Chapter 3. Basic concepts

Here we review basic aspects of the XSL-FO semantics and vocabulary, to gain a better understanding of how the technology works and how to use the specification itself.

Layout-based vs. content-based formatting. Two very different approaches to the formatting of information are contrasted. Layout-based formatting respects the constraints of the target medium, where limitations or capacities of the target may constrain the content or appearance of the information on a page. Content-based formatting respects the quantity and identity of the information, while the target medium is made to accommodate the information being formatted according to rules in a stylesheet.

Formatting is different from rendering. The distinction between formatting and rendering is the one between expressing what you want formatted vs. expressing how it is to be accomplished on the target device. This contrast is similar to the difference between declarative and imperative style programming methods , or the difference between XSLT's "transformation by example" paradigm vs. other algorithmic transformation approaches using programming languages.

Differing processing model concepts are expressed using unambiguous terminology. The XSL-FO specification, and this book as well, attempts to be very careful in using precise terminology when what is being referred to has similar concepts in other constructs that it could be confused with. For example, an XSL-FO instance contains elements and their attributes. This is similar to the corresponding formatting object tree with objects and their properties. This is, in turn , similar to the corresponding refined formatting object tree with objects and their area traits. This is, finally, similar to the corresponding area tree with areas and their traits.

The XSL-FO semantics and vocabulary properties address the requirements of arbitrary page boundaries imposed on the presentation of information, which is different from the needs of infinite-length web user agent windows . These new semantics are inspired by the Document Style Semantics and Specification Language (DSSSL) International Standard ISO/IEC 10179, but in practice diverge from DSSSL towards Cascading Style Sheets 2 (CSS2) for compatibility with web-based processing.

Formatting model and vocabulary properties extend what is currently available for web presentation. The semantics are classified based on their relationship to similar CSS properties:

• CSS properties by copy (unchanged CSS2 semantics),

• CSS properties with extended values,

• CSS properties "broken apart" to a finer granularity,

• XSL-FO-specific properties.

The XSL-FO support of multiple writing directions and a reference orientation are important concepts inherited from DSSSL that are not present in CSS2.

What's included in this chapter. This chapter includes discussion of the following XSL-FO objects:

• root ( 6.4.2 ):

  • is the document element of a XSL-FO instance,

• layout-master-set ( 6.4.6 ):

  • is the collection of definitions of page geometries and page sequencing and selection patterns,

• page-sequence ( 6.4.5 ):

  • is the specification of how information is to be paginated over a sequence of pages with common static information,

• flow ( 6.4.18 ):

  • is the content that is flowed to as many pages as required and formatted according to the appearance properties.