="text-align:left">Title</th>
<th style="text-align:left">Artist</th>
<xsl:for-each select="catalog/cd">
<td><xsl:value-of select="title"/></td>
<td><xsl:value-of select="artist"/></td>
XML Encoding
XML documents can contain international characters, like Norwegian æøå,
or French êèé.
To avoid errors, you should specify the encoding used, or save your XML
files as UTF-8.
Character Encoding
Character encoding defines a unique binary code for each different character
used in a document.
In computer terms, character encoding are also called character set,
character map, code set, and code page.
The Unicode Consortium
The Unicode Consortium develops the Unicode Standard. Their goal is to
replace the existing character sets with its standard Unicode Transformation
Format (UTF).
The Unicode Standard has become a success and is implemented in HTML,
XML, Java, JavaScript, E-mail, ASP, PHP, etc. The Unicode standard is also
supported in many operating systems and all modern browsers.
The Unicode Consortium cooperates with the leading standards development
organizations, like ISO, W3C, and ECMA.
The Unicode Character Sets
Unicode can be implemented by different character sets. The most commonly
used encodings are UTF-8 and UTF-16.
UTF-8 uses 1 byte (8-bits) to represent basic Latin characters, and two,
three, or four bytes for the rest.
UTF-16 uses 2 bytes (16 bits) for most characters, and four bytes for the
UTF-8 = The Web Standard
UTF-8 is the standard character encoding on the web.
UTF-8 is the default character encoding for HTML5, CSS, JavaScript, PHP,
SQL, and XML.
XML Encoding
The first line in an XML document is called the prolog:
<?xml version="1.0"?>
The prolog is optional. Normally it contains the XML version number.
It can also contain information about the encoding used in the document.
This prolog specifies UTF-8 encoding:
<?xml version="1.0" encoding="UTF-8"?>
The XML standard states that all XML software must understand both UTF-8
and UTF-16.
UTF-8 is the default for documents without encoding information.
In addition, most XML software systems understand encodings like ISO-
8859-1, Windows-1252, and ASCII.
XML Errors
Most often, XML documents are created on one computer, uploaded to a
server on a second computer, and displayed by a browser on a third
If the encoding is not correctly interpreted by all the three computers, the
browser might display meaningless text, or you might get an error message.
For high quality XML documents, UTF-8 encoding is the best to use. UTF-8
covers international characters, and it is also the default, if no encoding is
When you write an XML document:
Use an XML editor that supports encoding
Make sure you know what encoding the editor uses
Describe the encoding in the encoding attribute
UTF-8 is the safest encoding to use
UTF-8 is the web standard
Displaying XML
Raw XML files can be viewed in all major browsers.
Don't expect XML files to be displayed as HTML pages.
Viewing XML Files
<?xml version="1.0" encoding="UTF-8"?>
- <note>
<body>Don't forget me this weekend!</body>
Look at the XML file above in your browser: note.xml
Notice that an XML document will be displayed with color-coded root and
child elements. A plus (+) or minus sign (-) to the left of the elements can be
clicked to expand or collapse the element structure. To view the raw XML
source (without the + and - signs), select "View Page Source" or "View
Source" from the browser menu.
Note: In Safari 5 (and earlier), only the element text will be displayed. To
view the raw XML, you must right click the page and select "View Source".
Viewing an Invalid XML File
If an erroneous XML file is opened, some browsers report the error, and
some only display it incorrectly.
Try to open the following XML file in Chrome, IE, Firefox, Opera, and
Safari: note_error.xml.
Other XML Examples
Viewing some XML documents will help you get the XML feeling:
An XML CD catalog
This is a CD collection, stored as XML.
An XML plant catalog
This is a plant catalog from a plant shop, stored as XML.
An XML breakfast menu
This is a breakfast food menu from a restaurant, stored as XML.
Why Does XML Display Like This?
XML documents do not carry information about how to display the data.
Since XML tags are "invented" by the author of the XML document, browsers
do not know if a tag like <table> describes an HTML table or a dining table.
Without any information about how to display the data, most browsers will
just display the XML document as it is.
Displaying XML Files with CSS?
Below is an example of how to use CSS to format an XML document.
We can use an XML file like cd_catalog.xml and a style sheet
like cd_catalog.css
RESULT: The CD catalog formatted with the CSS file
Below is a fraction of the XML file. The second line links the XML file to the
CSS file:
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/css" href="cd_catalog.css"?>
<TITLE>Empire Burlesque</TITLE>
<TITLE>Hide your heart</TITLE>
<ARTIST>Bonnie Tyler</ARTIST>
Formatting XML with CSS is not recommended. Use JavaScript or XSLT instead.