I've released an improved version of xslthl today which, amongst other changes, adds support for Xalan and Saxon 8.5+.
To use this updated version the highlighting/common.xsl file needs to be adjusted. Attached is a patch of the required changes to the xsl-ns-1.74 release. The full file can also be found here:
The construction used makes the stylesheet safe to be used by xalan and the two saxon xslt processors.
An other change that affects docbook-xsl is the removal and addition of some xslthl elements. xslthl:section is removed and xslthl:annotation and xslthl:directive have been added.
All this information is also explained here: