XSL provides for two forms of output flow objects. The first set is set of displayable objects defined for HTML, which allows XML data to be mapped into HTML-aware browsers. The second set is based on DSSSL-O specifications (Document Style Semantics and Specification Language - Online), and allows XML data to be mapped to DSSSL-based text formatters, such as JADE. Both sets of flow objects are described using XML markup.

XSL defines a set of rules which define a set of actions that are to be associated with various patterns of target elements. The selection of target elements can be qualified in a number of ways. For example, XSL allows different rules to be applied to same element type dependent on what its ancestors, siblings or contents are. In addition, processing rules can be specified for application when particular attribute values have been associated with an element, or when element has specific contents. This means that specific rules can be applied to elements with unique identifiers or identified content types (classes).

XSL allows for definition of sharable sets of style rules. A style rule applies a set of processing characteristics to a target element without creating a new flow object. Where same style is to be applied to a number of elements, a uniquely named style can be defined for future reference. This provides XSL with facilities for creating cascading sets of style sheet specifications similar in effect to those defined in more limited Cascading Style Sheet specification used to process HTML documents.

XSL style sheets can use ECMAScript programming language to evaluate contents of elements or attributes prior to or during creation of flow objects. ECMAScript is a variant of JavaScript and Jscript that has been formally defined by European Computer Manufacturers Association. It allows tools containing a Java Virtual Machine to process data contained within an XML document. The language has been designed to support only a limited set of processing side-effects to ensure that evaluation cannot inhibit progressive rendering of large documents.

· Elements · Tags · Attributes · Entities · PCDATA · CDATA

This is a brief explanation of each of building blocks:

Elements

Elements are main building blocks of both XML and HTML documents.

Examples of HTML elements are "body" and "table". Examples of XML elements could be "my-schedule" and "date". Elements can contain text, other elements, or be empty. Examples of empty HTML elements are "hr", "br" and "img".

Tags

Tags are used to markup elements.

A starting tag like mark up beginning of an element, and an ending tag like mark up end of an element.

Examples:

A body element: body text in between. A message element: some message in between

Attributes

Attributes provide extra information about elements.

Attributes are placed inside start tag of an element. Attributes come in name/value pairs. The following "img" element has an additional information about a source file:

The name of element is "img". The name of attribute is "src". The value of attribute is "computer.gif". Since element itself is empty it is closed by a " /".

PCDATA

PCDATA means parsed character data.

Think of character data as text found between start tag and end tag of an XML element.

PCDATA is text that will be parsed by a parser. Tags inside text will be treated as markup and entities will be expanded.

CDATA

CDATA also means character data.

CDATA is text that will NOT be parsed by a parser. Tags inside text will NOT be treated as markup and entities will not be expanded.

Entities

Entities as variables used to define common text. Entity references are references to entities.