Unit 4: New Web Technologies: XML, XHTML, CSS

Lesson 1: XML

Reference: What is XML? by Norm Walsh

Describe the what XML is
Describe what an XML document looks like

What is XML?

eXtensible Markup Language
a simplified version of SGML (Standard Generalized Markup Language)
XML is really a meta-language for describing markup languages.
provides a facility to define tags and the structural relationships between them

Why XML?

XML is not a replacement for HTML
XML was designed to describe data and to focus on what data is.
XML was created so that richly structured documents could be used over the web

Why not HTML?

HTML was designed to display data and to focus on how data looks.
HTML is about displaying information, XML is about describing information.

Example

Example XML document:

      <?xml version="1.0"?>

      <oldjoke>

      <burns>Say <quote>goodnight</quote>,
      Gracie.</burns>

      <allen><quote>Goodnight, 
      Gracie.</quote></allen>

      <applause/>

      </oldjoke>

Components of XML

XML documents are composed of markup and content.
There are six kinds of markup:
- elements
- entity references
- comments
- processing instructions
- marked sections (not covered)
- document type declaration (DTD)

Elements

Most common form of markup
Delimited by angle brackets
Most elements identify the nature of the content they surround
Begins with a start-tag, <element>, and ends with an end-tag, </element>
Some elements may be empty, <element></element> or <element/> for short

Attributes

Attributes are name-value pairs that occur inside start-tags after the element name
All attribute values must be quoted
For example:
```
      <div class="preface">
```

Entity References

Some characters have been reserved to identify the start of markup
For example, the left angle bracket, <
Every entity must have a unique name
For example, the left angle bracket is <, the ampersand is &
You can define your own entities
A special form, called a character reference, can be used to insert arbitrary Unicode characters
For example, for decimal ℞ and hexadecimal references, ℞ are the character ℞

Comments

Comments begin with 
Comments can contain any data except the literal string --
For example:
```
        
```

Processing Instructions

Provide information to an application
Not textually part of the XML document but the XML processor is required to pass them to an application
Have the form: <?name pidata?>
Applications should process only the targets they recognize and ignore all other
Names beginning with xml are reserved for XML standardization

Document Type Declaration (DTD)

Define what entities are allows and where
Used to define an instance of a XML document type

Well Formed Documents

All sytax is proper
See article for full set of rules
All XML documents must be well formed (no sloppy coding like that allowed in HTML)

Example

An example of a well formed XML Document:

      <?xml version="1.0"?>
      <CDs>
        <CD>
          <artist>Bob Dylan</artist>
          <title>Blonde On Blonde</title>
          <release_date>1966</release_date>
        </CD>
        <CD>
          <artist>The Doors</artist>
          <title>L.A. Woman</title>
          <release_date>1971</release_date>
        </CD>
      </CDs>

Example

An example of an XML Document that is not well formed:

      <?xml version="1.0"?>
      <cds>
        <CD>
          <artist>Bob Dylan
          <title>Blonde On Blonde</artist></title>
          <release_date>1966</release_date>
        </cd>
        <cd>
          <artist>The Doors</artist>
          <title>L.A. Woman</title>
          <release_date>1971</release_date>
        </CD>
      </CDs>

Valid Documents

Documents that validate against a document type declaration (DTD)
It is not required that XML documents validate, therefore DTDs are optional