Our tests have shown that the XPS output on different versions of MS Office (even minor version changes) is different. Same, saving the XPS from different client machines, outputs different XPS files.
We want to be able to save the client version of XPS from an OfficeJS addin. We are aware of the possibilities to generate it on the server (using Open XML for example) but this won't do it for us.
It might be that Word Online will not support XPS but the desktop version must support it.