distinguishing between presentational and transformational stylesheets

[I filed bug 366746 
<https://bugzilla.mozilla.org/show_bug.cgi?id=366746> on this proposal 
but thought it might be useful to generate discussion in this group, so 
here's what I wrote in the bug.]

XSLT stylesheets seem to be used in two relatively distinct ways:

1. to format/style data for presentation to users;
2. to process/transform data for input into applications.

It would be useful to distinguish between these two uses so that 
Gecko-enabled applications can selectively apply stylesheets depending 
on whether or not the XSLT-processed data documents they load are being 
presented to users.

For example, when the microsummary service in Firefox retrieves web 
pages in order to extract microsummaries from them, it treats those 
pages as sources of data, not documents to be displayed to users.  So it 
would make sense for the service to apply transformational but not 
presentational stylesheets to those pages.

Jonas Sicking has suggested that this could be accomplished either by 
using separate processing instructions for the two cases or by adding an 
attribute to the "xml-stylesheet" processing instruction to identify the 
purpose of the stylesheet.

Presumably Gecko would always apply transformational stylesheets when 
loading documents (as both uses require the data to be transformed into 
its final form) but only apply presentational stylesheets for documents 
being displayed to users, so both kinds of stylesheets would be applied 
to XML documents loaded into a Firefox browser tab, but only 
transformational stylesheets would be applied to data loaded via 
XMLHttpRequest.

But perhaps it would be useful to allow calling code to specify which 
kinds of stylesheets to apply when initiating a load, if there are edge 
cases which deviate from this pattern.

As an aside, I wonder if this facility could also be used to resolve the 
conflict between Firefox developers and some site authors over Firefox's 
behavior around site-provided stylesheets for feeds.

Briefly, Firefox ignores such stylesheets when presenting users with a 
feed reading interface to feeds, because the vast majority of those 
stylesheets exist solely to improve the presentation of those feeds in 
browsers that don't support feed reading.

But a few sites use feeds and XSLT as two layers in the technology stack 
by which they construct web applications, and they resent having to 
employ hacks to prevent Firefox from breaking those applications by 
ignoring one component in that stack because it erroneously sees the 
feed document as "just a feed" with a presentational stylesheet.

Perhaps those sites could use this facility to identify their 
stylesheets as transformational, thus triggering Firefox to apply those 
stylesheets without the need for hacks, while Firefox continues to 
appropriately ignore presentational stylesheets when presenting "just a 
feed" feeds to users.