Hi Christoph,
I have also reported the problem with "onmouseover" and other events not  
recognised in XHTML modularization schemas
[http://lists.w3.org/Archives/Public/www-html-editor/2006AprJun/0014.html ]  
(without any answer so far).

I suggest a correction in  
[http://www.w3.org/TR/xhtml-modularization/SCHEMA/xhtml-attribs-1.xsd ]:

Lines 48-53 of xhtml-attribs-1.xsd:

<xs:attributeGroup name="xhtml.Common.attrib">
  <xs:attributeGroup ref="xhtml.Core.attrib"/>
  <xs:attributeGroup ref="xhtml.I18n.attrib"/>
  <xs:attributeGroup ref="xhtml.Common.extra"/>
  <xs:attributeGroup ref="xhtml.Events.attrib"/><!-- Line added -->
</xs:attributeGroup>

Cordially,
Alexandre
http://alexandre.alapetite.net