Referencing "Official" copy of stylesheets

We tend to include a line like the following near the top of our customization layer stylesheets:

<xsl:import href=""
http://docbook.sourceforge.net/release/xsl/1.72.0/html/docbook.xsl"/>

We then use catalogs to map the web address to something local on the users PC -- but it's nice to know that if there's no catalog, the information can still be found (albeit probably a bit slower) on the Internet. The problem comes in when there's a new release: right now the "
1.72.0" url doesn't exist; it's been replaced by "1.73.0" which means without a catalog, our stylesheets stop working.

I know one option would be to use "current" rather than a specific stylesheet version but we don't like to switch stylesheets without testing first. Perhaps there is a different public url we could use?