WWW6: eXtended Markup Language (XML)

overview:

the eXtended Markup Language (XML) is an alternative to the existing HTML to write documents that can be easily accessed over the Internet. XML is a subset of SGML. the term extendable refers to the fact, that an author can define his or her own tags and attributes. the limited set of tags in HTML has become one of its major problems. various vendors introduced new tags either in response to requests from the authors or simply to get an advantage over its competitors. this led to what became known as the "browser war". HTML V3.2 tried to address this problem by basically including all commonly used tags in the new standard.
another problem with HTML is the intermingling of contents and presentation. recently, this problem became addressed by the introduction of Cascading Style Sheets (CSS)

XML is a new approach to the same issues. some of the main features are:

• user definable tags and attributes
• clear separation between content and representation
• extended linking options

links:

XML linking elements are defined using the XML-LINK attribute. links are much more than just a pointer to another document, or maybe a pointer to a pre-defined position in such a document.

a link has one of the following values:

• SIMPLE: very similar to HTML's ANCHOR, pointing to one location
• EXTENDED: may refer to many different locations by using multiple LOCATOR values. the browser need to provide a means to select the desired location.
• the GROUP and DOCUMENT values may be used to define external documents which contain locators.

an author may also apply various policies to links. the SHOW attribute defines how the referred document shall be processed, the ACTUATE attributes defines when it shall be processed.

the SHOW attribute may take one of the following values:

1. EMBED: the referred document shall be embedded in the current document.
2. REPLACE: the referred document shall replace the current document. this is similar to HTML's behavior today.
3. NEW: the referred document shall appear in a new window.

the ACTUATE attribute may take one of the following values:

1. AUTO: when a document gets processed that includes links with the AUTO value, these links must be processed immediately without any user intervention.
2. USER: links with the value USER must not be followed until the user indicates that she or he wants to do so. this is similar to HTML's behavior today.

the combination of SHOW = "EMBED" and ACTUATE = "AUTO" provides a means to include one or more portions of text into a document, a feature that is missed by many authors in the current HTML standard.

in addition, XML provides means to access basically any character in any document without the need of having access to the referred document. it allows expressions to locate a particular position in a document such as "second chapter, third paragraph, forth occurrence of xyz".

reference documents:

see working drafts available from the W3C:

• XML part 1: syntax
• XML part 2: linking

to the WWW6 trip report main document