Turns out that SAXON 8 (including the latest version) does not properly build W3C DOMs. It uses empty strings for the namespace URI instead of null values. Batik's SVG DOM currently doesn't (defensively) convert empty strings to null values to avoid the problem. If anyone wants to use FOP Trunk with SAXON 8 and SVG in instream-foreign-objects, there will be trouble.
Jeremias Maerki
