XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (716 page)

As well as the

rule, there are similar rules for
[@diff=“add”]
and
[@diff=“del”]
.

There are a few other points to note about this stylesheet module.

Firstly, it contains the declaration

. This points up a nice little problem: How would one maintain a
diffspec.xsl
module that could be used as an overlay over a variety of different base stylesheets? To achieve this, the module could not import the underlying module, which means it could not use

to invoke the overridden template rules. An apparent solution would be to use a third module,
xmlspec-diff.xsl
, which imports both
xmlspec.xsl
and
diffspec.xsl
. Unfortunately, though, the rules for

don't allow this: this instruction will not consider all template rules of lower precedence, but only those in modules that have been directly imported. The new

instruction in XSLT 2.0 comes nicely to the rescue here.

Secondly, the stylesheet has a template rule, about 50 lines long, for

. This overrides the corresponding rule in the base stylesheet, and the only difference is that it outputs some text explaining the meaning of the change highlighting. Clearly duplicating so much code from the base stylesheet creates a problem in managing subsequent changes. There are a number of other ways to achieve the required effect. One way is to insert an

instruction to an empty template rule at the appropriate point in the base template; this can then be overridden in the overlay. If it is not possible to modify the base template, another solution is to write a template that invokes the base template by using

, capturing the results in a variable, and then perform a further transform on the contents of the variable. In this example, the XSLT 2.0 solution would look like this:

REC-xml.xsl

The third module in the stylesheet is called
REC-xml.xsl
. Compared with
xmlspec.xsl
, this module is rather poorly documented. The thinking is apparently that
xmlspec.xsl
should be a generic stylesheet that applies to all the W3C specifications (or at any rate, all those that are authored in XML), while
REC-xml.xsl
is a customization for the XML Recommendation in particular.

Looking through the contents of this stylesheet, most of it appears to be rather pragmatic. For example, it recognizes that an

element can have a
role
attribute with values such as
“2e”
,
“3e”
, and
“4e”
, and thus allows the published output to distinguish between authors of each edition of the specification. The
role
attribute is permitted on pretty well any element in the DTD, and has no defined values; a comment in the DTD makes it fairly clear that it is intended as a general-purpose extensibility mechanism to allow document editors to add information that can't otherwise be captured.