Saxon 9.3 as issued has a dependency on the interface javax.xml.stream.XMLStreamWriter, which is present in JDK 1.6 but not in JDK 1.5. It is possible to obtain JAXP 1.4 for use with JDK 1.5, but this is inconvenient. In fact Saxon 9.2 also used this interface, but only in less-commonly used parts of the product (such as XQJ), so most users were unaffected.
This is being fixed in the first maintenance release (220.127.116.11). There are two classes (net.sf.saxon.serializer.SerializerFactory and net.sf.saxon.s9api.Serializer) that contain methods whose declared return type is XMLStreamWriter. These will be changed to declare the return type as Saxon's implementation of this class, StreamWriterToReceiver. This is apparently sufficient to ensure that the interface does not need to be on the classpath unless the relevant method is actually called.