According to the following extract, aspose words support xhtml. But in the save options do not include xhtml.
1-How can I save a document in xhtml
2-What is the suggested mechanism for converting an html document to xhtml?
Load and save HTML/XHTML/MHTML for greater interoperability.
Aspose.Words HTML and XHTML import is resilient to documents that are not
well formed or that are invalid. Invalid or unrecognized elements or
attributes are safely ignored during import. Conversion to HTML and
XHTML in Aspose.Words is customizable, many parameters like the export
of document properties, headers and footers, image locations, scaling and
resolution, form field export options and others can all be specified.