Chapter 12: The Next Generation of Web Documents

12.1 Introduction

HTML (HyperText Markup Language) was developed as an implementation of SGML (Standard Generalized Markup Language) and was designed to allow markups of simple documents. As we move into the next stage of the use of the World Wide Web as a vehicle for electronic commerce, the limitations of HTML become apparent. Users of HTML are constrained to using only a limited number of pre-defined tags that only describe how to describe the features of the document. Attempts have been made to extend this capability with style sheets and scripting languages, but what is needed is a more robust language.

12.2 XML (http://csc.ColumbusState.edu/summers/notes/xml.html)

XML (Extensible Markup Language) is designed to be that language. It is considered extensible because it is not a fixed format like HTML. XML itself is not a single markup language: it is a metalanguage that lets you design your own markup language. A regular markup language defines a way to describe information in a certain class of documents (e.g. HTML). XML lets you define your own customized markup languages for many classes of document. It can do this because it is written in SGML, the international standard metalanguage for markup languages.

You can use XML to describe any type of data. XML can also be used to create new element types. XML is:

1. Extensible - flexibility

2. Portable

3. Structured

4. Descriptive.

Differences between HTML and XML

• XML is case sensitive
• Every tag must be paired with a close tag. Tags that don't normally have a close tag, must terminate with a forward slash, i.e. <BR />
• All attributes must be enclosed in single or double quotes.

Features of XML

• In XML, you can create new element types.
• Tags are used to describe element types.
• The root element contains all other elements in the document.
• A container element contains sub-elements.

12.3 Document Type Definition

XML documents should include a Document Type Definition (DTD). DTD files define the grammatical rules for a document. A DTD is not required for an XML document, but is recommended for uniformity of documents. The W3C defines a DTD as "a collection of declarations that, as a collection, defines the legal structure, elements, and attributes that are available for use in a document that complies to the DTD." XML documents end in the .xml extension while Document Type Definitions (DTD) documents end in the .dtd extension. XML parser (processor) combines an XML document with its DTD. IE 5.0 has a built-in XML processor = msxml. Netscape 6.0 has a built-in XML processor.

Features of Document Type Definitions

• !ELEMENT specifies the element that is being created. BNF rules are followed:
• + - Means one or more occurrences
• * - Means zero or more occurrences
• ? - Means zero or one occurrence
• Default is one occurrence
• !ATTLIST defines an attribute. #IMPLIED means that the attribute does not have a default value. CDATA means that the attribute contains a string.
• #PCDATA specifies that the element can store parsed character data.
• EMPTY means that the element does not contain content.
• When an XML document is created, the DTD for the XML document is declared in a document type declaration at the top of the document.
• The XML document's content is validated by checking its markup against its DTD and reporting markup errors. You can validate HTML, XHTML, and CSS
• Files by using the W3C's validator; an easy way is to include a link to http://validator.w3.org/check/referer in your document - click on it to validate the document.

12.4 XHTML (http://csc.ColumbusState.edu/summers/notes/xhtml.html)

XHTML = XML + HTML Extensible HyperText Markup Language (XHTML) is a reformulation of HTML 4.0 as an application of XML. XHTML was released as a Recommendation by the W3C on January 26, 2000. "XHTML 1.0 connects the present Web to the future Web, It provides the bridge to page and site authors for entering the structured data, XML world, while still being able to maintain operability with user agents that support HTML 4." -- Tim Berners-Lee, W3C Director.

12.5 Differences between HTML and XHTML

• The <head> and <body> elements cannot be omitted.
• XHTML is case sensitive
• XHTML tags are all in lowercase
• XHTML is a stricter, tidier version of HTML
• Every tag must be paired with a close tag. Tags that don't normally have a close tag, must terminate with a forward slash, i.e. <BR/>
• Elements must nest and not overlap
• All attributes must be enclosed in single or double quotes.
• Attribute-value pairs cannot be minimized, i.e. <td nowrap> must be written <td nowrap="nowrap">
• The name attribute is replaced by the id attribute (for a, applet, frame, iframe, and img elements)
• script and style elements need to have the contents enclosed in

<![CDATA[

...script/style content...

]]>

• An XHTML document consists of three main parts: the DOCTYPE, the Head, and the Body.
• The DOCTYPE definition specifies the document type:
• <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "DTD/xhtml1-transitional.dtd">
• The three document types are STRICT, TRANSITIONAL, and FRAMESET. STRICT should be used with clean documents that use Cascading Style Sheets
• The html tag follows the DOCTYPE tag and must have a namespace declaration <html xmlns = "http://www.w3.org/1999/xhtml">
• The head element contains the title element and any meta data for the document
• Pages written in XHTML should work well in most browsers.
• The tags and attributes for XHTML are almost identical to HTML
• XHTML is more portable

You can use the Quick Example (http://csc.ColumbusState.edu/summers/NOTES/XHTML/template.html) below as a template to create your XHTML documents. Once you have created your XHTML document, you can click on the image at the bottom or go to the address http://validator.w3.org/check/referer, which will connect with the W3C to validate whether your document conforms with the XHTML standards.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "DTD/xhtml1-transitional.dtd">

<html xmlns = "http://www.w3.org/1999/xhtml">

<head>

<title>Quick Example</title>

</head>

<body>

<h1>Quick Example</h1>

<p>

<a href="http://validator.w3.org/check/referer">

<img src="http://validator.w3.org/images/vxhtml10" alt="Valid XHTML 1.0!" height="31" width="88" />

</a>

</p>

<p>Note that the layout (with tabs and alignment) is purely for readability - XHTML doesn't require it.</p>

</body>

</html>

Some Definitions

• Element - An element has three parts: a start tag, the content, and an end tag. Most tags have 'closing tags' - </p>, which indicates where the effect of the 'opening' tag stops.
• URI - uniform resource identifier
• XML namespaces - An XML namespace is a collection of names that are used in XML documents as element types and attribute names. These are identified by a URI reference. I.e., the XHTML namespace is the list of valid tags that can be used in XHTML.

12.6 Other Markup Languages

VRML - Virtual Reality Modeling Language

(http://csc.ColumbusState.edu/summers/NOTES/vrml.htm) is a language for describing virtual worlds networked within the World Wide Web.

MathML (http://www.webeq.com/mathml/gitmml/) is a language for describing mathematical concepts over the Web.

CML – Chemical Markup Language

(http://ala.vsms.nottingham.ac.uk/vsms/talks/chemweb.html) is a language for describing chemical concepts over the Web.

SMIL - Synchronized Multimedia Integration Language

(http://csc.ColumbusState.edu/summers/SMIL.html) is a language designed to allow for the creation of synchronized multimedia delivery over the Web.

These are just a few of the emerging special purpose languages that have been developed over the last couple of years. There is even a Robotic Telescope Markup Language (RTML). Most of these new languages are special-purpose markup languages developed with XML. Expect more in the coming years.