XML - A Closer Look

The XML specification sets out the following goals for XML:

1. It shall be straightforward to use XML over the Internet.
2. XML shall support a wide variety of applications.
3. XML shall be compatible with SGML.
4. It shall be easy to write programs that process XML documents.
5. The number of optional features in XML is to be kept to an absolute minimum, ideally zero.
6. XML documents should be human-legible and reasonably clear.
7. The XML design should be prepared quickly.
8. The design of XML shall be formal and concise.
9. XML documents shall be easy to create.
10. Terseness in XML markup is of minimal importance.

How Is XML Defined?

1. XML, the Extensible Markup Language (02/10/98)
2. XLL, the Extensible Linking Language (???)
3. XSL, the Extensible Style Language (in progress)
4. XUA, the XML User Agent (not started yet)

Multidimensional Documents

• text file is one dimensional (contains no information about its content)
• HTML is two dimensional (contains basic markup that describes its content)
• XML is multidimensional - capable of being processed by different programs, delivered by different methods, and displayed in different views.
• XML is "object-oriented"
• Document Object Model
  • makes it possible to address all the elements of an HTML document
  • able to combine the content of the document, the logic that controls it, and the styles in one package

The Scripting Dimension

• JavaScript is an object-oriented language of sorts.
• JavaScript understands that the document is an object and that it has certain properties that you can reference or set. For instance you can reference the title or the location.

  Limitations of the JavaScript approach:

  • you cannot reference the contents of a document (the third paragraph, the top level headings, the table, etc.)
  • you cannot use the script to control the display of elements.
  • when content is embedded in an application, it requires a programmer to create and modify the application. Instead, we should separate the content from the scripting logic that controls the application. (Just as we separate style from the content).
  • the above limitation is not in the JavaScript language but in the document model underlying the Netscape browser.

• In the Document Object Model, we can address objects in an HTML document, but we are limited to the elements that HTML provides - which are fairly generic.
• This is where XML comes in nicely.

The Presentation Dimension

• use style sheets, either included in the document or external to it, to control page layout
• the current style sheet standard is CSS -- cascading style sheets
• HTML and XML will work with CSS
• CSS offers limited control of presentation, handling font changes and such.
• It lacks some important features that you'd associate with style sheets such as controlling the flow of text and positioning elements.
• There are efforts underway to replace CSS with a more substantial style sheet called XSL (DSSSL-lite).
• Presentations will need to be smart about the browser, the display device, the fonts available, and a number of other factors.
• Multidimensional documents will need to have multiple style sheets, reflecting different views or different devices.

The Server Independent Document

• XML helps to establish documents that can function independently of the server.
• Now you can look at the document as a container of information whose logic provides the user different views.
• It might be natural to create and manage the application in a database. Even so, you might extract the information and deliver an XML document that can exist independently of the server and the database.

The Primacy of Documents

• One reason for the success of the Web is that creating HTML documents is easy.
• Do we have to trade in the flexibility and control we have with documents for Java applets, plug-ins and proprietary formats that process information in mysterious ways.
• XML opens new dimensions for Web documents, ensuring that documents can be more functional.
• XML will help to see that documents continue to have a certain kind primacy on the Web.

Document Type Definition (DTD)

• full SGML uses a Document Type Definition (DTD) to describe the markup (elements) available in any specific type of document.
• the design and construction of a DTD can be a complex and non-trivial task, so XML has been designed so it can be used with or without a DTD.
• DTDless operation means you can invent markup without having to define it formally.

Well-Formed

• if there is no DTD in use, the document must start with a Standalone Document Declaration (SDD) saying so:
  
  <?XML Version="1.0" standalone="yes"?>
  <foo>

  <bar> . . . <blort/> . . . </bar>
  

  </foo>
  
  
• all tags must be balanced
• all attribute values must be in quotes
• any EMPTY element tags (e.g. those with no HTML end-tag like HTMLÆs <IMG>, <HR>, and <BR> and others) must either end with æ\>Æ or you have to make them non-EMPTY by adding a real-end tag;
  Example: <BR> would become either <BR/> or <BR></BR>.
  
  
• there must not be any isolated markup characters
• elements must nest inside each other properly (no overlapping of markup, same rules as for SGML);
• Well-formed files with no DTD may use attributes on any element, but the attributes must all be of type CDATA by default.

Valid

• Valid XML files are those which contain a Document Type Definition (DTD) like all other SGML applications, and which adhere to it.
• They must also be well-formed.
• A valid file begins like any other SGML file with a DTD, but may have an optional XML Declaration prepended:
  
  <?XML version="1.0"?>
  <!DOCTYPE advert SYSTEM "http://www.foo.org/ad.dtd">
  <advert>

  <headline> . . . <pic/> . . . </headline>
  <text> . . . </text>

  </advert>