Public/system ID for specific versions of stylesheets?

[Jon Leech <developer@oddhack.org>]

    I have a couple of versioning/import questions regarding
the Docbook modular stylesheets:

  - How much of an issue is backwards compatibility? If a document was
    written as a customization of the 1.68.1 stylesheets, is it likely
    to work with all later 1.x versions?
  - If not, are there canonical SYSTEM IDs for different released
    versions of stylesheets, that are likely to actually be present
    on-web well into the future?
  - Or is there a general approach to stylesheet customizations which
    avoids version/platform install dependencies?

    To be specific, one of our authors wrote a customization of the
modular stylesheets which looks like this at the top level:

<?xml version='1.0'?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"; version="1.0">
    <xsl:import
     href="http://docbook.sourceforge.net/release/xsl/1.68.1/html/docbook.xsl"/>
    <xsl:include href="common.xsl"/>
    <xsl:output method="html" encoding="us-ascii" indent="no"/>
</xsl:stylesheet>

    This works on his system with the 1.68.1 stylesheets installed,
since the XML catalog resolves the xsl:import URI to the local copy.

    However, the actual URI on docbook.sourceforge.net no longer exists
(was archived some time back). So when the document is built on another
system with a different version of the stylesheets installed, it doesn't
build at all - as contrasted to building slowly due to fetching the
document over the web.

    If the import is changed to

    <xsl:import
     href="http://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl"/>

    then it will work on more platforms, but at the risk of using an
incompatible version of the stylesheets. On my system "current" resolves
via the catalog to the installed 1.69.1 stylesheets, which work as well
as 1.68.1. But someone else might get the very latest db5 version of the
stylesheets, or a much earlier version, depending on what they happened
to have installed, with possible compatibility issues.

    Jon